What’s New In Python 3.6
************************

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

This article explains the new features in Python 3.6, compared to 3.5.
Python 3.6 was released on December 23, 2016.  See the changelog for a
full list of changes.

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 more compact
  representation based on a proposal by Raymond Hettinger and 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.

* 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 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 "PYTHONLEGACYWINDOWSSTDIO".

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 based on a proposal
by Raymond Hettinger which was first implemented 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 bpo-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 bpo-26516 and bpo-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 bpo-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 bpo-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 bpo-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 bpo-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 bpo-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 bpo-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 bpo-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 coroutines and
  callbacks. (Contributed by Yury Selivanov in bpo-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 bpo-27041.)

* New "loop.get_exception_handler()" method to get the current
  exception handler. (Contributed by Yury Selivanov in bpo-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 bpo-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 "loop.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 bpo-25593.)

* "Future.set_exception" will now raise "TypeError" when passed an
  instance of the "StopIteration" exception. (Contributed by Chris
  Angelico in bpo-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 bpo-27392.)

* "TCP_NODELAY" flag is now set for all TCP transports by default.
  (Contributed by Yury Selivanov in bpo-27456.)

* New "loop.shutdown_asyncgens()" to properly close pending
  asynchronous generators before closing the loop. (Contributed by
  Yury Selivanov in bpo-28003.)

* "Future" and "Task" classes now have an optimized C implementation
  which makes asyncio code up to 30% faster. (Contributed by Yury
  Selivanov and INADA Naoki in bpo-26081 and bpo-28544.)


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 bpo-25357.)


cmath
-----

The new "cmath.tau" (*τ*) constant has been added. (Contributed by
Lisa Roach in bpo-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 bpo-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 bpo-27598.)

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

The new "AsyncGenerator" abstract base class represents asynchronous
generators. (Contributed by Yury Selivanov in bpo-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 bpo-17941.)

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

Recursive "collections.deque" instances can now be pickled.
(Contributed by Serhiy Storchaka in bpo-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 bpo-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 bpo-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 bpo-24773.)

The "datetime.strftime()" and "date.strftime()" methods now support
ISO 8601 date directives "%G", "%u" and "%V". (Contributed by Ashley
Anderson in bpo-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 bpo-19475.)

The "datetime.combine()" now accepts an optional *tzinfo* argument.
(Contributed by Alexander Belopolsky in bpo-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 bpo-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 bpo-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 bpo-24277.)

The "email.mime" classes now all accept an optional *policy* keyword.
(Contributed by Berker Peksag in bpo-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 bpo-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 bpo-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
bpo-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 bpo-23848.)


fileinput
---------

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


hashlib
-------

"hashlib" supports OpenSSL 1.1.0.  The minimum recommend version is
1.0.2. (Contributed by Christian Heimes in bpo-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 bpo-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 bpo-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
bpo-27928.)


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

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

New in 3.6.2:

Multiple fixes for autocompletion. (Contributed by Louie Lu in
bpo-15786.)

New in 3.6.3:

Module Browser (on the File menu, formerly called Class Browser), now
displays nested functions and classes in addition to top-level
functions and classes. (Contributed by Guilherme Polo, Cheryl Sabella,
and Terry Jan Reedy in bpo-1612262.)

The IDLE features formerly implemented as extensions have been
reimplemented as normal features.  Their settings have been moved from
the Extensions tab to other dialog tabs. (Contributed by Charles
Wohlganger and Terry Jan Reedy in bpo-27099.)

The Settings dialog (Options, Configure IDLE) has been partly
rewritten to improve both appearance and function. (Contributed by
Cheryl Sabella and Terry Jan Reedy in multiple issues.)

New in 3.6.4:

The font sample now includes a selection of non-Latin characters so
that users can better see the effect of selecting a particular font.
(Contributed by Terry Jan Reedy in bpo-13802.) The sample can be
edited to include other characters. (Contributed by Serhiy Storchaka
in bpo-31860.)

New in 3.6.6:

Editor code context option revised.  Box displays all context lines up
to maxlines.  Clicking on a context line jumps the editor to that
line.  Context colors for custom themes is added to Highlights tab of
Settings dialog. (Contributed by Cheryl Sabella and Terry Jan Reedy in
bpo-33642, bpo-33768, and bpo-33679.)

On Windows, a new API call tells Windows that tk scales for DPI. On
Windows 8.1+ or 10, with DPI compatibility properties of the Python
binary unchanged, and a monitor resolution greater than 96 DPI, this
should make text and lines sharper.  It should otherwise have no
effect. (Contributed by Terry Jan Reedy in bpo-33656.)

New in 3.6.7:

Output over N lines (50 by default) is squeezed down to a button. N
can be changed in the PyShell section of the General page of the
Settings dialog.  Fewer, but possibly extra long, lines can be
squeezed by right clicking on the output.  Squeezed output can be
expanded in place by double-clicking the button or into the clipboard
or a separate window by right-clicking the button.  (Contributed by
Tal Einat in bpo-1529353.)


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 bpo-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 bpo-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 bpo-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 bpo-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 bpo-24884.)


math
----

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


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

Proxy Objects returned by "multiprocessing.Manager()" can now be
nested. (Contributed by Davin Potts in bpo-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 bpo-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 bpo-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 bpo-24164.)


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

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


pydoc
-----

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

"help()" and "pydoc" can now list named tuple fields in the order they
were defined rather than alphabetically. (Contributed by Raymond
Hettinger in bpo-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 bpo-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 bpo-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 bpo-24454.)

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


readline
--------

Added "set_auto_history()" to enable or disable automatic addition of
input to the history list.  (Contributed by Tyler Crompton in
bpo-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 bpo-25011 and
bpo-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 bpo-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 bpo-26587).


sqlite3
-------

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


socket
------

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

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

The "setsockopt()" now supports the "setsockopt(level, optname, None,
optlen: int)" form. (Contributed by Christian Heimes in bpo-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
bpo-27744 with support from Victor Stinner.)

New Linux constants "TCP_USER_TIMEOUT" and "TCP_CONGESTION" were
added. (Contributed by Omar Sandoval, issue:*26273*).


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 bpo-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 bpo-26721.)


ssl
---

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

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

"SSLContext" has better default configuration for options and ciphers.
(Contributed by Christian Heimes in bpo-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 bpo-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 bpo-28025.)

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


statistics
----------

A new "harmonic_mean()" function has been added. (Contributed by
Steven D’Aprano in bpo-27181.)


struct
------

"struct" now supports IEEE 754 half-precision floats via the "'e'"
format specifier. (Contributed by Eli Stevens, Mark Dickinson in
bpo-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
bpo-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
bpo-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 bpo-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 bpo-27932.)


telnetlib
---------

"Telnet" is now a context manager (contributed by Stéphane Wirtel in
bpo-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
bpo-6422.)

"timeit" now warns when there is substantial (4x) variance between
best and worst times. (Contributed by Serhiy Storchaka in bpo-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 bpo-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 bpo-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 bpo-26588.)


typing
------

Since the "typing" module is *provisional*, 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
bpo-25609.)

The "typing.Collection" class has been added for representing
"collections.abc.Collection". (Contributed by Ivan Levkivskyi in
bpo-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 checkers, 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 bpo-26323.)

* The "Mock.reset_mock()" method now has two optional keyword only
  arguments: *return_value* and *side_effect*. (Contributed by Kushal
  Das in bpo-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 bpo-12319.)


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

"RobotFileParser" now supports the "Crawl-delay" and "Request-rate"
extensions. (Contributed by Nikolay Bogoychev in bpo-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 bpo-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
bpo-26568 and bpo-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 bpo-23026.)


winsound
--------

Allowed keyword arguments to be passed to "Beep", "MessageBeep", and
"PlaySound" (bpo-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 bpo-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 bpo-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 bpo-26039.)


zlib
----

The "compress()" and "decompress()" functions now accept keyword
arguments. (Contributed by Aviv Palivoda in bpo-26243 and Xiang Zhang
in bpo-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 bpo-26647 and bpo-28050.)

* The "asyncio.Future" class now has an optimized C implementation.
  (Contributed by Yury Selivanov and INADA Naoki in bpo-26081.)

* The "asyncio.Task" class now has an optimized C implementation.
  (Contributed by Yury Selivanov in bpo-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 bpo-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 bpo-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 bpo-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 bpo-25301).

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

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

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

* Optimize "bytes.replace(b'', b'.')" and "bytearray.replace(b'',
  b'.')": up to 80% faster. (Contributed by Josh Snider in bpo-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 bpo-26249).

* "pickle.load()" and "pickle.loads()" are now up to 10% faster when
  deserializing many small objects (Contributed by Victor Stinner in
  bpo-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
  bpo-27574).

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

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

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

* Creation of "fractions.Fraction" instances from floats and decimals
  is now 2 to 3 times faster. (Contributed by Serhiy Storchaka in
  bpo-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 bpo-17884.

* Cross-compiling CPython with the Android NDK and the Android API
  level set to 21 (Android 5.0 Lollipop) 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 bpo-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 bpo-26359.)

* 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 bpo-5319.)

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

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

* The new "PyErr_SetImportErrorSubclass()" function allows for
  specifying a subclass of "ImportError" to raise. (Contributed by
  Eric Snow in bpo-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 bpo-26567.)

* The new "PyOS_FSPath()" function returns the file system
  representation of a *path-like object*. (Contributed by Brett Cannon
  in bpo-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 bpo-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 bpo-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 bpo-25791.)


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


asynchat
~~~~~~~~

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


asyncore
~~~~~~~~

The "asyncore" has been deprecated in favor of "asyncio". (Contributed
by Mariatta in bpo-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 bpo-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 bpo-27919 for details.


grp
~~~

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

The "importlib.machinery.WindowsRegistryFinder" class is now
deprecated. As of 3.6.0, it is still added to "sys.meta_path" by
default (on Windows), but this may change in future releases.


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 bpo-25791 and bpo-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 bpo-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 bpo-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 bpo-28022 and
bpo-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 bpo-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
  bpo-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 bpo-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 bpo-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
  bpo-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 bpo-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 bpo-23883.)

* When performing a relative import, if "__package__" does not compare
  equal to "__spec__.parent" then "ImportWarning" is raised.
  (Contributed by Brett Cannon in bpo-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 bpo-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 bpo-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 bpo-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 bpo-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 bpo-12319.)

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

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

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

* On Linux, "ctypes.util.find_library()" now looks in
  "LD_LIBRARY_PATH" for shared libraries. (Contributed by Vinay Sajip
  in bpo-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 bpo-21815.)

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

* The "pkgutil.iter_modules()" and "pkgutil.walk_packages()" functions
  now return "ModuleInfo" named tuples. (Contributed by Ramchandra
  Apte in bpo-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 bpo-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 Python 3.6 and a
  "RuntimeError" in Python 3.8.

* With the introduction of "ModuleNotFoundError", import system
  consumers may start expecting import system replacements to raise
  that more specific exception when appropriate, rather than the less-
  specific "ImportError". To provide future compatibility with such
  consumers, implementors of alternative import systems that
  completely replace "__import__()" will need to update their
  implementations to raise the new subclass when a module can’t be
  found at all. Implementors of compliant plugins to the default
  import system shouldn’t need to make any changes, as the default
  import system will raise the new subclass when appropriate.


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 bpo-26249.

* "Py_Exit()" (and the main interpreter) now override the exit status
  with 120 if flushing buffered data failed.  See bpo-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 bpo-26647 and bpo-28050.)

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

* The new "BUILD_CONST_KEY_MAP" opcode to optimize the creation of
  dictionaries with constant keys. (Contributed by Serhiy Storchaka in
  bpo-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 bpo-27095,
  and Serhiy Storchaka in bpo-27213, bpo-28257.)

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


Notable changes in Python 3.6.2
===============================


New "make regen-all" build target
---------------------------------

To simplify cross-compilation, and to ensure that CPython can reliably
be compiled without requiring an existing version of Python to already
be available, the autotools-based build system no longer attempts to
implicitly recompile generated files based on file modification times.

Instead, a new "make regen-all" command has been added to force
regeneration of these files when desired (e.g. after an initial
version of Python has already been built based on the pregenerated
versions).

More selective regeneration targets are also defined - see
Makefile.pre.in for details.

(Contributed by Victor Stinner in bpo-23404.)

New in version 3.6.2.


Removal of "make touch" build target
------------------------------------

The "make touch" build target previously used to request implicit
regeneration of generated files by updating their modification times
has been removed.

It has been replaced by the new "make regen-all" target.

(Contributed by Victor Stinner in bpo-23404.)

Changed in version 3.6.2.


Notable changes in Python 3.6.4
===============================

The "PyExc_RecursionErrorInst" singleton that was part of the public
API has been removed as its members being never cleared may cause a
segfault during finalization of the interpreter. (Contributed by
Xavier de Gaye in bpo-22898 and bpo-30697.)


Notable changes in Python 3.6.5
===============================

The "locale.localeconv()" function now sets temporarily the "LC_CTYPE"
locale to the "LC_NUMERIC" locale in some cases. (Contributed by
Victor Stinner in bpo-31900.)


Notable changes in Python 3.6.7
===============================

In 3.6.7 the "tokenize" module now implicitly emits a "NEWLINE" token
when provided with input that does not have a trailing new line.  This
behavior now matches what the C tokenizer does internally.
(Contributed by Ammar Askar in bpo-33899.)


Notable changes in Python 3.6.10
================================

Due to significant security concerns, the *reuse_address* parameter of
"asyncio.loop.create_datagram_endpoint()" is no longer supported. This
is because of the behavior of the socket option "SO_REUSEADDR" in UDP.
For more details, see the documentation for
"loop.create_datagram_endpoint()". (Contributed by Kyle Stanley,
Antoine Pitrou, and Yury Selivanov in bpo-37228.)


Notable changes in Python 3.6.13
================================

Earlier Python versions allowed using both ";" and "&" as query
parameter separators in "urllib.parse.parse_qs()" and
"urllib.parse.parse_qsl()".  Due to security concerns, and to conform
with newer W3C recommendations, this has been changed to allow only a
single separator key, with "&" as the default.  This change also
affects "cgi.parse()" and "cgi.parse_multipart()" as they use the
affected functions internally. For more details, please see their
respective documentation. (Contributed by Adam Goldschmidt, Senthil
Kumaran and Ken Jin in bpo-42967.)
