
What's New In Python 3.6
************************

Release:
   3.6.0rc1

Date:
   December 06, 2016

Editors:
   Elvis Pranskevichus <elvis@magic.io>, Yury Selivanov
   <yury@magic.io>

Note: Prerelease users should be aware that this document is
  currently in draft form. It will be updated substantially as Python
  3.6 moves towards release, so it's worth checking back even after
  reading earlier versions.

This article explains the new features in Python 3.6, compared to 3.5.

See also: **PEP 494** - Python 3.6 Release Schedule


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

New syntax features:

* PEP 498, formatted string literals.

* PEP 515, underscores in numeric literals.

* PEP 526, syntax for variable annotations.

* PEP 525, asynchronous generators.

* PEP 530: asynchronous comprehensions.

New library modules:

* "secrets": PEP 506 -- Adding A Secrets Module To The Standard
  Library.

CPython implementation improvements:

* The dict type has been reimplemented to use a faster, more compact
  representation similar to the PyPy dict implementation.  This
  resulted in dictionaries using 20% to 25% less memory when compared
  to Python 3.5.

* Customization of class creation has been simplified with the new
  protocol.

* The class attribute definition order is now preserved.

* The order of elements in "**kwargs" now corresponds to the order
  in which keyword arguments were passed to the function.

* DTrace and SystemTap probing support has been added.

* The new PYTHONMALLOC environment variable can now be used to debug
  the interpreter memory allocation and access errors.

Significant improvements in the standard library:

* The "asyncio" module has received new features, significant
  usability and performance improvements, and a fair amount of bug
  fixes. Starting with Python 3.6 the "asyncio" module is no longer
  provisional and its API is considered stable.

* A new file system path protocol has been implemented to support
  *path-like objects*. All standard library functions operating on
  paths have been updated to work with the new protocol.

* The "datetime" module has gained support for Local Time
  Disambiguation.

* The "typing" module received a number of improvements and is no
  longer provisional.

* The "tracemalloc" module has been significantly reworked and is
  now used to provide better output for "ResourceWarning" as well as
  provide better diagnostics for memory allocation errors. See the
  PYTHONMALLOC section for more information.

Security improvements:

* The new "secrets" module has been added to simplify the generation
  of cryptographically strong pseudo-random numbers suitable for
  managing secrets such as account authentication, tokens, and
  similar.

* On Linux, "os.urandom()" now blocks until the system urandom
  entropy pool is initialized to increase the security. See the **PEP
  524** for the rationale.

* The "hashlib" and "ssl" modules now support OpenSSL 1.1.0.

* The default settings and feature set of the "ssl" module have been
  improved.

* The "hashlib" module received support for the BLAKE2, SHA-3 and
  SHAKE hash algorithms and the "scrypt()" key derivation function.

Windows improvements:

* PEP 528 and PEP 529, Windows filesystem and console encoding
  changed to UTF-8.

* The "py.exe" launcher, when used interactively, no longer prefers
  Python 2 over Python 3 when the user doesn't specify a version (via
  command line arguments or a config file).  Handling of shebang lines
  remains unchanged - "python" refers to Python 2 in that case.

* "python.exe" and "pythonw.exe" have been marked as long-path
  aware, which means that the 260 character path limit may no longer
  apply. See removing the MAX_PATH limitation for details.

* A "._pth" file can be added to force isolated mode and fully
  specify all search paths to avoid registry and environment lookup.
  See the documentation for more information.

* A "python36.zip" file now works as a landmark to infer
  "PYTHONHOME". See the documentation for more information.


New Features
============


PEP 498: Formatted string literals
----------------------------------

**PEP 498** introduces a new kind of string literals: *f-strings*, or
formatted string literals.

Formatted string literals are prefixed with "'f'" and are similar to
the format strings accepted by "str.format()".  They contain
replacement fields surrounded by curly braces.  The replacement fields
are expressions, which are evaluated at run time, and then formatted
using the "format()" protocol:

   >>> name = "Fred"
   >>> f"He said his name is {name}."
   'He said his name is Fred.'
   >>> width = 10
   >>> precision = 4
   >>> value = decimal.Decimal("12.34567")
   >>> f"result: {value:{width}.{precision}}"  # nested fields
   'result:      12.35'

See also:

  **PEP 498** -- Literal String Interpolation.
     PEP written and implemented by Eric V. Smith.

  Feature documentation.


PEP 526: Syntax for variable annotations
----------------------------------------

**PEP 484** introduced the standard for type annotations of function
parameters, a.k.a. type hints. This PEP adds syntax to Python for
annotating the types of variables including class variables and
instance variables:

   primes: List[int] = []

   captain: str  # Note: no initial value!

   class Starship:
       stats: Dict[str, int] = {}

Just as for function annotations, the Python interpreter does not
attach any particular meaning to variable annotations and only stores
them in the "__annotations__" attribute of a class or module.

In contrast to variable declarations in statically typed languages,
the goal of annotation syntax is to provide an easy way to specify
structured type metadata for third party tools and libraries via the
abstract syntax tree and the "__annotations__" attribute.

See also:

  **PEP 526** -- Syntax for variable annotations.
     PEP written by Ryan Gonzalez, Philip House, Ivan Levkivskyi, Lisa
     Roach, and Guido van Rossum. Implemented by Ivan Levkivskyi.

  Tools that use or will use the new syntax: mypy, pytype, PyCharm,
  etc.


PEP 515: Underscores in Numeric Literals
----------------------------------------

**PEP 515** adds the ability to use underscores in numeric literals
for improved readability.  For example:

   >>> 1_000_000_000_000_000
   1000000000000000
   >>> 0x_FF_FF_FF_FF
   4294967295

Single underscores are allowed between digits and after any base
specifier.  Leading, trailing, or multiple underscores in a row are
not allowed.

The string formatting language also now has support for the "'_'"
option to signal the use of an underscore for a thousands separator
for floating point presentation types and for integer presentation
type "'d'".  For integer presentation types "'b'", "'o'", "'x'", and
"'X'", underscores will be inserted every 4 digits:

   >>> '{:_}'.format(1000000)
   '1_000_000'
   >>> '{:_x}'.format(0xFFFFFFFF)
   'ffff_ffff'

See also:

  **PEP 515** -- Underscores in Numeric Literals
     PEP written by Georg Brandl and Serhiy Storchaka.


PEP 525: Asynchronous Generators
--------------------------------

**PEP 492** introduced support for native coroutines and "async" /
"await" syntax to Python 3.5.  A notable limitation of the Python 3.5
implementation is that it was not possible to use "await" and "yield"
in the same function body.  In Python 3.6 this restriction has been
lifted, making it possible to define *asynchronous generators*:

   async def ticker(delay, to):
       """Yield numbers from 0 to *to* every *delay* seconds."""
       for i in range(to):
           yield i
           await asyncio.sleep(delay)

The new syntax allows for faster and more concise code.

See also:

  **PEP 525** -- Asynchronous Generators
     PEP written and implemented by Yury Selivanov.


PEP 530: Asynchronous Comprehensions
------------------------------------

**PEP 530** adds support for using "async for" in list, set, dict
comprehensions and generator expressions:

   result = [i async for i in aiter() if i % 2]

Additionally, "await" expressions are supported in all kinds of
comprehensions:

   result = [await fun() for fun in funcs if await condition()]

See also:

  **PEP 530** -- Asynchronous Comprehensions
     PEP written and implemented by Yury Selivanov.


PEP 487: Simpler customization of class creation
------------------------------------------------

It is now possible to customize subclass creation without using a
metaclass. The new "__init_subclass__" classmethod will be called on
the base class whenever a new subclass is created:

   class PluginBase:
       subclasses = []

       def __init_subclass__(cls, **kwargs):
           super().__init_subclass__(**kwargs)
           cls.subclasses.append(cls)

   class Plugin1(PluginBase):
       pass

   class Plugin2(PluginBase):
       pass

In order to allow zero-argument "super()" calls to work correctly from
"__init_subclass__()" implementations, custom metaclasses must ensure
that the new "__classcell__" namespace entry is propagated to
"type.__new__" (as described in Creating the class object).

See also:

  **PEP 487** -- Simpler customization of class creation
     PEP written and implemented by Martin Teichmann.

  Feature documentation


PEP 487: Descriptor Protocol Enhancements
-----------------------------------------

**PEP 487** extends the descriptor protocol has to include the new
optional "__set_name__()" method.  Whenever a new class is defined,
the new method will be called on all descriptors included in the
definition, providing them with a reference to the class being defined
and the name given to the descriptor within the class namespace.  In
other words, instances of descriptors can now know the attribute name
of the descriptor in the owner class:

   class IntField:
       def __get__(self, instance, owner):
           return instance.__dict__[self.name]

       def __set__(self, instance, value):
           if not isinstance(value, int):
               raise ValueError(f'expecting integer in {self.name}')
           instance.__dict__[self.name] = value

       # this is the new initializer:
       def __set_name__(self, owner, name):
           self.name = name

   class Model:
       int_field = IntField()

See also:

  **PEP 487** -- Simpler customization of class creation
     PEP written and implemented by Martin Teichmann.

  Feature documentation


PEP 519: Adding a file system path protocol
-------------------------------------------

File system paths have historically been represented as "str" or
"bytes" objects. This has led to people who write code which operate
on file system paths to assume that such objects are only one of those
two types (an "int" representing a file descriptor does not count as
that is not a file path). Unfortunately that assumption prevents
alternative object representations of file system paths like "pathlib"
from working with pre-existing code, including Python's standard
library.

To fix this situation, a new interface represented by "os.PathLike"
has been defined. By implementing the "__fspath__()" method, an object
signals that it represents a path. An object can then provide a low-
level representation of a file system path as a "str" or "bytes"
object. This means an object is considered *path-like* if it
implements "os.PathLike" or is a "str" or "bytes" object which
represents a file system path. Code can use "os.fspath()",
"os.fsdecode()", or "os.fsencode()" to explicitly get a "str" and/or
"bytes" representation of a path-like object.

The built-in "open()" function has been updated to accept
"os.PathLike" objects, as have all relevant functions in the "os" and
"os.path" modules, and most other functions and classes in the
standard library.  The "os.DirEntry" class and relevant classes in
"pathlib" have also been updated to implement "os.PathLike".

The hope is that updating the fundamental functions for operating on
file system paths will lead to third-party code to implicitly support
all *path-like objects* without any code changes, or at least very
minimal ones (e.g. calling "os.fspath()" at the beginning of code
before operating on a path-like object).

Here are some examples of how the new interface allows for
"pathlib.Path" to be used more easily and transparently with pre-
existing code:

   >>> import pathlib
   >>> with open(pathlib.Path("README")) as f:
   ...     contents = f.read()
   ...
   >>> import os.path
   >>> os.path.splitext(pathlib.Path("some_file.txt"))
   ('some_file', '.txt')
   >>> os.path.join("/a/b", pathlib.Path("c"))
   '/a/b/c'
   >>> import os
   >>> os.fspath(pathlib.Path("some_file.txt"))
   'some_file.txt'

(Implemented by Brett Cannon, Ethan Furman, Dusty Phillips, and Jelle
Zijlstra.)

See also:

  **PEP 519** -- Adding a file system path protocol
     PEP written by Brett Cannon and Koos Zevenhoven.


PEP 495: Local Time Disambiguation
----------------------------------

In most world locations, there have been and will be times when local
clocks are moved back.  In those times, intervals are introduced in
which local clocks show the same time twice in the same day. In these
situations, the information displayed on a local clock (or stored in a
Python datetime instance) is insufficient to identify a particular
moment in time.

**PEP 495** adds the new *fold* attribute to instances of
"datetime.datetime" and "datetime.time" classes to differentiate
between two moments in time for which local times are the same:

   >>> u0 = datetime(2016, 11, 6, 4, tzinfo=timezone.utc)
   >>> for i in range(4):
   ...     u = u0 + i*HOUR
   ...     t = u.astimezone(Eastern)
   ...     print(u.time(), 'UTC =', t.time(), t.tzname(), t.fold)
   ...
   04:00:00 UTC = 00:00:00 EDT 0
   05:00:00 UTC = 01:00:00 EDT 0
   06:00:00 UTC = 01:00:00 EST 1
   07:00:00 UTC = 02:00:00 EST 0

The values of the "fold" attribute have the value "0" for all
instances except those that represent the second (chronologically)
moment in time in an ambiguous case.

See also:

  **PEP 495** -- Local Time Disambiguation
     PEP written by Alexander Belopolsky and Tim Peters,
     implementation by Alexander Belopolsky.


PEP 529: Change Windows filesystem encoding to UTF-8
----------------------------------------------------

Representing filesystem paths is best performed with str (Unicode)
rather than bytes. However, there are some situations where using
bytes is sufficient and correct.

Prior to Python 3.6, data loss could result when using bytes paths on
Windows. With this change, using bytes to represent paths is now
supported on Windows, provided those bytes are encoded with the
encoding returned by "sys.getfilesystemencoding()", which now defaults
to "'utf-8'".

Applications that do not use str to represent paths should use
"os.fsencode()" and "os.fsdecode()" to ensure their bytes are
correctly encoded. To revert to the previous behaviour, set
"PYTHONLEGACYWINDOWSFSENCODING" or call
"sys._enablelegacywindowsfsencoding()".

See **PEP 529** for more information and discussion of code
modifications that may be required.


PEP 528: Change Windows console encoding to UTF-8
-------------------------------------------------

The default console on Windows will now accept all Unicode characters
and provide correctly read str objects to Python code. "sys.stdin",
"sys.stdout" and "sys.stderr" now default to utf-8 encoding.

This change only applies when using an interactive console, and not
when redirecting files or pipes. To revert to the previous behaviour
for interactive console use, set "PYTHONLEGACYWINDOWSIOENCODING".

See also:

  **PEP 528** -- Change Windows console encoding to UTF-8
     PEP written and implemented by Steve Dower.


PEP 520: Preserving Class Attribute Definition Order
----------------------------------------------------

Attributes in a class definition body have a natural ordering: the
same order in which the names appear in the source.  This order is now
preserved in the new class's "__dict__" attribute.

Also, the effective default class *execution* namespace (returned from
type.__prepare__()) is now an insertion-order-preserving mapping.

See also:

  **PEP 520** -- Preserving Class Attribute Definition Order
     PEP written and implemented by Eric Snow.


PEP 468: Preserving Keyword Argument Order
------------------------------------------

"**kwargs" in a function signature is now guaranteed to be an
insertion-order-preserving mapping.

See also:

  **PEP 468** -- Preserving Keyword Argument Order
     PEP written and implemented by Eric Snow.


New dict implementation
-----------------------

The dict type now uses a "compact" representation pioneered by PyPy.
The memory usage of the new "dict()" is between 20% and 25% smaller
compared to Python 3.5.

The order-preserving aspect of this new implementation is considered
an implementation detail and should not be relied upon (this may
change in the future, but it is desired to have this new dict
implementation in the language for a few releases before changing the
language spec to mandate order-preserving semantics for all current
and future Python implementations; this also helps preserve backwards-
compatibility with older versions of the language where random
iteration order is still in effect, e.g. Python 3.5).

(Contributed by INADA Naoki in issue 27350. Idea originally suggested
by Raymond Hettinger.)


PEP 523: Adding a frame evaluation API to CPython
-------------------------------------------------

While Python provides extensive support to customize how code
executes, one place it has not done so is in the evaluation of frame
objects.  If you wanted some way to intercept frame evaluation in
Python there really wasn't any way without directly manipulating
function pointers for defined functions.

**PEP 523** changes this by providing an API to make frame evaluation
pluggable at the C level. This will allow for tools such as debuggers
and JITs to intercept frame evaluation before the execution of Python
code begins. This enables the use of alternative evaluation
implementations for Python code, tracking frame evaluation, etc.

This API is not part of the limited C API and is marked as private to
signal that usage of this API is expected to be limited and only
applicable to very select, low-level use-cases. Semantics of the API
will change with Python as necessary.

See also:

  **PEP 523** -- Adding a frame evaluation API to CPython
     PEP written by Brett Cannon and Dino Viehland.


PYTHONMALLOC environment variable
---------------------------------

The new "PYTHONMALLOC" environment variable allows setting the Python
memory allocators and installing debug hooks.

It is now possible to install debug hooks on Python memory allocators
on Python compiled in release mode using "PYTHONMALLOC=debug". Effects
of debug hooks:

* Newly allocated memory is filled with the byte "0xCB"

* Freed memory is filled with the byte "0xDB"

* Detect violations of the Python memory allocator API. For example,
  "PyObject_Free()" called on a memory block allocated by
  "PyMem_Malloc()".

* Detect writes before the start of a buffer (buffer underflows)

* Detect writes after the end of a buffer (buffer overflows)

* Check that the *GIL* is held when allocator functions of
  "PYMEM_DOMAIN_OBJ" (ex: "PyObject_Malloc()") and "PYMEM_DOMAIN_MEM"
  (ex: "PyMem_Malloc()") domains are called.

Checking if the GIL is held is also a new feature of Python 3.6.

See the "PyMem_SetupDebugHooks()" function for debug hooks on Python
memory allocators.

It is now also possible to force the usage of the "malloc()" allocator
of the C library for all Python memory allocations using
"PYTHONMALLOC=malloc". This is helpful when using external memory
debuggers like Valgrind on a Python compiled in release mode.

On error, the debug hooks on Python memory allocators now use the
"tracemalloc" module to get the traceback where a memory block was
allocated.

Example of fatal error on buffer overflow using "python3.6 -X
tracemalloc=5" (store 5 frames in traces):

   Debug memory block at address p=0x7fbcd41666f8: API 'o'
       4 bytes originally requested
       The 7 pad bytes at p-7 are FORBIDDENBYTE, as expected.
       The 8 pad bytes at tail=0x7fbcd41666fc are not all FORBIDDENBYTE (0xfb):
           at tail+0: 0x02 *** OUCH
           at tail+1: 0xfb
           at tail+2: 0xfb
           at tail+3: 0xfb
           at tail+4: 0xfb
           at tail+5: 0xfb
           at tail+6: 0xfb
           at tail+7: 0xfb
       The block was made by call #1233329 to debug malloc/realloc.
       Data at p: 1a 2b 30 00

   Memory block allocated at (most recent call first):
     File "test/test_bytes.py", line 323
     File "unittest/case.py", line 600
     File "unittest/case.py", line 648
     File "unittest/suite.py", line 122
     File "unittest/suite.py", line 84

   Fatal Python error: bad trailing pad byte

   Current thread 0x00007fbcdbd32700 (most recent call first):
     File "test/test_bytes.py", line 323 in test_hex
     File "unittest/case.py", line 600 in run
     File "unittest/case.py", line 648 in __call__
     File "unittest/suite.py", line 122 in run
     File "unittest/suite.py", line 84 in __call__
     File "unittest/suite.py", line 122 in run
     File "unittest/suite.py", line 84 in __call__
     ...

(Contributed by Victor Stinner in issue 26516 and issue 26564.)


DTrace and SystemTap probing support
------------------------------------

Python can now be built "--with-dtrace" which enables static markers
for the following events in the interpreter:

* function call/return

* garbage collection started/finished

* line of code executed.

This can be used to instrument running interpreters in production,
without the need to recompile specific debug builds or providing
application-specific profiling/debugging code.

More details in Instrumenting CPython with DTrace and SystemTap.

The current implementation is tested on Linux and macOS.  Additional
markers may be added in the future.

(Contributed by Łukasz Langa in issue 21590, based on patches by Jesús
Cea Avión, David Malcolm, and Nikhil Benesch.)


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

Some smaller changes made to the core Python language are:

* A "global" or "nonlocal" statement must now textually appear
  before the first use of the affected name in the same scope.
  Previously this was a "SyntaxWarning".

* It is now possible to set a special method to "None" to indicate
  that the corresponding operation is not available. For example, if a
  class sets "__iter__()" to "None", the class is not iterable.
  (Contributed by Andrew Barnert and Ivan Levkivskyi in issue 25958.)

* Long sequences of repeated traceback lines are now abbreviated as
  ""[Previous line repeated {count} more times]"" (see traceback for
  an example). (Contributed by Emanuel Barry in issue 26823.)

* Import now raises the new exception "ModuleNotFoundError"
  (subclass of "ImportError") when it cannot find a module.  Code that
  currently checks for ImportError (in try-except) will still work.
  (Contributed by Eric Snow in issue 15767.)

* Class methods relying on zero-argument "super()" will now work
  correctly when called from metaclass methods during class creation.
  (Contributed by Martin Teichmann in issue 23722.)


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


secrets
-------

The main purpose of the new "secrets" module is to provide an obvious
way to reliably generate cryptographically strong pseudo-random values
suitable for managing secrets, such as account authentication, tokens,
and similar.

Warning: Note that the pseudo-random generators in the "random"
  module should *NOT* be used for security purposes.  Use "secrets" on
  Python 3.6+ and "os.urandom()" on Python 3.5 and earlier.

See also:

  **PEP 506** -- Adding A Secrets Module To The Standard Library
     PEP written and implemented by Steven D'Aprano.


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


array
-----

Exhausted iterators of "array.array" will now stay exhausted even if
the iterated array is extended.  This is consistent with the behavior
of other mutable sequences.

Contributed by Serhiy Storchaka in issue 26492.


ast
---

The new "ast.Constant" AST node has been added.  It can be used by
external AST optimizers for the purposes of constant folding.

Contributed by Victor Stinner in issue 26146.


asyncio
-------

Starting with Python 3.6 the "asyncio" module is no longer provisional
and its API is considered stable.

Notable changes in the "asyncio" module since Python 3.5.0 (all
backported to 3.5.x due to the provisional status):

* The "get_event_loop()" function has been changed to always return
  the currently running loop when called from couroutines and
  callbacks. (Contributed by Yury Selivanov in issue 28613.)

* The "ensure_future()" function and all functions that use it, such
  as "loop.run_until_complete()", now accept all kinds of *awaitable
  objects*. (Contributed by Yury Selivanov.)

* New "run_coroutine_threadsafe()" function to submit coroutines to
  event loops from other threads. (Contributed by Vincent Michel.)

* New "Transport.is_closing()" method to check if the transport is
  closing or closed. (Contributed by Yury Selivanov.)

* The "loop.create_server()" method can now accept a list of hosts.
  (Contributed by Yann Sionneau.)

* New "loop.create_future()" method to create Future objects.  This
  allows alternative event loop implementations, such as uvloop, to
  provide a faster "asyncio.Future" implementation. (Contributed by
  Yury Selivanov in issue 27041.)

* New "loop.get_exception_handler()" method to get the current
  exception handler. (Contributed by Yury Selivanov in issue 27040.)

* New "StreamReader.readuntil()" method to read data from the stream
  until a separator bytes sequence appears. (Contributed by Mark
  Korenberg.)

* The performance of "StreamReader.readexactly()" has been improved.
  (Contributed by Mark Korenberg in issue 28370.)

* The "loop.getaddrinfo()" method is optimized to avoid calling the
  system "getaddrinfo" function if the address is already resolved.
  (Contributed by A. Jesse Jiryu Davis.)

* The "BaseEventLoop.stop()" method has been changed to stop the
  loop immediately after the current iteration.  Any new callbacks
  scheduled as a result of the last iteration will be discarded.
  (Contributed by Guido van Rossum in issue 25593.)

* "Future.set_exception" will now raise "TypeError" when passed an
  instance of the "StopIteration" exception. (Contributed by Chris
  Angelico in issue 26221.)

* New "Loop.connect_accepted_socket()" method to be used by servers
  that accept connections outside of asyncio, but that use asyncio to
  handle them. (Contributed by Jim Fulton in issue 27392.)


binascii
--------

The "b2a_base64()" function now accepts an optional *newline* keyword
argument to control whether the newline character is appended to the
return value. (Contributed by Victor Stinner in issue 25357.)


cmath
-----

The new "cmath.tau" (τ) constant has been added. (Contributed by Lisa
Roach in issue 12345, see **PEP 628** for details.)

New constants: "cmath.inf" and "cmath.nan" to match "math.inf" and
"math.nan", and also "cmath.infj" and "cmath.nanj" to match the format
used by complex repr. (Contributed by Mark Dickinson in issue 23229.)


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

The new "Collection" abstract base class has been added to represent
sized iterable container classes. (Contributed by Ivan Levkivskyi,
docs by Neil Girdhar in issue 27598.)

The new "Reversible" abstract base class represents iterable classes
that also provide the "__reversed__()" method. (Contributed by Ivan
Levkivskyi in issue 25987.)

The new "AsyncGenerator" abstract base class represents asynchronous
generators. (Contributed by Yury Selivanov in issue 28720.)

The "namedtuple()" function now accepts an optional keyword argument
*module*, which, when specified, is used for the "__module__"
attribute of the returned named tuple class. (Contributed by Raymond
Hettinger in issue 17941.)

The *verbose* and *rename* arguments for "namedtuple()" are now
keyword-only. (Contributed by Raymond Hettinger in issue 25628.)

Recursive "collections.deque" instances can now be pickled.
(Contributed by Serhiy Storchaka in issue 26482.)


concurrent.futures
------------------

The "ThreadPoolExecutor" class constructor now accepts an optional
*thread_name_prefix* argument to make it possible to customize the
names of the threads created by the pool. (Contributed by Gregory P.
Smith in issue 27664.)


contextlib
----------

The "contextlib.AbstractContextManager" class has been added to
provide an abstract base class for context managers.  It provides a
sensible default implementation for *__enter__()* which returns "self"
and leaves *__exit__()* an abstract method.  A matching class has been
added to the "typing" module as "typing.ContextManager". (Contributed
by Brett Cannon in issue 25609.)


datetime
--------

The "datetime" and "time" classes have the new "fold" attribute used
to disambiguate local time when necessary.  Many functions in the
"datetime" have been updated to support local time disambiguation. See
Local Time Disambiguation section for more information. (Contributed
by Alexander Belopolsky in issue 24773.)

The "datetime.strftime()" and "date.strftime()" methods now support
ISO 8601 date directives "%G", "%u" and "%V". (Contributed by Ashley
Anderson in issue 12006.)

The "datetime.isoformat()" function now accepts an optional *timespec*
argument that specifies the number of additional components of the
time value to include. (Contributed by Alessandro Cucci and Alexander
Belopolsky in issue 19475.)

The "datetime.combine()" now accepts an optional *tzinfo* argument.
(Contributed by Alexander Belopolsky in issue 27661.)


decimal
-------

New "Decimal.as_integer_ratio()" method that returns a pair "(n, d)"
of integers that represent the given "Decimal" instance as a fraction,
in lowest terms and with a positive denominator:

   >>> Decimal('-3.14').as_integer_ratio()
   (-157, 50)

(Contributed by Stefan Krah amd Mark Dickinson in issue 25928.)


distutils
---------

The "default_format" attribute has been removed from
"distutils.command.sdist.sdist" and the "formats" attribute defaults
to "['gztar']". Although not anticipated, any code relying on the
presence of "default_format" may need to be adapted. See issue 27819
for more details.


email
-----

The new email API, enabled via the *policy* keyword to various
constructors, is no longer provisional.  The "email" documentation has
been reorganized and rewritten to focus on the new API, while
retaining the old documentation for the legacy API.  (Contributed by
R. David Murray in issue 24277.)

The "email.mime" classes now all accept an optional *policy* keyword.
(Contributed by Berker Peksag in issue 27331.)

The "DecodedGenerator" now supports the *policy* keyword.

There is a new "policy" attribute, "message_factory", that controls
what class is used by default when the parser creates new message
objects.  For the "email.policy.compat32" policy this is "Message",
for the new policies it is "EmailMessage". (Contributed by R. David
Murray in issue 20476.)


encodings
---------

On Windows, added the "'oem'" encoding to use "CP_OEMCP", and the
"'ansi'" alias for the existing "'mbcs'" encoding, which uses the
"CP_ACP" code page. (Contributed by Steve Dower in issue 27959.)


enum
----

Two new enumeration base classes have been added to the "enum" module:
"Flag" and "IntFlags".  Both are used to define constants that can be
combined using the bitwise operators. (Contributed by Ethan Furman in
issue 23591.)

Many standard library modules have been updated to use the "IntFlags"
class for their constants.

The new "enum.auto" value can be used to assign values to enum members
automatically:

   >>> from enum import Enum, auto
   >>> class Color(Enum):
   ...     red = auto()
   ...     blue = auto()
   ...     green = auto()
   ...
   >>> list(Color)
   [<Color.red: 1>, <Color.blue: 2>, <Color.green: 3>]


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

On Windows, the "faulthandler" module now installs a handler for
Windows exceptions: see "faulthandler.enable()". (Contributed by
Victor Stinner in issue 23848.)


fileinput
---------

"hook_encoded()" now supports the *errors* argument. (Contributed by
Joseph Hackman in issue 25788.)


hashlib
-------

"hashlib" supports OpenSSL 1.1.0.  The minimum recommend version is
1.0.2. (Contributed by Christian Heimes in issue 26470.)

BLAKE2 hash functions were added to the module. "blake2b()" and
"blake2s()" are always available and support the full feature set of
BLAKE2. (Contributed by Christian Heimes in issue 26798 based on code
by Dmitry Chestnykh and Samuel Neves. Documentation written by Dmitry
Chestnykh.)

The SHA-3 hash functions "sha3_224()", "sha3_256()", "sha3_384()",
"sha3_512()", and SHAKE hash functions "shake_128()" and "shake_256()"
were added. (Contributed by Christian Heimes in issue 16113. Keccak
Code Package by Guido Bertoni, Joan Daemen, Michaël Peeters, Gilles
Van Assche, and Ronny Van Keer.)

The password-based key derivation function "scrypt()" is now available
with OpenSSL 1.1.0 and newer. (Contributed by Christian Heimes in
issue 27928.)


http.client
-----------

"HTTPConnection.request()" and "endheaders()" both now support chunked
encoding request bodies. (Contributed by Demian Brecht and Rolf Krahl
in issue 12319.)


idlelib and IDLE
----------------

The idlelib package is being modernized and refactored to make IDLE
look and work better and to make the code easier to understand, test,
and improve. Part of making IDLE look better, especially on Linux and
Mac, is using ttk widgets, mostly in the dialogs.  As a result, IDLE
no longer runs with tcl/tk 8.4.  It now requires tcl/tk 8.5 or 8.6.
We recommend running the latest release of either.

'Modernizing' includes renaming and consolidation of idlelib modules.
The renaming of files with partial uppercase names is similar to the
renaming of, for instance, Tkinter and TkFont to tkinter and
tkinter.font in 3.0.  As a result, imports of idlelib files that
worked in 3.5 will usually not work in 3.6.  At least a module name
change will be needed (see idlelib/README.txt), sometimes more.  (Name
changes contributed by Al Swiegart and Terry Reedy in issue 24225.
Most idlelib patches since have been and will be part of the process.)

In compensation, the eventual result with be that some idlelib classes
will be easier to use, with better APIs and docstrings explaining
them.  Additional useful information will be added to idlelib when
available.


importlib
---------

Import now raises the new exception "ModuleNotFoundError" (subclass of
"ImportError") when it cannot find a module.  Code that current checks
for "ImportError" (in try-except) will still work. (Contributed by
Eric Snow in issue 15767.)

"importlib.util.LazyLoader" now calls "create_module()" on the wrapped
loader, removing the restriction that
"importlib.machinery.BuiltinImporter" and
"importlib.machinery.ExtensionFileLoader" couldn't be used with
"importlib.util.LazyLoader".

"importlib.util.cache_from_source()",
"importlib.util.source_from_cache()", and
"importlib.util.spec_from_file_location()" now accept a *path-like
object*.


inspect
-------

The "inspect.signature()" function now reports the implicit ".0"
parameters generated by the compiler for comprehension and generator
expression scopes as if they were positional-only parameters called
"implicit0". (Contributed by Jelle Zijlstra in issue 19611.)

To reduce code churn when upgrading from Python 2.7 and the legacy
"inspect.getargspec()" API, the previously documented deprecation of
"inspect.getfullargspec()" has been reversed. While this function is
convenient for single/source Python 2/3 code bases, the richer
"inspect.signature()" interface remains the recommended approach for
new code. (Contributed by Nick Coghlan in issue 27172)


json
----

"json.load()" and "json.loads()" now support binary input.  Encoded
JSON should be represented using either UTF-8, UTF-16, or UTF-32.
(Contributed by Serhiy Storchaka in issue 17909.)


logging
-------

The new "WatchedFileHandler.reopenIfNeeded()" method has been added to
add the ability to check if the log file needs to be reopened.
(Contributed by Marian Horban in issue 24884.)


math
----

The tau (τ) constant has been added to the "math" and "cmath" modules.
(Contributed by Lisa Roach in issue 12345, see **PEP 628** for
details.)


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

Proxy Objects returned by "multiprocessing.Manager()" can now be
nested. (Contributed by Davin Potts in issue 6766.)


os
--

See the summary of PEP 519 for details on how the "os" and "os.path"
modules now support *path-like objects*.

"scandir()" now supports "bytes" paths on Windows.

A new "close()" method allows explicitly closing a "scandir()"
iterator.  The "scandir()" iterator now supports the *context manager*
protocol.  If a "scandir()" iterator is neither exhausted nor
explicitly closed a "ResourceWarning" will be emitted in its
destructor. (Contributed by Serhiy Storchaka in issue 25994.)

On Linux, "os.urandom()" now blocks until the system urandom entropy
pool is initialized to increase the security. See the **PEP 524** for
the rationale.

The Linux "getrandom()" syscall (get random bytes) is now exposed as
the new "os.getrandom()" function. (Contributed by Victor Stinner,
part of the **PEP 524**)


pathlib
-------

"pathlib" now supports *path-like objects*. (Contributed by Brett
Cannon in issue 27186.)

See the summary of PEP 519 for details.


pdb
---

The "Pdb" class constructor has a new optional *readrc* argument to
control whether ".pdbrc" files should be read.


pickle
------

Objects that need "__new__" called with keyword arguments can now be
pickled using pickle protocols older than protocol version 4. Protocol
version 4 already supports this case.  (Contributed by Serhiy
Storchaka in issue 24164.)


pickletools
-----------

"pickletools.dis()" now outputs the implicit memo index for the
"MEMOIZE" opcode. (Contributed by Serhiy Storchaka in issue 25382.)


pydoc
-----

The "pydoc" module has learned to respect the "MANPAGER" environment
variable. (Contributed by Matthias Klose in issue 8637.)

"help()" and "pydoc" can now list named tuple fields in the order they
were defined rather than alphabetically. (Contributed by Raymond
Hettinger in issue 24879.)


random
------

The new "choices()" function returns a list of elements of specified
size from the given population with optional weights. (Contributed by
Raymond Hettinger in issue 18844.)


re
--

Added support of modifier spans in regular expressions.  Examples:
"'(?i:p)ython'" matches "'python'" and "'Python'", but not "'PYTHON'";
"'(?i)g(?-i:v)r'" matches "'GvR'" and "'gvr'", but not "'GVR'".
(Contributed by Serhiy Storchaka in issue 433028.)

Match object groups can be accessed by "__getitem__", which is
equivalent to "group()".  So "mo['name']" is now equivalent to
"mo.group('name')".  (Contributed by Eric Smith in issue 24454.)

"Match" objects now support "index-like objects" as group indices.
(Contributed by Jeroen Demeyer and Xiang Zhang in issue 27177.)


readline
--------

Added "set_auto_history()" to enable or disable automatic addition of
input to the history list.  (Contributed by Tyler Crompton in issue
26870.)


rlcompleter
-----------

Private and special attribute names now are omitted unless the prefix
starts with underscores.  A space or a colon is added after some
completed keywords. (Contributed by Serhiy Storchaka in issue 25011
and issue 25209.)


shlex
-----

The "shlex" has much improved shell compatibility through the new
*punctuation_chars* argument to control which characters are treated
as punctuation. (Contributed by Vinay Sajip in issue 1521950.)


site
----

When specifying paths to add to "sys.path" in a *.pth* file, you may
now specify file paths on top of directories (e.g. zip files).
(Contributed by Wolfgang Langner in issue 26587).


sqlite3
-------

"sqlite3.Cursor.lastrowid" now supports the "REPLACE" statement.
(Contributed by Alex LordThorsen in issue 16864.)


socket
------

The "ioctl()" function now supports the "SIO_LOOPBACK_FAST_PATH"
control code. (Contributed by Daniel Stokes in issue 26536.)

The "getsockopt()" constants "SO_DOMAIN", "SO_PROTOCOL", "SO_PEERSEC",
and "SO_PASSSEC" are now supported. (Contributed by Christian Heimes
in issue 26907.)

The "setsockopt()" now supports the "setsockopt(level, optname, None,
optlen: int)" form. (Contributed by Christian Heimes in issue 27744.)

The socket module now supports the address family "AF_ALG" to
interface with Linux Kernel crypto API. "ALG_*", "SOL_ALG" and
"sendmsg_afalg()" were added. (Contributed by Christian Heimes in
issue 27744 with support from Victor Stinner.)


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

Servers based on the "socketserver" module, including those defined in
"http.server", "xmlrpc.server" and "wsgiref.simple_server", now
support the *context manager* protocol. (Contributed by Aviv Palivoda
in issue 26404.)

The "wfile" attribute of "StreamRequestHandler" classes now implements
the "io.BufferedIOBase" writable interface.  In particular, calling
"write()" is now guaranteed to send the data in full.  (Contributed by
Martin Panter in issue 26721.)


ssl
---

"ssl" supports OpenSSL 1.1.0.  The minimum recommend version is 1.0.2.
(Contributed by Christian Heimes in issue 26470.)

3DES has been removed from the default cipher suites and ChaCha20
Poly1305 cipher suites have been added. (Contributed by Christian
Heimes in issue 27850 and issue 27766.)

"SSLContext" has better default configuration for options and ciphers.
(Contributed by Christian Heimes in issue 28043.)

SSL session can be copied from one client-side connection to another
with the new "SSLSession" class.  TLS session resumption can speed up
the initial handshake, reduce latency and improve performance
(Contributed by Christian Heimes in issue 19500 based on a draft by
Alex Warhawk.)

The new "get_ciphers()" method can be used to get a list of enabled
ciphers in order of cipher priority.

All constants and flags have been converted to "IntEnum" and
"IntFlags". (Contributed by Christian Heimes in issue 28025.)

Server and client-side specific TLS protocols for "SSLContext" were
added. (Contributed by Christian Heimes in issue 28085.)


statistics
----------

A new "harmonic_mean()" function has been added. (Contributed by
Steven D'Aprano in issue 27181.)


struct
------

"struct" now supports IEEE 754 half-precision floats via the "'e'"
format specifier. (Contributed by Eli Stevens, Mark Dickinson in issue
11734.)


subprocess
----------

"subprocess.Popen" destructor now emits a "ResourceWarning" warning if
the child process is still running. Use the context manager protocol
("with proc: ...") or explicitly call the "wait()" method to read the
exit status of the child process. (Contributed by Victor Stinner in
issue 26741.)

The "subprocess.Popen" constructor and all functions that pass
arguments through to it now accept *encoding* and *errors* arguments.
Specifying either of these will enable text mode for the *stdin*,
*stdout* and *stderr* streams. (Contributed by Steve Dower in issue
6135.)


sys
---

The new "getfilesystemencodeerrors()" function returns the name of the
error mode used to convert between Unicode filenames and bytes
filenames. (Contributed by Steve Dower in issue 27781.)

On Windows the return value of the "getwindowsversion()" function now
includes the *platform_version* field which contains the accurate
major version, minor version and build number of the current operating
system, rather than the version that is being emulated for the process
(Contributed by Steve Dower in issue 27932.)


telnetlib
---------

"Telnet" is now a context manager (contributed by Stéphane Wirtel in
issue 25485).


time
----

The "struct_time" attributes "tm_gmtoff" and "tm_zone" are now
available on all platforms.


timeit
------

The new "Timer.autorange()" convenience method has been added to call
"Timer.timeit()" repeatedly so that the total run time is greater or
equal to 200 milliseconds. (Contributed by Steven D'Aprano in issue
6422.)

"timeit" now warns when there is substantial (4x) variance between
best and worst times. (Contributed by Serhiy Storchaka in issue
23552.)


tkinter
-------

Added methods "trace_add()", "trace_remove()" and "trace_info()" in
the "tkinter.Variable" class.  They replace old methods
"trace_variable()", "trace()", "trace_vdelete()" and "trace_vinfo()"
that use obsolete Tcl commands and might not work in future versions
of Tcl. (Contributed by Serhiy Storchaka in issue 22115).


traceback
---------

Both the traceback module and the interpreter's builtin exception
display now abbreviate long sequences of repeated lines in tracebacks
as shown in the following example:

   >>> def f(): f()
   ...
   >>> f()
   Traceback (most recent call last):
     File "<stdin>", line 1, in <module>
     File "<stdin>", line 1, in f
     File "<stdin>", line 1, in f
     File "<stdin>", line 1, in f
     [Previous line repeated 995 more times]
   RecursionError: maximum recursion depth exceeded

(Contributed by Emanuel Barry in issue 26823.)


tracemalloc
-----------

The "tracemalloc" module now supports tracing memory allocations in
multiple different address spaces.

The new "DomainFilter" filter class has been added to filter block
traces by their address space (domain).

(Contributed by Victor Stinner in issue 26588.)


typing
------

Starting with Python 3.6 the "typing" module is no longer provisional
and its API is considered stable.

Since the "typing" module was *provisional* in Python 3.5, all changes
introduced in Python 3.6 have also been backported to Python 3.5.x.

The "typing" module has a much improved support for generic type
aliases.  For example "Dict[str, Tuple[S, T]]" is now a valid type
annotation. (Contributed by Guido van Rossum in Github #195.)

The "typing.ContextManager" class has been added for representing
"contextlib.AbstractContextManager". (Contributed by Brett Cannon in
issue 25609.)

The "typing.Collection" class has been added for representing
"collections.abc.Collection". (Contributed by Ivan Levkivskyi in issue
27598.)

The "typing.ClassVar" type construct has been added to mark class
variables.  As introduced in **PEP 526**, a variable annotation
wrapped in ClassVar indicates that a given attribute is intended to be
used as a class variable and should not be set on instances of that
class. (Contributed by Ivan Levkivskyi in Github #280.)

A new "TYPE_CHECKING" constant that is assumed to be "True" by the
static type chekers, but is "False" at runtime. (Contributed by Guido
van Rossum in Github #230.)

A new "NewType()" helper function has been added to create lightweight
distinct types for annotations:

   from typing import NewType

   UserId = NewType('UserId', int)
   some_id = UserId(524313)

The static type checker will treat the new type as if it were a
subclass of the original type.  (Contributed by Ivan Levkivskyi in
Github #189.)


unicodedata
-----------

The "unicodedata" module now uses data from Unicode 9.0.0.
(Contributed by Benjamin Peterson.)


unittest.mock
-------------

The "Mock" class has the following improvements:

* Two new methods, "Mock.assert_called()" and
  "Mock.assert_called_once()" to check if the mock object was called.
  (Contributed by Amit Saha in issue 26323.)

* The "Mock.reset_mock()" method now has two optional keyword only
  arguments: *return_value* and *side_effect*. (Contributed by Kushal
  Das in issue 21271.)


urllib.request
--------------

If a HTTP request has a file or iterable body (other than a bytes
object) but no "Content-Length" header, rather than throwing an error,
"AbstractHTTPHandler" now falls back to use chunked transfer encoding.
(Contributed by Demian Brecht and Rolf Krahl in issue 12319.)


urllib.robotparser
------------------

"RobotFileParser" now supports the "Crawl-delay" and "Request-rate"
extensions. (Contributed by Nikolay Bogoychev in issue 16099.)


venv
----

"venv" accepts a new parameter "--prompt". This parameter provides an
alternative prefix for the virtual environment. (Proposed by Łukasz
Balcerzak and ported to 3.6 by Stéphane Wirtel in issue 22829.)


warnings
--------

A new optional *source* parameter has been added to the
"warnings.warn_explicit()" function: the destroyed object which
emitted a "ResourceWarning". A *source* attribute has also been added
to "warnings.WarningMessage" (contributed by Victor Stinner in issue
26568 and issue 26567).

When a "ResourceWarning" warning is logged, the "tracemalloc" module
is now used to try to retrieve the traceback where the destroyed
object was allocated.

Example with the script "example.py":

   import warnings

   def func():
       return open(__file__)

   f = func()
   f = None

Output of the command "python3.6 -Wd -X tracemalloc=5 example.py":

   example.py:7: ResourceWarning: unclosed file <_io.TextIOWrapper name='example.py' mode='r' encoding='UTF-8'>
     f = None
   Object allocated at (most recent call first):
     File "example.py", lineno 4
       return open(__file__)
     File "example.py", lineno 6
       f = func()

The "Object allocated at" traceback is new and is only displayed if
"tracemalloc" is tracing Python memory allocations and if the
"warnings" module was already imported.


winreg
------

Added the 64-bit integer type "REG_QWORD". (Contributed by Clement
Rouault in issue 23026.)


winsound
--------

Allowed keyword arguments to be passed to "Beep", "MessageBeep", and
"PlaySound" (issue 27982).


xmlrpc.client
-------------

The "xmlrpc.client" module now supports unmarshalling additional data
types used by the Apache XML-RPC implementation for numerics and
"None". (Contributed by Serhiy Storchaka in issue 26885.)


zipfile
-------

A new "ZipInfo.from_file()" class method allows making a "ZipInfo"
instance from a filesystem file. A new "ZipInfo.is_dir()" method can
be used to check if the "ZipInfo" instance represents a directory.
(Contributed by Thomas Kluyver in issue 26039.)

The "ZipFile.open()" method can now be used to write data into a ZIP
file, as well as for extracting data. (Contributed by Thomas Kluyver
in issue 26039.)


zlib
----

The "compress()" and "decompress()" functions now accept keyword
arguments. (Contributed by Aviv Palivoda in issue 26243 and Xiang
Zhang in issue 16764 respectively.)


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

* The Python interpreter now uses a 16-bit wordcode instead of
  bytecode which made a number of opcode optimizations possible.
  (Contributed by Demur Rumed with input and reviews from Serhiy
  Storchaka and Victor Stinner in issue 26647 and issue 28050.)

* The "Future" class now has an optimized C implementation.
  (Contributed by Yury Selivanov and INADA Naoki in issue 26801.)

* The "Task" class now has an optimized C implementation.
  (Contributed by Yury Selivanov in issue 28544.)

* Various implementation improvements in the "typing" module (such
  as caching of generic types) allow up to 30 times performance
  improvements and reduced memory footprint.

* The ASCII decoder is now up to 60 times as fast for error handlers
  "surrogateescape", "ignore" and "replace" (Contributed by Victor
  Stinner in issue 24870).

* The ASCII and the Latin1 encoders are now up to 3 times as fast
  for the error handler "surrogateescape" (Contributed by Victor
  Stinner in issue 25227).

* The UTF-8 encoder is now up to 75 times as fast for error handlers
  "ignore", "replace", "surrogateescape", "surrogatepass" (Contributed
  by Victor Stinner in issue 25267).

* The UTF-8 decoder is now up to 15 times as fast for error handlers
  "ignore", "replace" and "surrogateescape" (Contributed by Victor
  Stinner in issue 25301).

* "bytes % args" is now up to 2 times faster. (Contributed by Victor
  Stinner in issue 25349).

* "bytearray % args" is now between 2.5 and 5 times faster.
  (Contributed by Victor Stinner in issue 25399).

* Optimize "bytes.fromhex()" and "bytearray.fromhex()": they are now
  between 2x and 3.5x faster. (Contributed by Victor Stinner in issue
  25401).

* Optimize "bytes.replace(b'', b'.')" and "bytearray.replace(b'',
  b'.')": up to 80% faster. (Contributed by Josh Snider in issue
  26574).

* Allocator functions of the "PyMem_Malloc()" domain
  ("PYMEM_DOMAIN_MEM") now use the pymalloc memory allocator instead
  of "malloc()" function of the C library. The pymalloc allocator is
  optimized for objects smaller or equal to 512 bytes with a short
  lifetime, and use "malloc()" for larger memory blocks. (Contributed
  by Victor Stinner in issue 26249).

* "pickle.load()" and "pickle.loads()" are now up to 10% faster when
  deserializing many small objects (Contributed by Victor Stinner in
  issue 27056).

* Passing *keyword arguments* to a function has an overhead in
  comparison with passing *positional arguments*.  Now in extension
  functions implemented with using Argument Clinic this overhead is
  significantly decreased. (Contributed by Serhiy Storchaka in issue
  27574).

* Optimized "glob()" and "iglob()" functions in the "glob" module;
  they are now about 3--6 times faster. (Contributed by Serhiy
  Storchaka in issue 25596).

* Optimized globbing in "pathlib" by using "os.scandir()"; it is now
  about 1.5--4 times faster. (Contributed by Serhiy Storchaka in issue
  26032).

* "xml.etree.ElementTree" parsing, iteration and deepcopy
  performance has been significantly improved. (Contributed by Serhiy
  Storchaka in issue 25638, issue 25873, and issue 25869.)

* Creation of "fractions.Fraction" instances from floats and
  decimals is now 2 to 3 times faster. (Contributed by Serhiy
  Storchaka in issue 25971.)


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

* Python now requires some C99 support in the toolchain to build.
  Most notably, Python now uses standard integer types and macros in
  place of custom macros like "PY_LONG_LONG". For more information,
  see **PEP 7** and issue 17884.

* Cross-compiling CPython with the Android NDK and the Android API
  level set to 21 (Android 5.0 Lollilop) or greater runs successfully.
  While Android is not yet a supported platform, the Python test suite
  runs on the Android emulator with only about 16 tests failures. See
  the Android meta-issue issue 26865.

* The "--enable-optimizations" configure flag has been added.
  Turning it on will activate expensive optimizations like PGO.
  (Original patch by Alecsandru Patrascu of Intel in issue 26539.)

* The *GIL* must now be held when allocator functions of
  "PYMEM_DOMAIN_OBJ" (ex: "PyObject_Malloc()") and "PYMEM_DOMAIN_MEM"
  (ex: "PyMem_Malloc()") domains are called.

* New "Py_FinalizeEx()" API which indicates if flushing buffered
  data failed. (Contributed by Martin Panter in issue 5319.)

* "PyArg_ParseTupleAndKeywords()" now supports positional-only
  parameters.  Positional-only parameters are defined by empty names.
  (Contributed by Serhiy Storchaka in issue 26282).

* "PyTraceback_Print" method now abbreviates long sequences of
  repeated lines as ""[Previous line repeated {count} more times]"".
  (Contributed by Emanuel Barry in issue 26823.)

* The new "PyErr_SetImportErrorSubclass()" function allows for
  specifying a subclass of "ImportError" to raise. (Contributed by
  Eric Snow in issue 15767.)

* The new "PyErr_ResourceWarning()" function can be used to generate
  a "ResourceWarning" providing the source of the resource allocation.
  (Contributed by Victor Stinner in issue 26567.)

* The new "PyOS_FSPath()" function returns the file system
  representation of a *path-like object*. (Contributed by Brett Cannon
  in issue 27186.)

* The "PyUnicode_FSConverter()" and "PyUnicode_FSDecoder()"
  functions will now accept *path-like objects*.


Other Improvements
==================

* When "--version" (short form: "-V") is supplied twice, Python
  prints "sys.version" for detailed information.

     $ ./python -VV
     Python 3.6.0b4+ (3.6:223967b49e49+, Nov 21 2016, 20:55:04)
     [GCC 4.2.1 Compatible Apple LLVM 8.0.0 (clang-800.0.42.1)]


Deprecated
==========


New Keywords
------------

"async" and "await" are not recommended to be used as variable, class,
function or module names.  Introduced by **PEP 492** in Python 3.5,
they will become proper keywords in Python 3.7.  Starting in Python
3.6, the use of "async" or "await" as names will generate a
"DeprecationWarning".


Deprecated Python behavior
--------------------------

Raising the "StopIteration" exception inside a generator will now
generate a "DeprecationWarning", and will trigger a "RuntimeError" in
Python 3.7.  See PEP 479: Change StopIteration handling inside
generators for details.

The "__aiter__()" method is now expected to return an asynchronous
iterator directly instead of returning an awaitable as previously.
Doing the former will trigger a "DeprecationWarning".  Backward
compatibility will be removed in Python 3.7. (Contributed by Yury
Selivanov in issue 27243.)

A backslash-character pair that is not a valid escape sequence now
generates a "DeprecationWarning".  Although this will eventually
become a "SyntaxError", that will not be for several Python releases.
(Contributed by Emanuel Barry in issue 27364.)

When performing a relative import, falling back on "__name__" and
"__path__" from the calling module when "__spec__" or "__package__"
are not defined now raises an "ImportWarning". (Contributed by Rose
Ames in issue 25791.)


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


asynchat
~~~~~~~~

The "asynchat" has been deprecated in favor of "asyncio". (Contributed
by Mariatta in issue 25002.)


asyncore
~~~~~~~~

The "asyncore" has been deprecated in favor of "asyncio". (Contributed
by Mariatta in issue 25002.)


dbm
~~~

Unlike other "dbm" implementations, the "dbm.dumb" module creates
databases with the "'rw'" mode and allows modifying the database
opened with the "'r'" mode.  This behavior is now deprecated and will
be removed in 3.8. (Contributed by Serhiy Storchaka in issue 21708.)


distutils
~~~~~~~~~

The undocumented "extra_path" argument to the "Distribution"
constructor is now considered deprecated and will raise a warning if
set.   Support for this parameter will be removed in a future Python
release.  See issue 27919 for details.


grp
~~~

The support of non-integer arguments in "getgrgid()" has been
deprecated. (Contributed by Serhiy Storchaka in issue 26129.)


importlib
~~~~~~~~~

The "importlib.machinery.SourceFileLoader.load_module()" and
"importlib.machinery.SourcelessFileLoader.load_module()" methods are
now deprecated. They were the only remaining implementations of
"importlib.abc.Loader.load_module()" in "importlib" that had not been
deprecated in previous versions of Python in favour of
"importlib.abc.Loader.exec_module()".


os
~~

Undocumented support of general *bytes-like objects* as paths in "os"
functions, "compile()" and similar functions is now deprecated.
(Contributed by Serhiy Storchaka in issue 25791 and issue 26754.)


re
~~

Support for inline flags "(?letters)" in the middle of the regular
expression has been deprecated and will be removed in a future Python
version.  Flags at the start of a regular expression are still
allowed. (Contributed by Serhiy Storchaka in issue 22493.)


ssl
~~~

OpenSSL 0.9.8, 1.0.0 and 1.0.1 are deprecated and no longer supported.
In the future the "ssl" module will require at least OpenSSL 1.0.2 or
1.1.0.

SSL-related arguments like "certfile", "keyfile" and "check_hostname"
in "ftplib", "http.client", "imaplib", "poplib", and "smtplib" have
been deprecated in favor of "context". (Contributed by Christian
Heimes in issue 28022.)

A couple of protocols and functions of the "ssl" module are now
deprecated. Some features will no longer be available in future
versions of OpenSSL. Other features are deprecated in favor of a
different API. (Contributed by Christian Heimes in issue 28022 and
issue 26470.)


tkinter
~~~~~~~

The "tkinter.tix" module is now deprecated.  "tkinter" users should
use "tkinter.ttk" instead.


venv
~~~~

The "pyvenv" script has been deprecated in favour of "python3 -m
venv". This prevents confusion as to what Python interpreter "pyvenv"
is connected to and thus what Python interpreter will be used by the
virtual environment.  (Contributed by Brett Cannon in issue 25154.)


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

Undocumented functions "PyUnicode_AsEncodedObject()",
"PyUnicode_AsDecodedObject()", "PyUnicode_AsEncodedUnicode()" and
"PyUnicode_AsDecodedUnicode()" are deprecated now. Use the generic
codec based API instead.


Deprecated Build Options
------------------------

The "--with-system-ffi" configure flag is now on by default on non-
macOS UNIX platforms.  It may be disabled by using "--without-system-
ffi", but using the flag is deprecated and will not be accepted in
Python 3.7. macOS is unaffected by this change.  Note that many OS
distributors already use the "--with-system-ffi" flag when building
their system Python.


Removed
=======


API and Feature Removals
------------------------

* Unknown escapes consisting of "'\'" and an ASCII letter in regular
  expressions will now cause an error.  In replacement templates for
  "re.sub()" they are still allowed, but deprecated. The "re.LOCALE"
  flag can now only be used with binary patterns.

* "inspect.getmoduleinfo()" was removed (was deprecated since
  CPython 3.3). "inspect.getmodulename()" should be used for obtaining
  the module name for a given path. (Contributed by Yury Selivanov in
  issue 13248.)

* "traceback.Ignore" class and "traceback.usage",
  "traceback.modname", "traceback.fullmodname",
  "traceback.find_lines_from_code", "traceback.find_lines",
  "traceback.find_strings", "traceback.find_executable_lines" methods
  were removed from the "traceback" module. They were undocumented
  methods deprecated since Python 3.2 and equivalent functionality is
  available from private methods.

* The "tk_menuBar()" and "tk_bindForTraversal()" dummy methods in
  "tkinter" widget classes were removed (corresponding Tk commands
  were obsolete since Tk 4.0).

* The "open()" method of the "zipfile.ZipFile" class no longer
  supports the "'U'" mode (was deprecated since Python 3.4). Use
  "io.TextIOWrapper" for reading compressed text files in *universal
  newlines* mode.

* The undocumented "IN", "CDROM", "DLFCN", "TYPES", "CDIO", and
  "STROPTS" modules have been removed.  They had been available in the
  platform specific "Lib/plat-*/" directories, but were chronically
  out of date, inconsistently available across platforms, and
  unmaintained.  The script that created these modules is still
  available in the source distribution at Tools/scripts/h2py.py.

* The deprecated "asynchat.fifo" class has been removed.


Porting to Python 3.6
=====================

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


Changes in 'python' Command Behavior
------------------------------------

* The output of a special Python build with defined "COUNT_ALLOCS",
  "SHOW_ALLOC_COUNT" or "SHOW_TRACK_COUNT" macros is now off by
  default.  It can be re-enabled using the "-X showalloccount" option.
  It now outputs to "stderr" instead of "stdout". (Contributed by
  Serhiy Storchaka in issue 23034.)


Changes in the Python API
-------------------------

* "open()" will no longer allow combining the "'U'" mode flag with
  "'+'". (Contributed by Jeff Balogh and John O'Connor in issue 2091.)

* "sqlite3" no longer implicitly commits an open transaction before
  DDL statements.

* On Linux, "os.urandom()" now blocks until the system urandom
  entropy pool is initialized to increase the security.

* When "importlib.abc.Loader.exec_module()" is defined,
  "importlib.abc.Loader.create_module()" must also be defined.

* "PyErr_SetImportError()" now sets "TypeError" when its **msg**
  argument is not set. Previously only "NULL" was returned.

* The format of the "co_lnotab" attribute of code objects changed to
  support a negative line number delta. By default, Python does not
  emit bytecode with a negative line number delta. Functions using
  "frame.f_lineno", "PyFrame_GetLineNumber()" or "PyCode_Addr2Line()"
  are not affected. Functions directly decoding "co_lnotab" should be
  updated to use a signed 8-bit integer type for the line number
  delta, but this is only required to support applications using a
  negative line number delta. See "Objects/lnotab_notes.txt" for the
  "co_lnotab" format and how to decode it, and see the **PEP 511** for
  the rationale.

* The functions in the "compileall" module now return booleans
  instead of "1" or "0" to represent success or failure, respectively.
  Thanks to booleans being a subclass of integers, this should only be
  an issue if you were doing identity checks for "1" or "0". See issue
  25768.

* Reading the "port" attribute of "urllib.parse.urlsplit()" and
  "urlparse()" results now raises "ValueError" for out-of-range
  values, rather than returning "None".  See issue 20059.

* The "imp" module now raises a "DeprecationWarning" instead of
  "PendingDeprecationWarning".

* The following modules have had missing APIs added to their
  "__all__" attributes to match the documented APIs: "calendar",
  "cgi", "csv", "ElementTree", "enum", "fileinput", "ftplib",
  "logging", "mailbox", "mimetypes", "optparse", "plistlib", "smtpd",
  "subprocess", "tarfile", "threading" and "wave".  This means they
  will export new symbols when "import *" is used. (Contributed by
  Joel Taddei and Jacek Kołodziej in issue 23883.)

* When performing a relative import, if "__package__" does not
  compare equal to "__spec__.parent" then "ImportWarning" is raised.
  (Contributed by Brett Cannon in issue 25791.)

* When a relative import is performed and no parent package is
  known, then "ImportError" will be raised. Previously, "SystemError"
  could be raised. (Contributed by Brett Cannon in issue 18018.)

* Servers based on the "socketserver" module, including those
  defined in "http.server", "xmlrpc.server" and
  "wsgiref.simple_server", now only catch exceptions derived from
  "Exception". Therefore if a request handler raises an exception like
  "SystemExit" or "KeyboardInterrupt", "handle_error()" is no longer
  called, and the exception will stop a single-threaded server.
  (Contributed by Martin Panter in issue 23430.)

* "spwd.getspnam()" now raises a "PermissionError" instead of
  "KeyError" if the user doesn't have privileges.

* The "socket.socket.close()" method now raises an exception if an
  error (e.g. "EBADF") was reported by the underlying system call.
  (Contributed by Martin Panter in issue 26685.)

* The *decode_data* argument for the "smtpd.SMTPChannel" and
  "smtpd.SMTPServer" constructors is now "False" by default. This
  means that the argument passed to "process_message()" is now a bytes
  object by default, and "process_message()" will be passed keyword
  arguments. Code that has already been updated in accordance with the
  deprecation warning generated by 3.5 will not be affected.

* All optional arguments of the "dump()", "dumps()", "load()" and
  "loads()" functions and "JSONEncoder" and "JSONDecoder" class
  constructors in the "json" module are now keyword-only. (Contributed
  by Serhiy Storchaka in issue 18726.)

* Subclasses of "type" which don't override "type.__new__" may no
  longer use the one-argument form to get the type of an object.

* As part of **PEP 487**, the handling of keyword arguments passed
  to "type" (other than the metaclass hint, "metaclass") is now
  consistently delegated to "object.__init_subclass__()". This means
  that "type.__new__()" and "type.__init__()" both now accept
  arbitrary keyword arguments, but "object.__init_subclass__()" (which
  is called from "type.__new__()") will reject them by default. Custom
  metaclasses accepting additional keyword arguments will need to
  adjust their calls to "type.__new__()" (whether direct or via
  "super") accordingly.

* In "distutils.command.sdist.sdist", the "default_format" attribute
  has been removed and is no longer honored. Instead, the gzipped
  tarfile format is the default on all platforms and no platform-
  specific selection is made. In environments where distributions are
  built on Windows and zip distributions are required, configure the
  project with a "setup.cfg" file containing the following:

     [sdist]
     formats=zip

  This behavior has also been backported to earlier Python versions by
  Setuptools 26.0.0.

* In the "urllib.request" module and the
  "http.client.HTTPConnection.request()" method, if no Content-Length
  header field has been specified and the request body is a file
  object, it is now sent with HTTP 1.1 chunked encoding. If a file
  object has to be sent to a HTTP 1.0 server, the Content-Length value
  now has to be specified by the caller. (Contributed by Demian Brecht
  and Rolf Krahl with tweaks from Martin Panter in issue 12319.)

* The "DictReader" now returns rows of type "OrderedDict".
  (Contributed by Steve Holden in issue 27842.)

* The "crypt.METHOD_CRYPT" will no longer be added to
  "crypt.methods" if unsupported by the platform. (Contributed by
  Victor Stinner in issue 25287.)

* The *verbose* and *rename* arguments for "namedtuple()" are now
  keyword-only. (Contributed by Raymond Hettinger in issue 25628.)

* On Linux, "ctypes.util.find_library()" now looks in
  "LD_LIBRARY_PATH" for shared libraries. (Contributed by Vinay Sajip
  in issue 9998.)

* The "imaplib.IMAP4" class now handles flags containing the "']'"
  character in messages sent from the server to improve real-world
  compatibility. (Contributed by Lita Cho in issue 21815.)

* The "mmap.write()" function now returns the number of bytes
  written like other write methods. (Contributed by Jakub Stasiak in
  issue 26335.)

* The "pkgutil.iter_modules()" and "pkgutil.walk_packages()"
  functions now return "ModuleInfo" named tuples. (Contributed by
  Ramchandra Apte in issue 17211.)

* "re.sub()" now raises an error for invalid numerical group
  references in replacement templates even if the pattern is not found
  in the string.  The error message for invalid group references now
  includes the group index and the position of the reference.
  (Contributed by SilentGhost, Serhiy Storchaka in issue 25953.)

* "zipfile.ZipFile" will now raise "NotImplementedError" for
  unrecognized compression values.  Previously a plain "RuntimeError"
  was raised.  Additionally, calling "ZipFile" methods on a closed
  ZipFile or calling the "write()" method on a ZipFile created with
  mode "'r'" will raise a "ValueError". Previously, a "RuntimeError"
  was raised in those scenarios.

* when custom metaclasses are combined with zero-argument "super()"
  or direct references from methods to the implicit "__class__"
  closure variable, the implicit "__classcell__" namespace entry must
  now be passed up to "type.__new__" for initialisation. Failing to do
  so will result in a "DeprecationWarning" in 3.6 and a
  "RuntimeWarning" in the future.


Changes in the C API
--------------------

* The "PyMem_Malloc()" allocator family now uses the pymalloc
  allocator rather than the system "malloc()". Applications calling
  "PyMem_Malloc()" without holding the GIL can now crash. Set the
  "PYTHONMALLOC" environment variable to "debug" to validate the usage
  of memory allocators in your application. See issue 26249.

* "Py_Exit()" (and the main interpreter) now override the exit
  status with 120 if flushing buffered data failed.  See issue 5319.


CPython bytecode changes
------------------------

There have been several major changes to the *bytecode* in Python 3.6.

* The Python interpreter now uses a 16-bit wordcode instead of
  bytecode. (Contributed by Demur Rumed with input and reviews from
  Serhiy Storchaka and Victor Stinner in issue 26647 and issue 28050.)

* The new "FORMAT_VALUE" and "BUILD_STRING" opcodes as part of the
  formatted string literal implementation. (Contributed by Eric Smith
  in issue 25483 and Serhiy Storchaka in issue 27078.)

* The new "BUILD_CONST_KEY_MAP" opcode to optimize the creation of
  dictionaries with constant keys. (Contributed by Serhiy Storchaka in
  issue 27140.)

* The function call opcodes have been heavily reworked for better
  performance and simpler implementation. The "MAKE_FUNCTION",
  "CALL_FUNCTION", "CALL_FUNCTION_KW" and "BUILD_MAP_UNPACK_WITH_CALL"
  opcodes have been modified, the new "CALL_FUNCTION_EX" and
  "BUILD_TUPLE_UNPACK_WITH_CALL" have been added, and
  "CALL_FUNCTION_VAR", "CALL_FUNCTION_VAR_KW" and "MAKE_CLOSURE"
  opcodes have been removed. (Contributed by Demur Rumed in issue
  27095, and Serhiy Storchaka in issue 27213, issue 28257.)

* The new "SETUP_ANNOTATIONS" and "STORE_ANNOTATION" opcodes have
  been added to support the new *variable annotation* syntax.
  (Contributed by Ivan Levkivskyi in issue 27985.)
