What's New In Python 3.3
************************

This article explains the new features in Python 3.3, compared to 3.2.
Python 3.3 was released on September 29, 2012.  For full details, see
the changelog.

See also: **PEP 398** - Python 3.3 Release Schedule


Summary -- Release highlights
=============================

New syntax features:

* New "yield from" expression for generator delegation.

* The "u'unicode'" syntax is accepted again for "str" objects.

New library modules:

* "faulthandler" (helps debugging low-level crashes)

* "ipaddress" (high-level objects representing IP addresses and masks)

* "lzma" (compress data using the XZ / LZMA algorithm)

* "unittest.mock" (replace parts of your system under test with mock
  objects)

* "venv" (Python virtual environments, as in the popular "virtualenv"
  package)

New built-in features:

* Reworked I/O exception hierarchy.

Implementation improvements:

* Rewritten import machinery based on "importlib".

* More compact unicode strings.

* More compact attribute dictionaries.

Significantly Improved Library Modules:

* C Accelerator for the decimal module.

* Better unicode handling in the email module (*provisional*).

Security improvements:

* Hash randomization is switched on by default.

Please read on for a comprehensive list of user-facing changes.


PEP 405: Virtual Environments
=============================

Virtual environments help create separate Python setups while sharing
a system-wide base install, for ease of maintenance.  Virtual
environments have their own set of private site packages (i.e. locally
installed libraries), and are optionally segregated from the system-
wide site packages.  Their concept and implementation are inspired by
the popular "virtualenv" third-party package, but benefit from tighter
integration with the interpreter core.

This PEP adds the "venv" module for programmatic access, and the
"pyvenv" script for command-line access and administration.  The
Python interpreter checks for a "pyvenv.cfg", file whose existence
signals the base of a virtual environment's directory tree.

See also:

  **PEP 405** - Python Virtual Environments
     PEP written by Carl Meyer; implementation by Carl Meyer and Vinay
     Sajip


PEP 420: Implicit Namespace Packages
====================================

Native support for package directories that don't require
"__init__.py" marker files and can automatically span multiple path
segments (inspired by various third party approaches to namespace
packages, as described in **PEP 420**)

See also:

  **PEP 420** - Implicit Namespace Packages
     PEP written by Eric V. Smith; implementation by Eric V. Smith and
     Barry Warsaw


PEP 3118: New memoryview implementation and buffer protocol documentation
=========================================================================

The implementation of **PEP 3118** has been significantly improved.

The new memoryview implementation comprehensively fixes all ownership
and lifetime issues of dynamically allocated fields in the Py_buffer
struct that led to multiple crash reports. Additionally, several
functions that crashed or returned incorrect results for non-
contiguous or multi-dimensional input have been fixed.

The memoryview object now has a PEP-3118 compliant getbufferproc()
that checks the consumer's request type. Many new features have been
added, most of them work in full generality for non-contiguous arrays
and arrays with suboffsets.

The documentation has been updated, clearly spelling out
responsibilities for both exporters and consumers. Buffer request
flags are grouped into basic and compound flags. The memory layout of
non-contiguous and multi-dimensional NumPy-style arrays is explained.


Features
--------

* All native single character format specifiers in struct module
  syntax (optionally prefixed with '@') are now supported.

* With some restrictions, the cast() method allows changing of format
  and shape of C-contiguous arrays.

* Multi-dimensional list representations are supported for any array
  type.

* Multi-dimensional comparisons are supported for any array type.

* One-dimensional memoryviews of hashable (read-only) types with
  formats B, b or c are now hashable.  (Contributed by Antoine Pitrou
  in bpo-13411.)

* Arbitrary slicing of any 1-D arrays type is supported. For example,
  it is now possible to reverse a memoryview in *O*(1) by using a
  negative step.


API changes
-----------

* The maximum number of dimensions is officially limited to 64.

* The representation of empty shape, strides and suboffsets is now an
  empty tuple instead of "None".

* Accessing a memoryview element with format 'B' (unsigned bytes) now
  returns an integer (in accordance with the struct module syntax).
  For returning a bytes object the view must be cast to 'c' first.

* memoryview comparisons now use the logical structure of the operands
  and compare all array elements by value. All format strings in
  struct module syntax are supported. Views with unrecognised format
  strings are still permitted, but will always compare as unequal,
  regardless of view contents.

* For further changes see Build and C API Changes and Porting C code.

(Contributed by Stefan Krah in bpo-10181.)

See also: **PEP 3118** - Revising the Buffer Protocol


PEP 393: Flexible String Representation
=======================================

The Unicode string type is changed to support multiple internal
representations, depending on the character with the largest Unicode
ordinal (1, 2, or 4 bytes) in the represented string.  This allows a
space-efficient representation in common cases, but gives access to
full UCS-4 on all systems.  For compatibility with existing APIs,
several representations may exist in parallel; over time, this
compatibility should be phased out.

On the Python side, there should be no downside to this change.

On the C API side, **PEP 393** is fully backward compatible.  The
legacy API should remain available at least five years.  Applications
using the legacy API will not fully benefit of the memory reduction,
or - worse - may use a bit more memory, because Python may have to
maintain two versions of each string (in the legacy format and in the
new efficient storage).


Functionality
-------------

Changes introduced by **PEP 393** are the following:

* Python now always supports the full range of Unicode code points,
  including non-BMP ones (i.e. from "U+0000" to "U+10FFFF").  The
  distinction between narrow and wide builds no longer exists and
  Python now behaves like a wide build, even under Windows.

* With the death of narrow builds, the problems specific to narrow
  builds have also been fixed, for example:

  * "len()" now always returns 1 for non-BMP characters, so
    "len('\U0010FFFF') == 1";

  * surrogate pairs are not recombined in string literals, so
    "'\uDBFF\uDFFF' != '\U0010FFFF'";

  * indexing or slicing non-BMP characters returns the expected value,
    so "'\U0010FFFF'[0]" now returns "'\U0010FFFF'" and not
    "'\uDBFF'";

  * all other functions in the standard library now correctly handle
    non-BMP code points.

* The value of "sys.maxunicode" is now always "1114111" ("0x10FFFF" in
  hexadecimal).  The "PyUnicode_GetMax()" function still returns
  either "0xFFFF" or "0x10FFFF" for backward compatibility, and it
  should not be used with the new Unicode API (see bpo-13054).

* The "./configure" flag "--with-wide-unicode" has been removed.


Performance and resource usage
------------------------------

The storage of Unicode strings now depends on the highest code point
in the string:

* pure ASCII and Latin1 strings ("U+0000-U+00FF") use 1 byte per code
  point;

* BMP strings ("U+0000-U+FFFF") use 2 bytes per code point;

* non-BMP strings ("U+10000-U+10FFFF") use 4 bytes per code point.

The net effect is that for most applications, memory usage of string
storage should decrease significantly - especially compared to former
wide unicode builds - as, in many cases, strings will be pure ASCII
even in international contexts (because many strings store non-human
language data, such as XML fragments, HTTP headers, JSON-encoded data,
etc.).  We also hope that it will, for the same reasons, increase CPU
cache efficiency on non-trivial applications. The memory usage of
Python 3.3 is two to three times smaller than Python 3.2, and a little
bit better than Python 2.7, on a Django benchmark (see the PEP for
details).

See also:

  **PEP 393** - Flexible String Representation
     PEP written by Martin von Löwis; implementation by Torsten Becker
     and Martin von Löwis.


PEP 397: Python Launcher for Windows
====================================

The Python 3.3 Windows installer now includes a "py" launcher
application that can be used to launch Python applications in a
version independent fashion.

This launcher is invoked implicitly when double-clicking "*.py" files.
If only a single Python version is installed on the system, that
version will be used to run the file. If multiple versions are
installed, the most recent version is used by default, but this can be
overridden by including a Unix-style "shebang line" in the Python
script.

The launcher can also be used explicitly from the command line as the
"py" application. Running "py" follows the same version selection
rules as implicitly launching scripts, but a more specific version can
be selected by passing appropriate arguments (such as "-3" to request
Python 3 when Python 2 is also installed, or "-2.6" to specifically
request an earlier Python version when a more recent version is
installed).

In addition to the launcher, the Windows installer now includes an
option to add the newly installed Python to the system PATH.
(Contributed by Brian Curtin in bpo-3561.)

See also:

  **PEP 397** - Python Launcher for Windows
     PEP written by Mark Hammond and Martin v. Löwis; implementation
     by Vinay Sajip.

  Launcher documentation: Python Launcher for Windows

  Installer PATH modification: Finding the Python executable


PEP 3151: Reworking the OS and IO exception hierarchy
=====================================================

The hierarchy of exceptions raised by operating system errors is now
both simplified and finer-grained.

You don't have to worry anymore about choosing the appropriate
exception type between "OSError", "IOError", "EnvironmentError",
"WindowsError", "mmap.error", "socket.error" or "select.error".  All
these exception types are now only one: "OSError".  The other names
are kept as aliases for compatibility reasons.

Also, it is now easier to catch a specific error condition.  Instead
of inspecting the "errno" attribute (or "args[0]") for a particular
constant from the "errno" module, you can catch the adequate "OSError"
subclass.  The available subclasses are the following:

* "BlockingIOError"

* "ChildProcessError"

* "ConnectionError"

* "FileExistsError"

* "FileNotFoundError"

* "InterruptedError"

* "IsADirectoryError"

* "NotADirectoryError"

* "PermissionError"

* "ProcessLookupError"

* "TimeoutError"

And the "ConnectionError" itself has finer-grained subclasses:

* "BrokenPipeError"

* "ConnectionAbortedError"

* "ConnectionRefusedError"

* "ConnectionResetError"

Thanks to the new exceptions, common usages of the "errno" can now be
avoided.  For example, the following code written for Python 3.2:

   from errno import ENOENT, EACCES, EPERM

   try:
       with open("document.txt") as f:
           content = f.read()
   except IOError as err:
       if err.errno == ENOENT:
           print("document.txt file is missing")
       elif err.errno in (EACCES, EPERM):
           print("You are not allowed to read document.txt")
       else:
           raise

can now be written without the "errno" import and without manual
inspection of exception attributes:

   try:
       with open("document.txt") as f:
           content = f.read()
   except FileNotFoundError:
       print("document.txt file is missing")
   except PermissionError:
       print("You are not allowed to read document.txt")

See also:

  **PEP 3151** - Reworking the OS and IO Exception Hierarchy
     PEP written and implemented by Antoine Pitrou


PEP 380: Syntax for Delegating to a Subgenerator
================================================

PEP 380 adds the "yield from" expression, allowing a *generator* to
delegate part of its operations to another generator. This allows a
section of code containing "yield" to be factored out and placed in
another generator. Additionally, the subgenerator is allowed to return
with a value, and the value is made available to the delegating
generator.

While designed primarily for use in delegating to a subgenerator, the
"yield from" expression actually allows delegation to arbitrary
subiterators.

For simple iterators, "yield from iterable" is essentially just a
shortened form of "for item in iterable: yield item":

   >>> def g(x):
   ...     yield from range(x, 0, -1)
   ...     yield from range(x)
   ...
   >>> list(g(5))
   [5, 4, 3, 2, 1, 0, 1, 2, 3, 4]

However, unlike an ordinary loop, "yield from" allows subgenerators to
receive sent and thrown values directly from the calling scope, and
return a final value to the outer generator:

   >>> def accumulate():
   ...     tally = 0
   ...     while 1:
   ...         next = yield
   ...         if next is None:
   ...             return tally
   ...         tally += next
   ...
   >>> def gather_tallies(tallies):
   ...     while 1:
   ...         tally = yield from accumulate()
   ...         tallies.append(tally)
   ...
   >>> tallies = []
   >>> acc = gather_tallies(tallies)
   >>> next(acc)  # Ensure the accumulator is ready to accept values
   >>> for i in range(4):
   ...     acc.send(i)
   ...
   >>> acc.send(None)  # Finish the first tally
   >>> for i in range(5):
   ...     acc.send(i)
   ...
   >>> acc.send(None)  # Finish the second tally
   >>> tallies
   [6, 10]

The main principle driving this change is to allow even generators
that are designed to be used with the "send" and "throw" methods to be
split into multiple subgenerators as easily as a single large function
can be split into multiple subfunctions.

See also:

  **PEP 380** - Syntax for Delegating to a Subgenerator
     PEP written by Greg Ewing; implementation by Greg Ewing,
     integrated into 3.3 by Renaud Blanch, Ryan Kelly and Nick
     Coghlan; documentation by Zbigniew Jędrzejewski-Szmek and Nick
     Coghlan


PEP 409: Suppressing exception context
======================================

PEP 409 introduces new syntax that allows the display of the chained
exception context to be disabled. This allows cleaner error messages
in applications that convert between exception types:

   >>> class D:
   ...     def __init__(self, extra):
   ...         self._extra_attributes = extra
   ...     def __getattr__(self, attr):
   ...         try:
   ...             return self._extra_attributes[attr]
   ...         except KeyError:
   ...             raise AttributeError(attr) from None
   ...
   >>> D({}).x
   Traceback (most recent call last):
     File "<stdin>", line 1, in <module>
     File "<stdin>", line 8, in __getattr__
   AttributeError: x

Without the "from None" suffix to suppress the cause, the original
exception would be displayed by default:

   >>> class C:
   ...     def __init__(self, extra):
   ...         self._extra_attributes = extra
   ...     def __getattr__(self, attr):
   ...         try:
   ...             return self._extra_attributes[attr]
   ...         except KeyError:
   ...             raise AttributeError(attr)
   ...
   >>> C({}).x
   Traceback (most recent call last):
     File "<stdin>", line 6, in __getattr__
   KeyError: 'x'

   During handling of the above exception, another exception occurred:

   Traceback (most recent call last):
     File "<stdin>", line 1, in <module>
     File "<stdin>", line 8, in __getattr__
   AttributeError: x

No debugging capability is lost, as the original exception context
remains available if needed (for example, if an intervening library
has incorrectly suppressed valuable underlying details):

   >>> try:
   ...     D({}).x
   ... except AttributeError as exc:
   ...     print(repr(exc.__context__))
   ...
   KeyError('x',)

See also:

  **PEP 409** - Suppressing exception context
     PEP written by Ethan Furman; implemented by Ethan Furman and Nick
     Coghlan.


PEP 414: Explicit Unicode literals
==================================

To ease the transition from Python 2 for Unicode aware Python
applications that make heavy use of Unicode literals, Python 3.3 once
again supports the ""u"" prefix for string literals. This prefix has
no semantic significance in Python 3, it is provided solely to reduce
the number of purely mechanical changes in migrating to Python 3,
making it easier for developers to focus on the more significant
semantic changes (such as the stricter default separation of binary
and text data).

See also:

  **PEP 414** - Explicit Unicode literals
     PEP written by Armin Ronacher.


PEP 3155: Qualified name for classes and functions
==================================================

Functions and class objects have a new "__qualname__" attribute
representing the "path" from the module top-level to their definition.
For global functions and classes, this is the same as "__name__". For
other functions and classes, it provides better information about
where they were actually defined, and how they might be accessible
from the global scope.

Example with (non-bound) methods:

   >>> class C:
   ...     def meth(self):
   ...         pass
   ...
   >>> C.meth.__name__
   'meth'
   >>> C.meth.__qualname__
   'C.meth'

Example with nested classes:

   >>> class C:
   ...     class D:
   ...         def meth(self):
   ...             pass
   ...
   >>> C.D.__name__
   'D'
   >>> C.D.__qualname__
   'C.D'
   >>> C.D.meth.__name__
   'meth'
   >>> C.D.meth.__qualname__
   'C.D.meth'

Example with nested functions:

   >>> def outer():
   ...     def inner():
   ...         pass
   ...     return inner
   ...
   >>> outer().__name__
   'inner'
   >>> outer().__qualname__
   'outer.<locals>.inner'

The string representation of those objects is also changed to include
the new, more precise information:

   >>> str(C.D)
   "<class '__main__.C.D'>"
   >>> str(C.D.meth)
   '<function C.D.meth at 0x7f46b9fe31e0>'

See also:

  **PEP 3155** - Qualified name for classes and functions
     PEP written and implemented by Antoine Pitrou.


PEP 412: Key-Sharing Dictionary
===============================

Dictionaries used for the storage of objects' attributes are now able
to share part of their internal storage between each other (namely,
the part which stores the keys and their respective hashes).  This
reduces the memory consumption of programs creating many instances of
non-builtin types.

See also:

  **PEP 412** - Key-Sharing Dictionary
     PEP written and implemented by Mark Shannon.


PEP 362: Function Signature Object
==================================

A new function "inspect.signature()" makes introspection of python
callables easy and straightforward.  A broad range of callables is
supported: python functions, decorated or not, classes, and
"functools.partial()" objects.  New classes "inspect.Signature",
"inspect.Parameter" and "inspect.BoundArguments" hold information
about the call signatures, such as, annotations, default values,
parameters kinds, and bound arguments, which considerably simplifies
writing decorators and any code that validates or amends calling
signatures or arguments.

See also:

  **PEP 362**: -  Function Signature Object
     PEP written by Brett Cannon, Yury Selivanov, Larry Hastings,
     Jiwon Seo; implemented by Yury Selivanov.


PEP 421: Adding sys.implementation
==================================

A new attribute on the "sys" module exposes details specific to the
implementation of the currently running interpreter.  The initial set
of attributes on "sys.implementation" are "name", "version",
"hexversion", and "cache_tag".

The intention of "sys.implementation" is to consolidate into one
namespace the implementation-specific data used by the standard
library.  This allows different Python implementations to share a
single standard library code base much more easily.  In its initial
state, "sys.implementation" holds only a small portion of the
implementation-specific data.  Over time that ratio will shift in
order to make the standard library more portable.

One example of improved standard library portability is "cache_tag".
As of Python 3.3, "sys.implementation.cache_tag" is used by
"importlib" to support **PEP 3147** compliance.  Any Python
implementation that uses "importlib" for its built-in import system
may use "cache_tag" to control the caching behavior for modules.


SimpleNamespace
---------------

The implementation of "sys.implementation" also introduces a new type
to Python: "types.SimpleNamespace".  In contrast to a mapping-based
namespace, like "dict", "SimpleNamespace" is attribute-based, like
"object".  However, unlike "object", "SimpleNamespace" instances are
writable.  This means that you can add, remove, and modify the
namespace through normal attribute access.

See also:

  **PEP 421** - Adding sys.implementation
     PEP written and implemented by Eric Snow.


Using importlib as the Implementation of Import
===============================================

bpo-2377 - Replace __import__ w/ importlib.__import__ bpo-13959 - Re-
implement parts of "imp" in pure Python bpo-14605 - Make import
machinery explicit bpo-14646 - Require loaders set __loader__ and
__package__

The "__import__()" function is now powered by
"importlib.__import__()". This work leads to the completion of "phase
2" of **PEP 302**. There are multiple benefits to this change. First,
it has allowed for more of the machinery powering import to be exposed
instead of being implicit and hidden within the C code. It also
provides a single implementation for all Python VMs supporting Python
3.3 to use, helping to end any VM-specific deviations in import
semantics. And finally it eases the maintenance of import, allowing
for future growth to occur.

For the common user, there should be no visible change in semantics.
For those whose code currently manipulates import or calls import
programmatically, the code changes that might possibly be required are
covered in the Porting Python code section of this document.


New APIs
--------

One of the large benefits of this work is the exposure of what goes
into making the import statement work. That means the various
importers that were once implicit are now fully exposed as part of the
"importlib" package.

The abstract base classes defined in "importlib.abc" have been
expanded to properly delineate between *meta path finders* and *path
entry finders* by introducing "importlib.abc.MetaPathFinder" and
"importlib.abc.PathEntryFinder", respectively. The old ABC of
"importlib.abc.Finder" is now only provided for backwards-
compatibility and does not enforce any method requirements.

In terms of finders, "importlib.machinery.FileFinder" exposes the
mechanism used to search for source and bytecode files of a module.
Previously this class was an implicit member of "sys.path_hooks".

For loaders, the new abstract base class "importlib.abc.FileLoader"
helps write a loader that uses the file system as the storage
mechanism for a module's code. The loader for source files
("importlib.machinery.SourceFileLoader"), sourceless bytecode files
("importlib.machinery.SourcelessFileLoader"), and extension modules
("importlib.machinery.ExtensionFileLoader") are now available for
direct use.

"ImportError" now has "name" and "path" attributes which are set when
there is relevant data to provide. The message for failed imports will
also provide the full name of the module now instead of just the tail
end of the module's name.

The "importlib.invalidate_caches()" function will now call the method
with the same name on all finders cached in "sys.path_importer_cache"
to help clean up any stored state as necessary.


Visible Changes
---------------

For potential required changes to code, see the Porting Python code
section.

Beyond the expanse of what "importlib" now exposes, there are other
visible changes to import. The biggest is that "sys.meta_path" and
"sys.path_hooks" now store all of the meta path finders and path entry
hooks used by import.  Previously the finders were implicit and hidden
within the C code of import instead of being directly exposed. This
means that one can now easily remove or change the order of the
various finders to fit one's needs.

Another change is that all modules have a "__loader__" attribute,
storing the loader used to create the module. **PEP 302** has been
updated to make this attribute mandatory for loaders to implement, so
in the future once 3rd-party loaders have been updated people will be
able to rely on the existence of the attribute. Until such time,
though, import is setting the module post-load.

Loaders are also now expected to set the "__package__" attribute from
**PEP 366**. Once again, import itself is already setting this on all
loaders from "importlib" and import itself is setting the attribute
post-load.

"None" is now inserted into "sys.path_importer_cache" when no finder
can be found on "sys.path_hooks". Since "imp.NullImporter" is not
directly exposed on "sys.path_hooks" it could no longer be relied upon
to always be available to use as a value representing no finder found.

All other changes relate to semantic changes which should be taken
into consideration when updating code for Python 3.3, and thus should
be read about in the Porting Python code section of this document.

(Implementation by Brett Cannon)


Other Language Changes
======================

Some smaller changes made to the core Python language are:

* Added support for Unicode name aliases and named sequences. Both
  "unicodedata.lookup()" and "'\N{...}'" now resolve name aliases, and
  "unicodedata.lookup()" resolves named sequences too.

  (Contributed by Ezio Melotti in bpo-12753.)

* Unicode database updated to UCD version 6.1.0

* Equality comparisons on "range()" objects now return a result
  reflecting the equality of the underlying sequences generated by
  those range objects. (bpo-13201)

* The "count()", "find()", "rfind()", "index()" and "rindex()" methods
  of "bytes" and "bytearray" objects now accept an integer between 0
  and 255 as their first argument.

  (Contributed by Petri Lehtinen in bpo-12170.)

* The "rjust()", "ljust()", and "center()" methods of "bytes" and
  "bytearray" now accept a "bytearray" for the "fill" argument.
  (Contributed by Petri Lehtinen in bpo-12380.)

* New methods have been added to "list" and "bytearray": "copy()" and
  "clear()" (bpo-10516).  Consequently, "MutableSequence" now also
  defines a "clear()" method (bpo-11388).

* Raw bytes literals can now be written "rb"..."" as well as
  "br"..."".

  (Contributed by Antoine Pitrou in bpo-13748.)

* "dict.setdefault()" now does only one lookup for the given key,
  making it atomic when used with built-in types.

  (Contributed by Filip Gruszczyński in bpo-13521.)

* The error messages produced when a function call does not match the
  function signature have been significantly improved.

  (Contributed by Benjamin Peterson.)


A Finer-Grained Import Lock
===========================

Previous versions of CPython have always relied on a global import
lock. This led to unexpected annoyances, such as deadlocks when
importing a module would trigger code execution in a different thread
as a side-effect. Clumsy workarounds were sometimes employed, such as
the "PyImport_ImportModuleNoBlock()" C API function.

In Python 3.3, importing a module takes a per-module lock.  This
correctly serializes importation of a given module from multiple
threads (preventing the exposure of incompletely initialized modules),
while eliminating the aforementioned annoyances.

(Contributed by Antoine Pitrou in bpo-9260.)


Builtin functions and types
===========================

* "open()" gets a new *opener* parameter: the underlying file
  descriptor for the file object is then obtained by calling *opener*
  with (*file*, *flags*). It can be used to use custom flags like
  "os.O_CLOEXEC" for example. The "'x'" mode was added: open for
  exclusive creation, failing if the file already exists.

* "print()": added the *flush* keyword argument. If the *flush*
  keyword argument is true, the stream is forcibly flushed.

* "hash()": hash randomization is enabled by default, see
  "object.__hash__()" and "PYTHONHASHSEED".

* The "str" type gets a new "casefold()" method: return a casefolded
  copy of the string, casefolded strings may be used for caseless
  matching. For example, "'ß'.casefold()" returns "'ss'".

* The sequence documentation has been substantially rewritten to
  better explain the binary/text sequence distinction and to provide
  specific documentation sections for the individual builtin sequence
  types (bpo-4966).


New Modules
===========


faulthandler
------------

This new debug module "faulthandler" contains functions to dump Python
tracebacks explicitly, on a fault (a crash like a segmentation fault),
after a timeout, or on a user signal. Call "faulthandler.enable()" to
install fault handlers for the "SIGSEGV", "SIGFPE", "SIGABRT",
"SIGBUS", and "SIGILL" signals. You can also enable them at startup by
setting the "PYTHONFAULTHANDLER" environment variable or by using "-X"
"faulthandler" command line option.

Example of a segmentation fault on Linux:

   $ python -q -X faulthandler
   >>> import ctypes
   >>> ctypes.string_at(0)
   Fatal Python error: Segmentation fault

   Current thread 0x00007fb899f39700:
     File "/home/python/cpython/Lib/ctypes/__init__.py", line 486 in string_at
     File "<stdin>", line 1 in <module>
   Segmentation fault


ipaddress
---------

The new "ipaddress" module provides tools for creating and
manipulating objects representing IPv4 and IPv6 addresses, networks
and interfaces (i.e. an IP address associated with a specific IP
subnet).

(Contributed by Google and Peter Moody in **PEP 3144**.)


lzma
----

The newly added "lzma" module provides data compression and
decompression using the LZMA algorithm, including support for the
".xz" and ".lzma" file formats.

(Contributed by Nadeem Vawda and Per Øyvind Karlsen in bpo-6715.)


Improved Modules
================


abc
---

Improved support for abstract base classes containing descriptors
composed with abstract methods. The recommended approach to declaring
abstract descriptors is now to provide "__isabstractmethod__" as a
dynamically updated property. The built-in descriptors have been
updated accordingly.

* "abc.abstractproperty" has been deprecated, use "property" with
  "abc.abstractmethod()" instead.

* "abc.abstractclassmethod" has been deprecated, use "classmethod"
  with "abc.abstractmethod()" instead.

* "abc.abstractstaticmethod" has been deprecated, use "staticmethod"
  with "abc.abstractmethod()" instead.

(Contributed by Darren Dale in bpo-11610.)

"abc.ABCMeta.register()" now returns the registered subclass, which
means it can now be used as a class decorator (bpo-10868).


array
-----

The "array" module supports the long long type using "q" and "Q" type
codes.

(Contributed by Oren Tirosh and Hirokazu Yamamoto in bpo-1172711.)


base64
------

ASCII-only Unicode strings are now accepted by the decoding functions
of the "base64" modern interface. For example,
"base64.b64decode('YWJj')" returns "b'abc'".  (Contributed by Catalin
Iacob in bpo-13641.)


binascii
--------

In addition to the binary objects they normally accept, the "a2b_"
functions now all also accept ASCII-only strings as input.
(Contributed by Antoine Pitrou in bpo-13637.)


bz2
---

The "bz2" module has been rewritten from scratch. In the process,
several new features have been added:

* New "bz2.open()" function: open a bzip2-compressed file in binary or
  text mode.

* "bz2.BZ2File" can now read from and write to arbitrary file-like
  objects, by means of its constructor's *fileobj* argument.

  (Contributed by Nadeem Vawda in bpo-5863.)

* "bz2.BZ2File" and "bz2.decompress()" can now decompress multi-stream
  inputs (such as those produced by the **pbzip2** tool).
  "bz2.BZ2File" can now also be used to create this type of file,
  using the "'a'" (append) mode.

  (Contributed by Nir Aides in bpo-1625.)

* "bz2.BZ2File" now implements all of the "io.BufferedIOBase" API,
  except for the "detach()" and "truncate()" methods.


codecs
------

The "mbcs" codec has been rewritten to handle correctly "replace" and
"ignore" error handlers on all Windows versions.  The "mbcs" codec now
supports all error handlers, instead of only "replace" to encode and
"ignore" to decode.

A new Windows-only codec has been added: "cp65001" (bpo-13216). It is
the Windows code page 65001 (Windows UTF-8, "CP_UTF8").  For example,
it is used by "sys.stdout" if the console output code page is set to
cp65001 (e.g., using "chcp 65001" command).

Multibyte CJK decoders now resynchronize faster.  They only ignore the
first byte of an invalid byte sequence. For example,
"b'\xff\n'.decode('gb2312', 'replace')" now returns a "\n" after the
replacement character.

(bpo-12016)

Incremental CJK codec encoders are no longer reset at each call to
their encode() methods. For example:

   >>> import codecs
   >>> encoder = codecs.getincrementalencoder('hz')('strict')
   >>> b''.join(encoder.encode(x) for x in '\u52ff\u65bd\u65bc\u4eba\u3002 Bye.')
   b'~{NpJ)l6HK!#~} Bye.'

This example gives "b'~{Np~}~{J)~}~{l6~}~{HK~}~{!#~} Bye.'" with older
Python versions.

(bpo-12100)

The "unicode_internal" codec has been deprecated.


collections
-----------

Addition of a new "ChainMap" class to allow treating a number of
mappings as a single unit.  (Written by Raymond Hettinger for
bpo-11089, made public in bpo-11297.)

The abstract base classes have been moved in a new "collections.abc"
module, to better differentiate between the abstract and the concrete
collections classes.  Aliases for ABCs are still present in the
"collections" module to preserve existing imports.  (bpo-11085)

The "Counter" class now supports the unary "+" and "-" operators, as
well as the in-place operators "+=", "-=", "|=", and "&=".
(Contributed by Raymond Hettinger in bpo-13121.)


contextlib
----------

"ExitStack" now provides a solid foundation for programmatic
manipulation of context managers and similar cleanup functionality.
Unlike the previous "contextlib.nested" API (which was deprecated and
removed), the new API is designed to work correctly regardless of
whether context managers acquire their resources in their "__init__"
method (for example, file objects) or in their "__enter__" method (for
example, synchronisation objects from the "threading" module).

(bpo-13585)


crypt
-----

Addition of salt and modular crypt format (hashing method) and the
"mksalt()" function to the "crypt" module.

(bpo-10924)


curses
------

* If the "curses" module is linked to the ncursesw library, use
  Unicode functions when Unicode strings or characters are passed
  (e.g. "waddwstr()"), and bytes functions otherwise (e.g.
  "waddstr()").

* Use the locale encoding instead of "utf-8" to encode Unicode
  strings.

* "curses.window" has a new "curses.window.encoding" attribute.

* The "curses.window" class has a new "get_wch()" method to get a wide
  character

* The "curses" module has a new "unget_wch()" function to push a wide
  character so the next "get_wch()" will return it

(Contributed by Iñigo Serna in bpo-6755.)


datetime
--------

* Equality comparisons between naive and aware "datetime" instances
  now return "False" instead of raising "TypeError" (bpo-15006).

* New "datetime.datetime.timestamp()" method: Return POSIX timestamp
  corresponding to the "datetime" instance.

* The "datetime.datetime.strftime()" method supports formatting years
  older than 1000.

* The "datetime.datetime.astimezone()" method can now be called
  without arguments to convert datetime instance to the system
  timezone.


decimal
-------

bpo-7652 - integrate fast native decimal arithmetic.
   C-module and libmpdec written by Stefan Krah.

The new C version of the decimal module integrates the high speed
libmpdec library for arbitrary precision correctly rounded decimal
floating-point arithmetic. libmpdec conforms to IBM's General Decimal
Arithmetic Specification.

Performance gains range from 10x for database applications to 100x for
numerically intensive applications. These numbers are expected gains
for standard precisions used in decimal floating-point arithmetic.
Since the precision is user configurable, the exact figures may vary.
For example, in integer bignum arithmetic the differences can be
significantly higher.

The following table is meant as an illustration. Benchmarks are
available at https://www.bytereef.org/mpdecimal/quickstart.html.

   +-----------+---------------+----------------+---------------+
   |           | decimal.py    | _decimal       | speedup       |
   |===========|===============|================|===============|
   | pi        | 42.02s        | 0.345s         | 120x          |
   +-----------+---------------+----------------+---------------+
   | telco     | 172.19s       | 5.68s          | 30x           |
   +-----------+---------------+----------------+---------------+
   | psycopg   | 3.57s         | 0.29s          | 12x           |
   +-----------+---------------+----------------+---------------+


Features
~~~~~~~~

* The "FloatOperation" signal optionally enables stricter semantics
  for mixing floats and Decimals.

* If Python is compiled without threads, the C version automatically
  disables the expensive thread local context machinery. In this case,
  the variable "HAVE_THREADS" is set to "False".


API changes
~~~~~~~~~~~

* The C module has the following context limits, depending on the
  machine architecture:

     +---------------------+------------------+---------------------------+
     |                     | 32-bit           | 64-bit                    |
     |=====================|==================|===========================|
     | "MAX_PREC"          | "425000000"      | "999999999999999999"      |
     +---------------------+------------------+---------------------------+
     | "MAX_EMAX"          | "425000000"      | "999999999999999999"      |
     +---------------------+------------------+---------------------------+
     | "MIN_EMIN"          | "-425000000"     | "-999999999999999999"     |
     +---------------------+------------------+---------------------------+

* In the context templates ("DefaultContext", "BasicContext" and
  "ExtendedContext") the magnitude of "Emax" and "Emin" has changed to
  "999999".

* The "Decimal" constructor in decimal.py does not observe the context
  limits and converts values with arbitrary exponents or precision
  exactly. Since the C version has internal limits, the following
  scheme is used: If possible, values are converted exactly, otherwise
  "InvalidOperation" is raised and the result is NaN. In the latter
  case it is always possible to use "create_decimal()" in order to
  obtain a rounded or inexact value.

* The power function in decimal.py is always correctly rounded. In the
  C version, it is defined in terms of the correctly rounded "exp()"
  and "ln()" functions, but the final result is only "almost always
  correctly rounded".

* In the C version, the context dictionary containing the signals is a
  "MutableMapping".  For speed reasons, "flags" and "traps" always
  refer to the same "MutableMapping" that the context was initialized
  with. If a new signal dictionary is assigned, "flags" and "traps"
  are updated with the new values, but they do not reference the RHS
  dictionary.

* Pickling a "Context" produces a different output in order to have a
  common interchange format for the Python and C versions.

* The order of arguments in the "Context" constructor has been changed
  to match the order displayed by "repr()".

* The "watchexp" parameter in the "quantize()" method is deprecated.


email
-----


Policy Framework
~~~~~~~~~~~~~~~~

The email package now has a "policy" framework.  A "Policy" is an
object with several methods and properties that control how the email
package behaves.  The primary policy for Python 3.3 is the "Compat32"
policy, which provides backward compatibility with the email package
in Python 3.2.  A "policy" can be specified when an email message is
parsed by a "parser", or when a "Message" object is created, or when
an email is serialized using a "generator".  Unless overridden, a
policy passed to a "parser" is inherited by all the "Message" object
and sub-objects created by the "parser".  By default a "generator"
will use the policy of the "Message" object it is serializing.  The
default policy is "compat32".

The minimum set of controls implemented by all "policy" objects are:

+-----------------+---------------------------------------------------------+
| max_line_length | The maximum length, excluding the linesep character(s), |
|                 | individual lines may have when a "Message" is           |
|                 | serialized.  Defaults to 78.                            |
+-----------------+---------------------------------------------------------+
| linesep         | The character used to separate individual lines when a  |
|                 | "Message" is serialized.  Defaults to "\n".             |
+-----------------+---------------------------------------------------------+
| cte_type        | "7bit" or "8bit".  "8bit" applies only to a "Bytes"     |
|                 | "generator", and means that non-ASCII may be used where |
|                 | allowed by the protocol (or where it exists in the      |
|                 | original input).                                        |
+-----------------+---------------------------------------------------------+
| raise_on_defect | Causes a "parser" to raise error when defects are       |
|                 | encountered instead of adding them to the "Message"     |
|                 | object's "defects" list.                                |
+-----------------+---------------------------------------------------------+

A new policy instance, with new settings, is created using the
"clone()" method of policy objects.  "clone" takes any of the above
controls as keyword arguments.  Any control not specified in the call
retains its default value.  Thus you can create a policy that uses
"\r\n" linesep characters like this:

   mypolicy = compat32.clone(linesep='\r\n')

Policies can be used to make the generation of messages in the format
needed by your application simpler.  Instead of having to remember to
specify "linesep='\r\n'" in all the places you call a "generator", you
can specify it once, when you set the policy used by the "parser" or
the "Message", whichever your program uses to create "Message"
objects.  On the other hand, if you need to generate messages in
multiple forms, you can still specify the parameters in the
appropriate "generator" call.  Or you can have custom policy instances
for your different cases, and pass those in when you create the
"generator".


Provisional Policy with New Header API
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

While the policy framework is worthwhile all by itself, the main
motivation for introducing it is to allow the creation of new policies
that implement new features for the email package in a way that
maintains backward compatibility for those who do not use the new
policies.  Because the new policies introduce a new API, we are
releasing them in Python 3.3 as a *provisional policy*.  Backwards
incompatible changes (up to and including removal of the code) may
occur if deemed necessary by the core developers.

The new policies are instances of "EmailPolicy", and add the following
additional controls:

+-----------------+---------------------------------------------------------+
| refold_source   | Controls whether or not headers parsed by a "parser"    |
|                 | are refolded by the "generator".  It can be "none",     |
|                 | "long", or "all".  The default is "long", which means   |
|                 | that source headers with a line longer than             |
|                 | "max_line_length" get refolded.  "none" means no line   |
|                 | get refolded, and "all" means that all lines get        |
|                 | refolded.                                               |
+-----------------+---------------------------------------------------------+
| header_factory  | A callable that take a "name" and "value" and produces  |
|                 | a custom header object.                                 |
+-----------------+---------------------------------------------------------+

The "header_factory" is the key to the new features provided by the
new policies.  When one of the new policies is used, any header
retrieved from a "Message" object is an object produced by the
"header_factory", and any time you set a header on a "Message" it
becomes an object produced by "header_factory".  All such header
objects have a "name" attribute equal to the header name.  Address and
Date headers have additional attributes that give you access to the
parsed data of the header.  This means you can now do things like
this:

   >>> m = Message(policy=SMTP)
   >>> m['To'] = 'Éric <foo@example.com>'
   >>> m['to']
   'Éric <foo@example.com>'
   >>> m['to'].addresses
   (Address(display_name='Éric', username='foo', domain='example.com'),)
   >>> m['to'].addresses[0].username
   'foo'
   >>> m['to'].addresses[0].display_name
   'Éric'
   >>> m['Date'] = email.utils.localtime()
   >>> m['Date'].datetime
   datetime.datetime(2012, 5, 25, 21, 39, 24, 465484, tzinfo=datetime.timezone(datetime.timedelta(-1, 72000), 'EDT'))
   >>> m['Date']
   'Fri, 25 May 2012 21:44:27 -0400'
   >>> print(m)
   To: =?utf-8?q?=C3=89ric?= <foo@example.com>
   Date: Fri, 25 May 2012 21:44:27 -0400

You will note that the unicode display name is automatically encoded
as "utf-8" when the message is serialized, but that when the header is
accessed directly, you get the unicode version.  This eliminates any
need to deal with the "email.header" "decode_header()" or
"make_header()" functions.

You can also create addresses from parts:

   >>> m['cc'] = [Group('pals', [Address('Bob', 'bob', 'example.com'),
   ...                           Address('Sally', 'sally', 'example.com')]),
   ...            Address('Bonzo', addr_spec='bonz@laugh.com')]
   >>> print(m)
   To: =?utf-8?q?=C3=89ric?= <foo@example.com>
   Date: Fri, 25 May 2012 21:44:27 -0400
   cc: pals: Bob <bob@example.com>, Sally <sally@example.com>;, Bonzo <bonz@laugh.com>

Decoding to unicode is done automatically:

   >>> m2 = message_from_string(str(m))
   >>> m2['to']
   'Éric <foo@example.com>'

When you parse a message, you can use the "addresses" and "groups"
attributes of the header objects to access the groups and individual
addresses:

   >>> m2['cc'].addresses
   (Address(display_name='Bob', username='bob', domain='example.com'), Address(display_name='Sally', username='sally', domain='example.com'), Address(display_name='Bonzo', username='bonz', domain='laugh.com'))
   >>> m2['cc'].groups
   (Group(display_name='pals', addresses=(Address(display_name='Bob', username='bob', domain='example.com'), Address(display_name='Sally', username='sally', domain='example.com')), Group(display_name=None, addresses=(Address(display_name='Bonzo', username='bonz', domain='laugh.com'),))

In summary, if you use one of the new policies, header manipulation
works the way it ought to:  your application works with unicode
strings, and the email package transparently encodes and decodes the
unicode to and from the RFC standard Content Transfer Encodings.


Other API Changes
~~~~~~~~~~~~~~~~~

New "BytesHeaderParser", added to the "parser" module to complement
"HeaderParser" and complete the Bytes API.

New utility functions:

* "format_datetime()": given a "datetime", produce a string formatted
  for use in an email header.

* "parsedate_to_datetime()": given a date string from an email header,
  convert it into an aware "datetime", or a naive "datetime" if the
  offset is "-0000".

* "localtime()": With no argument, returns the current local time as
  an aware "datetime" using the local "timezone".  Given an aware
  "datetime", converts it into an aware "datetime" using the local
  "timezone".


ftplib
------

* "ftplib.FTP" now accepts a "source_address" keyword argument to
  specify the "(host, port)" to use as the source address in the bind
  call when creating the outgoing socket.  (Contributed by Giampaolo
  Rodolà in bpo-8594.)

* The "FTP_TLS" class now provides a new "ccc()" function to revert
  control channel back to plaintext.  This can be useful to take
  advantage of firewalls that know how to handle NAT with non-secure
  FTP without opening fixed ports.  (Contributed by Giampaolo Rodolà
  in bpo-12139.)

* Added "ftplib.FTP.mlsd()" method which provides a parsable directory
  listing format and deprecates "ftplib.FTP.nlst()" and
  "ftplib.FTP.dir()".  (Contributed by Giampaolo Rodolà in bpo-11072.)


functools
---------

The "functools.lru_cache()" decorator now accepts a "typed" keyword
argument (that defaults to "False" to ensure that it caches values of
different types that compare equal in separate cache slots.
(Contributed by Raymond Hettinger in bpo-13227.)


gc
--

It is now possible to register callbacks invoked by the garbage
collector before and after collection using the new "callbacks" list.


hmac
----

A new "compare_digest()" function has been added to prevent side
channel attacks on digests through timing analysis.  (Contributed by
Nick Coghlan and Christian Heimes in bpo-15061.)


http
----

"http.server.BaseHTTPRequestHandler" now buffers the headers and
writes them all at once when "end_headers()" is called.  A new method
"flush_headers()" can be used to directly manage when the accumulated
headers are sent. (Contributed by Andrew Schaaf in bpo-3709.)

"http.server" now produces valid "HTML 4.01 strict" output.
(Contributed by Ezio Melotti in bpo-13295.)

"http.client.HTTPResponse" now has a "readinto()" method, which means
it can be used as an "io.RawIOBase" class.  (Contributed by John Kuhn
in bpo-13464.)


html
----

"html.parser.HTMLParser" is now able to parse broken markup without
raising errors, therefore the *strict* argument of the constructor and
the "HTMLParseError" exception are now deprecated. The ability to
parse broken markup is the result of a number of bug fixes that are
also available on the latest bug fix releases of Python 2.7/3.2.
(Contributed by Ezio Melotti in bpo-15114, and bpo-14538, bpo-13993,
bpo-13960, bpo-13358, bpo-1745761, bpo-755670, bpo-13357, bpo-12629,
bpo-1200313, bpo-670664, bpo-13273, bpo-12888, bpo-7311.)

A new "html5" dictionary that maps HTML5 named character references to
the equivalent Unicode character(s) (e.g. "html5['gt;'] == '>'") has
been added to the "html.entities" module.  The dictionary is now also
used by "HTMLParser".  (Contributed by Ezio Melotti in bpo-11113 and
bpo-15156.)


imaplib
-------

The "IMAP4_SSL" constructor now accepts an SSLContext parameter to
control parameters of the secure channel.

(Contributed by Sijin Joseph in bpo-8808.)


inspect
-------

A new "getclosurevars()" function has been added. This function
reports the current binding of all names referenced from the function
body and where those names were resolved, making it easier to verify
correct internal state when testing code that relies on stateful
closures.

(Contributed by Meador Inge and Nick Coghlan in bpo-13062.)

A new "getgeneratorlocals()" function has been added. This function
reports the current binding of local variables in the generator's
stack frame, making it easier to verify correct internal state when
testing generators.

(Contributed by Meador Inge in bpo-15153.)


io
--

The "open()" function has a new "'x'" mode that can be used to
exclusively create a new file, and raise a "FileExistsError" if the
file already exists. It is based on the C11 'x' mode to fopen().

(Contributed by David Townshend in bpo-12760.)

The constructor of the "TextIOWrapper" class has a new *write_through*
optional argument. If *write_through* is "True", calls to "write()"
are guaranteed not to be buffered: any data written on the
"TextIOWrapper" object is immediately handled to its underlying binary
buffer.


itertools
---------

"accumulate()" now takes an optional "func" argument for providing a
user-supplied binary function.


logging
-------

The "basicConfig()" function now supports an optional "handlers"
argument taking an iterable of handlers to be added to the root
logger.

A class level attribute "append_nul" has been added to "SysLogHandler"
to allow control of the appending of the "NUL" ("\000") byte to syslog
records, since for some daemons it is required while for others it is
passed through to the log.


math
----

The "math" module has a new function, "log2()",  which returns the
base-2 logarithm of *x*.

(Written by Mark Dickinson in bpo-11888.)


mmap
----

The "read()" method is now more compatible with other file-like
objects: if the argument is omitted or specified as "None", it returns
the bytes from the current file position to the end of the mapping.
(Contributed by Petri Lehtinen in bpo-12021.)


multiprocessing
---------------

The new "multiprocessing.connection.wait()" function allows polling
multiple objects (such as connections, sockets and pipes) with a
timeout. (Contributed by Richard Oudkerk in bpo-12328.)

"multiprocessing.Connection" objects can now be transferred over
multiprocessing connections. (Contributed by Richard Oudkerk in
bpo-4892.)

"multiprocessing.Process" now accepts a "daemon" keyword argument to
override the default behavior of inheriting the "daemon" flag from the
parent process (bpo-6064).

New attribute "multiprocessing.Process.sentinel" allows a program to
wait on multiple "Process" objects at one time using the appropriate
OS primitives (for example, "select" on posix systems).

New methods "multiprocessing.pool.Pool.starmap()" and
"starmap_async()" provide "itertools.starmap()" equivalents to the
existing "multiprocessing.pool.Pool.map()" and "map_async()"
functions.  (Contributed by Hynek Schlawack in bpo-12708.)


nntplib
-------

The "nntplib.NNTP" class now supports the context management protocol
to unconditionally consume "socket.error" exceptions and to close the
NNTP connection when done:

   >>> from nntplib import NNTP
   >>> with NNTP('news.gmane.org') as n:
   ...     n.group('gmane.comp.python.committers')
   ...
   ('211 1755 1 1755 gmane.comp.python.committers', 1755, 1, 1755, 'gmane.comp.python.committers')
   >>>

(Contributed by Giampaolo Rodolà in bpo-9795.)


os
--

* The "os" module has a new "pipe2()" function that makes it possible
  to create a pipe with "O_CLOEXEC" or "O_NONBLOCK" flags set
  atomically. This is especially useful to avoid race conditions in
  multi-threaded programs.

* The "os" module has a new "sendfile()" function which provides an
  efficient "zero-copy" way for copying data from one file (or socket)
  descriptor to another. The phrase "zero-copy" refers to the fact
  that all of the copying of data between the two descriptors is done
  entirely by the kernel, with no copying of data into userspace
  buffers. "sendfile()" can be used to efficiently copy data from a
  file on disk to a network socket, e.g. for downloading a file.

  (Patch submitted by Ross Lagerwall and Giampaolo Rodolà in
  bpo-10882.)

* To avoid race conditions like symlink attacks and issues with
  temporary files and directories, it is more reliable (and also
  faster) to manipulate file descriptors instead of file names. Python
  3.3 enhances existing functions and introduces new functions to work
  on file descriptors (bpo-4761, bpo-10755 and bpo-14626).

  * The "os" module has a new "fwalk()" function similar to "walk()"
    except that it also yields file descriptors referring to the
    directories visited. This is especially useful to avoid symlink
    races.

  * The following functions get new optional *dir_fd* (paths relative
    to directory descriptors) and/or *follow_symlinks* (not following
    symlinks): "access()", "chflags()", "chmod()", "chown()",
    "link()", "lstat()", "mkdir()", "mkfifo()", "mknod()", "open()",
    "readlink()", "remove()", "rename()", "replace()", "rmdir()",
    "stat()", "symlink()", "unlink()", "utime()".  Platform support
    for using these parameters can be checked via the sets
    "os.supports_dir_fd" and "os.supports_follows_symlinks".

  * The following functions now support a file descriptor for their
    path argument: "chdir()", "chmod()", "chown()", "execve()",
    "listdir()", "pathconf()", "exists()", "stat()", "statvfs()",
    "utime()".  Platform support for this can be checked via the
    "os.supports_fd" set.

* "access()" accepts an "effective_ids" keyword argument to turn on
  using the effective uid/gid rather than the real uid/gid in the
  access check. Platform support for this can be checked via the
  "supports_effective_ids" set.

* The "os" module has two new functions: "getpriority()" and
  "setpriority()". They can be used to get or set process
  niceness/priority in a fashion similar to "os.nice()" but extended
  to all processes instead of just the current one.

  (Patch submitted by Giampaolo Rodolà in bpo-10784.)

* The new "os.replace()" function allows cross-platform renaming of a
  file with overwriting the destination.  With "os.rename()", an
  existing destination file is overwritten under POSIX, but raises an
  error under Windows. (Contributed by Antoine Pitrou in bpo-8828.)

* The stat family of functions ("stat()", "fstat()", and "lstat()")
  now support reading a file's timestamps with nanosecond precision.
  Symmetrically, "utime()" can now write file timestamps with
  nanosecond precision.  (Contributed by Larry Hastings in bpo-14127.)

* The new "os.get_terminal_size()" function queries the size of the
  terminal attached to a file descriptor. See also
  "shutil.get_terminal_size()". (Contributed by Zbigniew Jędrzejewski-
  Szmek in bpo-13609.)

* New functions to support Linux extended attributes (bpo-12720):
  "getxattr()", "listxattr()", "removexattr()", "setxattr()".

* New interface to the scheduler. These functions control how a
  process is allocated CPU time by the operating system. New
  functions: "sched_get_priority_max()", "sched_get_priority_min()",
  "sched_getaffinity()", "sched_getparam()", "sched_getscheduler()",
  "sched_rr_get_interval()", "sched_setaffinity()",
  "sched_setparam()", "sched_setscheduler()", "sched_yield()",

* New functions to control the file system:

  * "posix_fadvise()": Announces an intention to access data in a
    specific pattern thus allowing the kernel to make optimizations.

  * "posix_fallocate()": Ensures that enough disk space is allocated
    for a file.

  * "sync()": Force write of everything to disk.

* Additional new  posix functions:

  * "lockf()": Apply, test or remove a POSIX lock on an open file
    descriptor.

  * "pread()": Read from a file descriptor at an offset, the file
    offset remains unchanged.

  * "pwrite()": Write to a file descriptor from an offset, leaving the
    file offset unchanged.

  * "readv()": Read from a file descriptor into a number of writable
    buffers.

  * "truncate()": Truncate the file corresponding to *path*, so that
    it is at most *length* bytes in size.

  * "waitid()": Wait for the completion of one or more child
    processes.

  * "writev()": Write the contents of *buffers* to a file descriptor,
    where *buffers* is an arbitrary sequence of buffers.

  * "getgrouplist()" (bpo-9344): Return list of group ids that
    specified user belongs to.

* "times()" and "uname()": Return type changed from a tuple to a
  tuple-like object with named attributes.

* Some platforms now support additional constants for the "lseek()"
  function, such as "os.SEEK_HOLE" and "os.SEEK_DATA".

* New constants "RTLD_LAZY", "RTLD_NOW", "RTLD_GLOBAL", "RTLD_LOCAL",
  "RTLD_NODELETE", "RTLD_NOLOAD", and "RTLD_DEEPBIND" are available on
  platforms that support them.   These are for use with the
  "sys.setdlopenflags()" function, and supersede the similar constants
  defined in "ctypes" and "DLFCN".  (Contributed by Victor Stinner in
  bpo-13226.)

* "os.symlink()" now accepts (and ignores) the "target_is_directory"
  keyword argument on non-Windows platforms, to ease cross-platform
  support.


pdb
---

Tab-completion is now available not only for command names, but also
their arguments.  For example, for the "break" command, function and
file names are completed.

(Contributed by Georg Brandl in bpo-14210)


pickle
------

"pickle.Pickler" objects now have an optional "dispatch_table"
attribute allowing per-pickler reduction functions to be set.

(Contributed by Richard Oudkerk in bpo-14166.)


pydoc
-----

The Tk GUI and the "serve()" function have been removed from the
"pydoc" module: "pydoc -g" and "serve()" have been deprecated in
Python 3.2.


re
--

"str" regular expressions now support "\u" and "\U" escapes.

(Contributed by Serhiy Storchaka in bpo-3665.)


sched
-----

* "run()" now accepts a *blocking* parameter which when set to false
  makes the method execute the scheduled events due to expire soonest
  (if any) and then return immediately. This is useful in case you
  want to use the "scheduler" in non-blocking applications.
  (Contributed by Giampaolo Rodolà in bpo-13449.)

* "scheduler" class can now be safely used in multi-threaded
  environments.  (Contributed by Josiah Carlson and Giampaolo Rodolà
  in bpo-8684.)

* *timefunc* and *delayfunct* parameters of "scheduler" class
  constructor are now optional and defaults to "time.time()" and
  "time.sleep()" respectively.  (Contributed by Chris Clark in
  bpo-13245.)

* "enter()" and "enterabs()" *argument* parameter is now optional.
  (Contributed by Chris Clark in bpo-13245.)

* "enter()" and "enterabs()" now accept a *kwargs* parameter.
  (Contributed by Chris Clark in bpo-13245.)


select
------

Solaris and derivative platforms have a new class "select.devpoll" for
high performance asynchronous sockets via "/dev/poll". (Contributed by
Jesús Cea Avión in bpo-6397.)


shlex
-----

The previously undocumented helper function "quote" from the "pipes"
modules has been moved to the "shlex" module and documented.
"quote()" properly escapes all characters in a string that might be
otherwise given special meaning by the shell.


shutil
------

* New functions:

  * "disk_usage()": provides total, used and free disk space
    statistics.  (Contributed by Giampaolo Rodolà in bpo-12442.)

  * "chown()": allows one to change user and/or group of the given
    path also specifying the user/group names and not only their
    numeric ids.  (Contributed by Sandro Tosi in bpo-12191.)

  * "shutil.get_terminal_size()": returns the size of the terminal
    window to which the interpreter is attached.  (Contributed by
    Zbigniew Jędrzejewski-Szmek in bpo-13609.)

* "copy2()" and "copystat()" now preserve file timestamps with
  nanosecond precision on platforms that support it. They also
  preserve file "extended attributes" on Linux.  (Contributed by Larry
  Hastings in bpo-14127 and  bpo-15238.)

* Several functions now take an optional "symlinks" argument: when
  that parameter is true, symlinks aren't dereferenced and the
  operation instead acts on the symlink itself (or creates one, if
  relevant). (Contributed by Hynek Schlawack in bpo-12715.)

* When copying files to a different file system, "move()" now handles
  symlinks the way the posix "mv" command does, recreating the symlink
  rather than copying the target file contents.  (Contributed by
  Jonathan Niehof in bpo-9993.)  "move()" now also returns the "dst"
  argument as its result.

* "rmtree()" is now resistant to symlink attacks on platforms which
  support the new "dir_fd" parameter in "os.open()" and "os.unlink()".
  (Contributed by Martin von Löwis and Hynek Schlawack in bpo-4489.)


signal
------

* The "signal" module has new functions:

  * "pthread_sigmask()": fetch and/or change the signal mask of the
    calling thread (Contributed by Jean-Paul Calderone in bpo-8407);

  * "pthread_kill()": send a signal to a thread;

  * "sigpending()": examine pending functions;

  * "sigwait()": wait a signal;

  * "sigwaitinfo()": wait for a signal, returning detailed information
    about it;

  * "sigtimedwait()": like "sigwaitinfo()" but with a timeout.

* The signal handler writes the signal number as a single byte instead
  of a nul byte into the wakeup file descriptor. So it is possible to
  wait more than one signal and know which signals were raised.

* "signal.signal()" and "signal.siginterrupt()" raise an OSError,
  instead of a RuntimeError: OSError has an errno attribute.


smtpd
-----

The "smtpd" module now supports **RFC 5321** (extended SMTP) and **RFC
1870** (size extension).  Per the standard, these extensions are
enabled if and only if the client initiates the session with an "EHLO"
command.

(Initial "ELHO" support by Alberto Trevino.  Size extension by Juhana
Jauhiainen.  Substantial additional work on the patch contributed by
Michele Orrù and Dan Boswell.  bpo-8739)


smtplib
-------

The "SMTP", "SMTP_SSL", and "LMTP" classes now accept a
"source_address" keyword argument to specify the "(host, port)" to use
as the source address in the bind call when creating the outgoing
socket.  (Contributed by Paulo Scardine in bpo-11281.)

"SMTP" now supports the context management protocol, allowing an
"SMTP" instance to be used in a "with" statement.  (Contributed by
Giampaolo Rodolà in bpo-11289.)

The "SMTP_SSL" constructor and the "starttls()" method now accept an
SSLContext parameter to control parameters of the secure channel.
(Contributed by Kasun Herath in bpo-8809.)


socket
------

* The "socket" class now exposes additional methods to process
  ancillary data when supported by the underlying platform:

  * "sendmsg()"

  * "recvmsg()"

  * "recvmsg_into()"

  (Contributed by David Watson in bpo-6560, based on an earlier patch
  by Heiko Wundram)

* The "socket" class now supports the PF_CAN protocol family
  (https://en.wikipedia.org/wiki/Socketcan), on Linux
  (https://lwn.net/Articles/253425).

  (Contributed by Matthias Fuchs, updated by Tiago Gonçalves in
  bpo-10141.)

* The "socket" class now supports the PF_RDS protocol family
  (https://en.wikipedia.org/wiki/Reliable_Datagram_Sockets and
  https://oss.oracle.com/projects/rds).

* The "socket" class now supports the "PF_SYSTEM" protocol family on
  OS X.  (Contributed by Michael Goderbauer in bpo-13777.)

* New function "sethostname()" allows the hostname to be set on Unix
  systems if the calling process has sufficient privileges.
  (Contributed by Ross Lagerwall in bpo-10866.)


socketserver
------------

"BaseServer" now has an overridable method "service_actions()" that is
called by the "serve_forever()" method in the service loop.
"ForkingMixIn" now uses this to clean up zombie child processes.
(Contributed by Justin Warkentin in bpo-11109.)


sqlite3
-------

New "sqlite3.Connection" method "set_trace_callback()" can be used to
capture a trace of all sql commands processed by sqlite.  (Contributed
by Torsten Landschoff in bpo-11688.)


ssl
---

* The "ssl" module has two new random generation functions:

  * "RAND_bytes()": generate cryptographically strong pseudo-random
    bytes.

  * "RAND_pseudo_bytes()": generate pseudo-random bytes.

  (Contributed by Victor Stinner in bpo-12049.)

* The "ssl" module now exposes a finer-grained exception hierarchy in
  order to make it easier to inspect the various kinds of errors.
  (Contributed by Antoine Pitrou in bpo-11183.)

* "load_cert_chain()" now accepts a *password* argument to be used if
  the private key is encrypted. (Contributed by Adam Simpkins in
  bpo-12803.)

* Diffie-Hellman key exchange, both regular and Elliptic Curve-based,
  is now supported through the "load_dh_params()" and
  "set_ecdh_curve()" methods. (Contributed by Antoine Pitrou in
  bpo-13626 and bpo-13627.)

* SSL sockets have a new "get_channel_binding()" method allowing the
  implementation of certain authentication mechanisms such as SCRAM-
  SHA-1-PLUS.  (Contributed by Jacek Konieczny in bpo-12551.)

* You can query the SSL compression algorithm used by an SSL socket,
  thanks to its new "compression()" method.  The new attribute
  "OP_NO_COMPRESSION" can be used to disable compression. (Contributed
  by Antoine Pitrou in bpo-13634.)

* Support has been added for the Next Protocol Negotiation extension
  using the "ssl.SSLContext.set_npn_protocols()" method. (Contributed
  by Colin Marc in bpo-14204.)

* SSL errors can now be introspected more easily thanks to "library"
  and "reason" attributes. (Contributed by Antoine Pitrou in
  bpo-14837.)

* The "get_server_certificate()" function now supports IPv6.
  (Contributed by Charles-François Natali in bpo-11811.)

* New attribute "OP_CIPHER_SERVER_PREFERENCE" allows setting SSLv3
  server sockets to use the server's cipher ordering preference rather
  than the client's (bpo-13635).


stat
----

The undocumented tarfile.filemode function has been moved to
"stat.filemode()". It can be used to convert a file's mode to a string
of the form '-rwxrwxrwx'.

(Contributed by Giampaolo Rodolà in bpo-14807.)


struct
------

The "struct" module now supports "ssize_t" and "size_t" via the new
codes "n" and "N", respectively.  (Contributed by Antoine Pitrou in
bpo-3163.)


subprocess
----------

Command strings can now be bytes objects on posix platforms.
(Contributed by Victor Stinner in bpo-8513.)

A new constant "DEVNULL" allows suppressing output in a platform-
independent fashion.  (Contributed by Ross Lagerwall in bpo-5870.)


sys
---

The "sys" module has a new "thread_info" *named tuple* holding
information about the thread implementation (bpo-11223).


tarfile
-------

"tarfile" now supports "lzma" encoding via the "lzma" module.
(Contributed by Lars Gustäbel in bpo-5689.)


tempfile
--------

"tempfile.SpooledTemporaryFile"'s "truncate()" method now accepts a
"size" parameter.  (Contributed by Ryan Kelly in bpo-9957.)


textwrap
--------

The "textwrap" module has a new "indent()" that makes it
straightforward to add a common prefix to selected lines in a block of
text  (bpo-13857).


threading
---------

"threading.Condition", "threading.Semaphore",
"threading.BoundedSemaphore", "threading.Event", and
"threading.Timer", all of which used to be factory functions returning
a class instance, are now classes and may be subclassed.  (Contributed
by Éric Araujo in bpo-10968.)

The "threading.Thread" constructor now accepts a "daemon" keyword
argument to override the default behavior of inheriting the "daemon"
flag value from the parent thread (bpo-6064).

The formerly private function "_thread.get_ident" is now available as
the public function "threading.get_ident()".  This eliminates several
cases of direct access to the "_thread" module in the stdlib.  Third
party code that used "_thread.get_ident" should likewise be changed to
use the new public interface.


time
----

The **PEP 418** added new functions to the "time" module:

* "get_clock_info()": Get information on a clock.

* "monotonic()": Monotonic clock (cannot go backward), not affected by
  system clock updates.

* "perf_counter()": Performance counter with the highest available
  resolution to measure a short duration.

* "process_time()": Sum of the system and user CPU time of the current
  process.

Other new functions:

* "clock_getres()", "clock_gettime()" and "clock_settime()" functions
  with "CLOCK_*xxx*" constants. (Contributed by Victor Stinner in
  bpo-10278.)

To improve cross platform consistency, "sleep()" now raises a
"ValueError" when passed a negative sleep value.  Previously this was
an error on posix, but produced an infinite sleep on Windows.


types
-----

Add a new "types.MappingProxyType" class: Read-only proxy of a
mapping. (bpo-14386)

The new functions "types.new_class()" and "types.prepare_class()"
provide support for **PEP 3115** compliant dynamic type creation.
(bpo-14588)


unittest
--------

"assertRaises()", "assertRaisesRegex()", "assertWarns()", and
"assertWarnsRegex()" now accept a keyword argument *msg* when used as
context managers.  (Contributed by Ezio Melotti and Winston Ewert in
bpo-10775.)

"unittest.TestCase.run()" now returns the "TestResult" object.


urllib
------

The "Request" class, now accepts a *method* argument used by
"get_method()" to determine what HTTP method should be used.  For
example, this will send a "'HEAD'" request:

   >>> urlopen(Request('https://www.python.org', method='HEAD'))

(bpo-1673007)


webbrowser
----------

The "webbrowser" module supports more "browsers": Google Chrome (named
**chrome**, **chromium**, **chrome-browser** or **chromium-browser**
depending on the version and operating system), and the generic
launchers **xdg-open**, from the FreeDesktop.org project, and **gvfs-
open**, which is the default URI handler for GNOME 3.  (The former
contributed by Arnaud Calmettes in bpo-13620, the latter by Matthias
Klose in bpo-14493.)


xml.etree.ElementTree
---------------------

The "xml.etree.ElementTree" module now imports its C accelerator by
default; there is no longer a need to explicitly import
"xml.etree.cElementTree" (this module stays for backwards
compatibility, but is now deprecated).  In addition,  the "iter"
family of methods of "Element" has been optimized (rewritten in C).
The module's documentation has also been greatly improved with added
examples and a more detailed reference.


zlib
----

New attribute "zlib.Decompress.eof" makes it possible to distinguish
between a properly formed compressed stream and an incomplete or
truncated one. (Contributed by Nadeem Vawda in bpo-12646.)

New attribute "zlib.ZLIB_RUNTIME_VERSION" reports the version string
of the underlying "zlib" library that is loaded at runtime.
(Contributed by Torsten Landschoff in bpo-12306.)


Optimizations
=============

Major performance enhancements have been added:

* Thanks to **PEP 393**, some operations on Unicode strings have been
  optimized:

  * the memory footprint is divided by 2 to 4 depending on the text

  * encode an ASCII string to UTF-8 doesn't need to encode characters
    anymore, the UTF-8 representation is shared with the ASCII
    representation

  * the UTF-8 encoder has been optimized

  * repeating a single ASCII letter and getting a substring of an
    ASCII string is 4 times faster

* UTF-8 is now 2x to 4x faster.  UTF-16 encoding is now up to 10x
  faster.

  (Contributed by Serhiy Storchaka, bpo-14624, bpo-14738 and
  bpo-15026.)


Build and C API Changes
=======================

Changes to Python's build process and to the C API include:

* New **PEP 3118** related function:

  * "PyMemoryView_FromMemory()"

* **PEP 393** added new Unicode types, macros and functions:

  * High-level API:

    * "PyUnicode_CopyCharacters()"

    * "PyUnicode_FindChar()"

    * "PyUnicode_GetLength()", "PyUnicode_GET_LENGTH"

    * "PyUnicode_New()"

    * "PyUnicode_Substring()"

    * "PyUnicode_ReadChar()", "PyUnicode_WriteChar()"

  * Low-level API:

    * "Py_UCS1", "Py_UCS2", "Py_UCS4" types

    * "PyASCIIObject" and "PyCompactUnicodeObject" structures

    * "PyUnicode_READY"

    * "PyUnicode_FromKindAndData()"

    * "PyUnicode_AsUCS4()", "PyUnicode_AsUCS4Copy()"

    * "PyUnicode_DATA", "PyUnicode_1BYTE_DATA",
      "PyUnicode_2BYTE_DATA", "PyUnicode_4BYTE_DATA"

    * "PyUnicode_KIND" with "PyUnicode_Kind" enum:
      "PyUnicode_WCHAR_KIND", "PyUnicode_1BYTE_KIND",
      "PyUnicode_2BYTE_KIND", "PyUnicode_4BYTE_KIND"

    * "PyUnicode_READ", "PyUnicode_READ_CHAR", "PyUnicode_WRITE"

    * "PyUnicode_MAX_CHAR_VALUE"

* "PyArg_ParseTuple" now accepts a "bytearray" for the "c" format
  (bpo-12380).


Deprecated
==========


Unsupported Operating Systems
-----------------------------

OS/2 and VMS are no longer supported due to the lack of a maintainer.

Windows 2000 and Windows platforms which set "COMSPEC" to
"command.com" are no longer supported due to maintenance burden.

OSF support, which was deprecated in 3.2, has been completely removed.


Deprecated Python modules, functions and methods
------------------------------------------------

* Passing a non-empty string to "object.__format__()" is deprecated,
  and will produce a "TypeError" in Python 3.4 (bpo-9856).

* The "unicode_internal" codec has been deprecated because of the
  **PEP 393**, use UTF-8, UTF-16 ("utf-16-le" or "utf-16-be"), or
  UTF-32 ("utf-32-le" or "utf-32-be")

* "ftplib.FTP.nlst()" and "ftplib.FTP.dir()": use "ftplib.FTP.mlsd()"

* "platform.popen()": use the "subprocess" module. Check especially
  the Replacing Older Functions with the subprocess Module section
  (bpo-11377).

* bpo-13374: The Windows bytes API has been deprecated in the "os"
  module. Use Unicode filenames, instead of bytes filenames, to not
  depend on the ANSI code page anymore and to support any filename.

* bpo-13988: The "xml.etree.cElementTree" module is deprecated.  The
  accelerator is used automatically whenever available.

* The behaviour of "time.clock()" depends on the platform: use the new
  "time.perf_counter()" or "time.process_time()" function instead,
  depending on your requirements, to have a well defined behaviour.

* The "os.stat_float_times()" function is deprecated.

* "abc" module:

  * "abc.abstractproperty" has been deprecated, use "property" with
    "abc.abstractmethod()" instead.

  * "abc.abstractclassmethod" has been deprecated, use "classmethod"
    with "abc.abstractmethod()" instead.

  * "abc.abstractstaticmethod" has been deprecated, use "staticmethod"
    with "abc.abstractmethod()" instead.

* "importlib" package:

  * "importlib.abc.SourceLoader.path_mtime()" is now deprecated in
    favour of "importlib.abc.SourceLoader.path_stats()" as bytecode
    files now store both the modification time and size of the source
    file the bytecode file was compiled from.


Deprecated functions and types of the C API
-------------------------------------------

The "Py_UNICODE" has been deprecated by **PEP 393** and will be
removed in Python 4. All functions using this type are deprecated:

Unicode functions and methods using "Py_UNICODE" and Py_UNICODE*
types:

* "PyUnicode_FromUnicode": use "PyUnicode_FromWideChar()" or
  "PyUnicode_FromKindAndData()"

* "PyUnicode_AS_UNICODE", "PyUnicode_AsUnicode()",
  "PyUnicode_AsUnicodeAndSize()": use "PyUnicode_AsWideCharString()"

* "PyUnicode_AS_DATA": use "PyUnicode_DATA" with "PyUnicode_READ" and
  "PyUnicode_WRITE"

* "PyUnicode_GET_SIZE", "PyUnicode_GetSize()": use
  "PyUnicode_GET_LENGTH" or "PyUnicode_GetLength()"

* "PyUnicode_GET_DATA_SIZE": use "PyUnicode_GET_LENGTH(str) *
  PyUnicode_KIND(str)" (only work on ready strings)

* "PyUnicode_AsUnicodeCopy()": use "PyUnicode_AsUCS4Copy()" or
  "PyUnicode_AsWideCharString()"

* "PyUnicode_GetMax()"

Functions and macros manipulating Py_UNICODE* strings:

* "Py_UNICODE_strlen()": use "PyUnicode_GetLength()" or
  "PyUnicode_GET_LENGTH"

* "Py_UNICODE_strcat()": use "PyUnicode_CopyCharacters()" or
  "PyUnicode_FromFormat()"

* "Py_UNICODE_strcpy()", "Py_UNICODE_strncpy()", "Py_UNICODE_COPY()":
  use "PyUnicode_CopyCharacters()" or "PyUnicode_Substring()"

* "Py_UNICODE_strcmp()": use "PyUnicode_Compare()"

* "Py_UNICODE_strncmp()": use "PyUnicode_Tailmatch()"

* "Py_UNICODE_strchr()", "Py_UNICODE_strrchr()": use
  "PyUnicode_FindChar()"

* "Py_UNICODE_FILL()": use "PyUnicode_Fill()"

* "Py_UNICODE_MATCH"

Encoders:

* "PyUnicode_Encode()": use "PyUnicode_AsEncodedObject()"

* "PyUnicode_EncodeUTF7()"

* "PyUnicode_EncodeUTF8()": use "PyUnicode_AsUTF8()" or
  "PyUnicode_AsUTF8String()"

* "PyUnicode_EncodeUTF32()"

* "PyUnicode_EncodeUTF16()"

* "PyUnicode_EncodeUnicodeEscape()" use
  "PyUnicode_AsUnicodeEscapeString()"

* "PyUnicode_EncodeRawUnicodeEscape()" use
  "PyUnicode_AsRawUnicodeEscapeString()"

* "PyUnicode_EncodeLatin1()": use "PyUnicode_AsLatin1String()"

* "PyUnicode_EncodeASCII()": use "PyUnicode_AsASCIIString()"

* "PyUnicode_EncodeCharmap()"

* "PyUnicode_TranslateCharmap()"

* "PyUnicode_EncodeMBCS()": use "PyUnicode_AsMBCSString()" or
  "PyUnicode_EncodeCodePage()" (with "CP_ACP" code_page)

* "PyUnicode_EncodeDecimal()", "PyUnicode_TransformDecimalToASCII()"


Deprecated features
-------------------

The "array" module's "'u'" format code is now deprecated and will be
removed in Python 4 together with the rest of the ("Py_UNICODE") API.


Porting to Python 3.3
=====================

This section lists previously described changes and other bugfixes
that may require changes to your code.


Porting Python code
-------------------

* Hash randomization is enabled by default. Set the "PYTHONHASHSEED"
  environment variable to "0" to disable hash randomization. See also
  the "object.__hash__()" method.

* bpo-12326: On Linux, sys.platform doesn't contain the major version
  anymore. It is now always 'linux', instead of 'linux2' or 'linux3'
  depending on the Linux version used to build Python. Replace
  sys.platform == 'linux2' with sys.platform.startswith('linux'), or
  directly sys.platform == 'linux' if you don't need to support older
  Python versions.

* bpo-13847, bpo-14180: "time" and "datetime": "OverflowError" is now
  raised instead of "ValueError" if a timestamp is out of range.
  "OSError" is now raised if C functions "gmtime()" or "localtime()"
  failed.

* The default finders used by import now utilize a cache of what is
  contained within a specific directory. If you create a Python source
  file or sourceless bytecode file, make sure to call
  "importlib.invalidate_caches()" to clear out the cache for the
  finders to notice the new file.

* "ImportError" now uses the full name of the module that was
  attempted to be imported. Doctests that check ImportErrors' message
  will need to be updated to use the full name of the module instead
  of just the tail of the name.

* The *index* argument to "__import__()" now defaults to 0 instead of
  -1 and no longer support negative values. It was an oversight when
  **PEP 328** was implemented that the default value remained -1. If
  you need to continue to perform a relative import followed by an
  absolute import, then perform the relative import using an index of
  1, followed by another import using an index of 0. It is preferred,
  though, that you use "importlib.import_module()" rather than call
  "__import__()" directly.

* "__import__()" no longer allows one to use an index value other than
  0 for top-level modules. E.g. "__import__('sys', level=1)" is now an
  error.

* Because "sys.meta_path" and "sys.path_hooks" now have finders on
  them by default, you will most likely want to use "list.insert()"
  instead of "list.append()" to add to those lists.

* Because "None" is now inserted into "sys.path_importer_cache", if
  you are clearing out entries in the dictionary of paths that do not
  have a finder, you will need to remove keys paired with values of
  "None" **and** "imp.NullImporter" to be backwards-compatible. This
  will lead to extra overhead on older versions of Python that re-
  insert "None" into "sys.path_importer_cache" where it represents the
  use of implicit finders, but semantically it should not change
  anything.

* "importlib.abc.Finder" no longer specifies a "find_module()"
  abstract method that must be implemented. If you were relying on
  subclasses to implement that method, make sure to check for the
  method's existence first. You will probably want to check for
  "find_loader()" first, though, in the case of working with *path
  entry finders*.

* "pkgutil" has been converted to use "importlib" internally. This
  eliminates many edge cases where the old behaviour of the **PEP
  302** import emulation failed to match the behaviour of the real
  import system. The import emulation itself is still present, but is
  now deprecated. The "pkgutil.iter_importers()" and
  "pkgutil.walk_packages()" functions special case the standard import
  hooks so they are still supported even though they do not provide
  the non-standard "iter_modules()" method.

* A longstanding RFC-compliance bug (bpo-1079) in the parsing done by
  "email.header.decode_header()" has been fixed.  Code that uses the
  standard idiom to convert encoded headers into unicode
  ("str(make_header(decode_header(h))") will see no change, but code
  that looks at the individual tuples returned by decode_header will
  see that whitespace that precedes or follows "ASCII" sections is now
  included in the "ASCII" section.  Code that builds headers using
  "make_header" should also continue to work without change, since
  "make_header" continues to add whitespace between "ASCII" and
  non-"ASCII" sections if it is not already present in the input
  strings.

* "email.utils.formataddr()" now does the correct content transfer
  encoding when passed non-"ASCII" display names.  Any code that
  depended on the previous buggy behavior that preserved the
  non-"ASCII" unicode in the formatted output string will need to be
  changed (bpo-1690608).

* "poplib.POP3.quit()" may now raise protocol errors like all other
  "poplib" methods.  Code that assumes "quit" does not raise
  "poplib.error_proto" errors may need to be changed if errors on
  "quit" are encountered by a particular application (bpo-11291).

* The "strict" argument to "email.parser.Parser", deprecated since
  Python 2.4, has finally been removed.

* The deprecated method "unittest.TestCase.assertSameElements" has
  been removed.

* The deprecated variable "time.accept2dyear" has been removed.

* The deprecated "Context._clamp" attribute has been removed from the
  "decimal" module.  It was previously replaced by the public
  attribute "clamp".  (See bpo-8540.)

* The undocumented internal helper class "SSLFakeFile" has been
  removed from "smtplib", since its functionality has long been
  provided directly by "socket.socket.makefile()".

* Passing a negative value to "time.sleep()" on Windows now raises an
  error instead of sleeping forever.  It has always raised an error on
  posix.

* The "ast.__version__" constant has been removed.  If you need to
  make decisions affected by the AST version, use "sys.version_info"
  to make the decision.

* Code that used to work around the fact that the "threading" module
  used factory functions by subclassing the private classes will need
  to change to subclass the now-public classes.

* The undocumented debugging machinery in the threading module has
  been removed, simplifying the code.  This should have no effect on
  production code, but is mentioned here in case any application debug
  frameworks were interacting with it (bpo-13550).


Porting C code
--------------

* In the course of changes to the buffer API the undocumented
  "smalltable" member of the "Py_buffer" structure has been removed
  and the layout of the "PyMemoryViewObject" has changed.

  All extensions relying on the relevant parts in "memoryobject.h" or
  "object.h" must be rebuilt.

* Due to PEP 393, the "Py_UNICODE" type and all functions using this
  type are deprecated (but will stay available for at least five
  years).  If you were using low-level Unicode APIs to construct and
  access unicode objects and you want to benefit of the memory
  footprint reduction provided by **PEP 393**, you have to convert
  your code to the new Unicode API.

  However, if you only have been using high-level functions such as
  "PyUnicode_Concat()", "PyUnicode_Join()" or
  "PyUnicode_FromFormat()", your code will automatically take
  advantage of the new unicode representations.

* "PyImport_GetMagicNumber()" now returns "-1" upon failure.

* As a negative value for the *level* argument to "__import__()" is no
  longer valid, the same now holds for "PyImport_ImportModuleLevel()".
  This also means that the value of *level* used by
  "PyImport_ImportModuleEx()" is now "0" instead of "-1".


Building C extensions
---------------------

* The range of possible file names for C extensions has been narrowed.
  Very rarely used spellings have been suppressed: under POSIX, files
  named "xxxmodule.so", "xxxmodule.abi3.so" and
  "xxxmodule.cpython-*.so" are no longer recognized as implementing
  the "xxx" module.  If you had been generating such files, you have
  to switch to the other spellings (i.e., remove the "module" string
  from the file names).

  (implemented in bpo-14040.)


Command Line Switch Changes
---------------------------

* The -Q command-line flag and related artifacts have been removed.
  Code checking sys.flags.division_warning will need updating.

  (bpo-10998, contributed by Éric Araujo.)

* When **python** is started with "-S", "import site" will no longer
  add site-specific paths to the module search paths.  In previous
  versions, it did.

  (bpo-11591, contributed by Carl Meyer with editions by Éric Araujo.)
