原文：What’s New In Python 3.6

What's New In Python 3.6¶

Release:	3.6.0
Date:	December 15, 2016
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.

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 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](https://docs.p ython.org/3.6/reference/lexical_analysis.html#f-strings).

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](https://docs.python.org/3.6/reference/lexical_analysis .html#f-strings).

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__()](https://docs.python.org/3 .6/reference/datamodel.html#object.init_subclass "object.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](https://docs.python.org/3.6/reference/datamodel.html#class-object- creation)).

See also

PEP 487 - Simpler customization of class creation

PEP written and implemented by Martin Teichmann.

[Feature documentation](https://docs.python.org/3.6/reference/datamodel.html #class-customization)

PEP 487: Descriptor Protocol Enhancements¶

PEP 487 extends the descriptor protocol has to include the new optional [__set_name__()](https://docs.pytho n.org/3.6/reference/datamodel.html#object.set_name "object.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](https://docs.python.org/3.6/reference/datamodel.html#d escriptors)

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__()](https:/ /docs.python.org/3.6/library/os.html#os.PathLike.fspath "os.PathLike.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](https://docs.python.org/3.6/li brary/datetime.html#datetime.datetime "datetime.datetime" ) and datetime.tim e 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](https://docs.python.org/3.6/library/datetime.html#d atetime.datetime.fold "datetime.datetime.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: 更改windows下文件系统编码格式为UTF-8¶

使用str (Unicode) 表示文件系统路径比bytes能获得更好的效果。尽管如此，在某些情况下bytes也足以胜任并且也是正确的。

在3.6之前,使用bytes路径可能导致数据丢失。改进后, windows下现在支持使用bytes表示路径,这些bytes将以sys.getfilesy stemencoding()的方式编码，默认编码格式为'utf-8'。

不使用str方式表示路径的应用程序应当使用os.fsencode()和os.fsdecode() 以确保他们的bytes被正确编码。要回复到之前的状态, 设置 PYTHONLEGACYWINDOWSFSENCODING 或者调用 sys._enablelegacywindowsfsencoding()。

查看 PEP 529以获取更多信息并讨论可能需要变更的代码。

PEP 528: 更改windows控制台编码为UTF-8¶

windows下的默认控制台现在支持所有的Unicode字符并可以正确读取Python代码中的str对象。 sys.stdin, sys.stdout 以及 sys.stderr 现在的默认使用utf-8编码。

这一变化仅适用于使用交互控制台之时，而非重定向文件或者管道。如果要使用之前的交互控制台, 需设置 PYTHONLEGACYWINDOWSIOENCODING。

另请参阅

PEP 528 - 修改windows控制台编码为UTF-8

PEP 由Steve Dower编写和实现。

PEP 520: 保存类属性定义顺序¶

类的定义体中的属性有一个自然顺序：即源码中属性名出现的顺序。这个顺序现在保存在新的类__dict__ 的属性中.

同样，有效的缺省类和执行空间 (从type.prepare()返回)是一个保存插入顺序的映射。

另请参阅

PEP 520 - 保存类属性定义顺序

该PEP由Eric Snow编写和实现。

PEP 468: 保存关键字参数顺序¶

函数声明中的**kwargs 的顺序现在被保证是插入顺序的映射。

另请参阅

PEP 468 - 保存关键字参数顺序

该PEP由Eric Snow编写和实现。

新的字典dict类型的实现¶

字典dict类型现在使用 PyPy首创的 "紧凑" 表达方式。新字典dict() 的内存占用比Python3.5中减少20%到25%。

新的实现中保存顺序的功能被认为是不可过于依赖的(未来也许会改变，不过在将所有当前和未来的Python实现的语言规范转换为保证顺序的语法之前的几个版本中，新的dict有望被实现的; 这也能帮助保证对那些仍旧是随机迭代顺序的旧版本的向后兼容，比如Python 3.5)。

(由INADA Naoki在issue 27350提供。想法最初由Raymond Hettinger提出.)

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](https://docs.python.org/3.6/using/cmdline.html #envvar-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()](https://docs.python.org/3.6/c-api/memory.h tml#c.PyMem_SetupDebugHooks "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](https://docs.python.org/3.6/library/tracemalloc.html#module- tracemalloc "tracemalloc: Trace memory allocations." ) 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](https://docs .python.org/3.6/howto/instrumentation.html#instrumentation).

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 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 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.)
TCP_NODELAY flag is now set for all TCP transports by default. (Contributed by Yury Selivanov in issue 27456.)
New loop.shutdown_asyncgens() to properly close pending asynchronous generators before closing the loop. (Contributed by Yury Selivanov in issue 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 issue 26081 and issue 28544.)

binascii¶

The [b2a_base64()](https://docs.python.org/3.6/library/binascii.html#binasci i.b2a_base64 "binascii.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](https://docs.python.org/3.6/library/collections.abc.htm l#collections.abc.Collection "collections.abc.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](https://docs.python.org/3.6/library/collections.abc.htm l#collections.abc.Reversible "collections.abc.Reversible" ) abstract base class represents iterable classes that also provide the [__reversed__()](htt ps://docs.python.org/3.6/reference/datamodel.html#object.reversed "object.reversed" ) method. (Contributed by Ivan Levkivskyi in issue 25987.)

The new [AsyncGenerator](https://docs.python.org/3.6/library/collections.abc .html#collections.abc.AsyncGenerator "collections.abc.AsyncGenerator" ) abstract base class represents asynchronous generators. (Contributed by Yury Selivanov in issue 28720.)

The [namedtuple()](https://docs.python.org/3.6/library/collections.html#coll ections.namedtuple "collections.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()](https://docs.python. org/3.6/library/collections.html#collections.namedtuple "collections.namedtuple" ) are now keyword-only. (Contributed by Raymond Hettinger in issue 25628.)

Recursive [collections.deque](https://docs.python.org/3.6/library/collection s.html#collections.deque "collections.deque" ) instances can now be pickled. (Contributed by Serhiy Storchaka in issue 26482.)

concurrent.futures¶

The [ThreadPoolExecutor](https://docs.python.org/3.6/library/concurrent.futu res.html#concurrent.futures.ThreadPoolExecutor "concurrent.futures.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](https://docs.python.org/3.6/library/ contextlib.html#contextlib.AbstractContextManager "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.Context Manager](https://docs.python.org/3.6/library/typing.html#typing.ContextManage r "typing.ContextManager" ). (Contributed by Brett Cannon in issue 25609.)

datetime¶

The [datetime](https://docs.python.org/3.6/library/datetime.html#datetime.da tetime "datetime.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()](https://docs.python.org/3.6/library/datetime.html# datetime.datetime.strftime "datetime.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()](https://docs.python.org/3.6/library/datetime.html #datetime.datetime.isoformat "datetime.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()](https://docs.python.org/3.6/library/datetime.html#d atetime.datetime.combine "datetime.datetime.combine" ) now accepts an optional tzinfo argument. (Contributed by Alexander Belopolsky in issue 27661.)

decimal¶

New [Decimal.as_integer_ratio()](https://docs.python.org/3.6/library/decimal .html#decimal.Decimal.as_integer_ratio "decimal.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](https://docs.python.org/3.6/library/email.mime.html#module- email.mime "email.mime: Build MIME messages." ) classes now all accept an optional policy keyword. (Contributed by Berker Peksag in issue 27331.)

The [DecodedGenerator](https://docs.python.org/3.6/library/email.generator.h tml#email.generator.DecodedGenerator "email.generator.DecodedGenerator" ) now supports the policy keyword.

There is a new [policy](https://docs.python.org/3.6/library/email.policy.html#module- email.policy "email.policy: Controlling the parsing and generating of messages" ) attribute, [message_factory](https://docs.python.org/3.6/library /email.policy.html#email.policy.Policy.message_factory "email.policy.Policy.message_factory" ), that controls what class is used by default when the parser creates new message objects. For the [email.policy.co mpat32](https://docs.python.org/3.6/library/email.policy.html#email.policy.co mpat32 "email.policy.compat32" ) policy this is [Message](https://docs.pytho n.org/3.6/library/email.compat32-message.html#email.message.Message "email.message.Message" ), for the new policies it is [EmailMessage](https:/ /docs.python.org/3.6/library/email.message.html#email.message.EmailMessage "email.message.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](https://docs.python.org/3.6/library/faulthandler.html#module- faulthandler "faulthandler: Dump the Python traceback." ) module now installs a handler for Windows exceptions: see [faulthandler.enable()](https://docs.p ython.org/3.6/library/faulthandler.html#faulthandler.enable "faulthandler.enable" ). (Contributed by Victor Stinner in issue 23848.)

fileinput¶

[hook_encoded()](https://docs.python.org/3.6/library/fileinput.html#fileinpu t.hook_encoded "fileinput.hook_encoded" ) now supports the errors argument. (Contributed by Joseph Hackman in issue 25788.)

hashlib¶

[hashlib](https://docs.python.org/3.6/library/hashlib-blake2.html#module- hashlib "hashlib: BLAKE2 hash function for Python" ) 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()](https://docs.python.org/3.6/library/hashlib- blake2.html#hashlib.blake2b "hashlib.blake2b" ) and [blake2s()](https://docs.python.org/3.6/library/hashlib- blake2.html#hashlib.blake2s "hashlib.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()](https://docs.python.org/3.6/library/http.client.h tml#http.client.HTTPConnection.request "http.client.HTTPConnection.request" ) and [endheaders()](https://docs.python.org/3.6/library/http.client.html#http .client.HTTPConnection.endheaders "http.client.HTTPConnection.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](https://docs.pytho n.org/3.6/library/exceptions.html#ModuleNotFoundError "ModuleNotFoundError" ) (subclass of [ImportError](https://docs.python.org/3.6/library/exceptions.ht ml#ImportError "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](https://docs.python.org/3.6/library/importlib.ht ml#importlib.util.LazyLoader "importlib.util.LazyLoader" ) now calls [create_ module()](https://docs.python.org/3.6/library/importlib.html#importlib.abc.Lo ader.create_module "importlib.abc.Loader.create_module" ) on the wrapped loader, removing the restriction that [importlib.machinery.BuiltinImporter]( https://docs.python.org/3.6/library/importlib.html#importlib.machinery.Builtin Importer "importlib.machinery.BuiltinImporter" ) and [importlib.machinery.Ext ensionFileLoader](https://docs.python.org/3.6/library/importlib.html#importli b.machinery.ExtensionFileLoader "importlib.machinery.ExtensionFileLoader" ) couldn't be used with [importlib.util.LazyLoader](https://docs.python.org/3. 6/library/importlib.html#importlib.util.LazyLoader "importlib.util.LazyLoader" ).

[importlib.util.cache_from_source()](https://docs.python.org/3.6/library/imp ortlib.html#importlib.util.cache_from_source "importlib.util.cache_from_source" ), [importlib.util.source_from_cache()](h ttps://docs.python.org/3.6/library/importlib.html#importlib.util.source_from_c ache "importlib.util.source_from_cache" ), and [importlib.util.spec_from_file _location()](https://docs.python.org/3.6/library/importlib.html#importlib.uti l.spec_from_file_location "importlib.util.spec_from_file_location" ) now accept a [path-like object](https://docs.python.org/3.6/glossary.html#term- path-like-object).

inspect¶

The [inspect.signature()](https://docs.python.org/3.6/library/inspect.html#i nspect.signature "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.g etargspec()](https://docs.python.org/3.6/library/inspect.html#inspect.getargs pec "inspect.getargspec" ) API, the previously documented deprecation of [ins pect.getfullargspec()](https://docs.python.org/3.6/library/inspect.html#inspe ct.getfullargspec "inspect.getfullargspec" ) has been reversed. While this function is convenient for single/source Python 2/3 code bases, the richer [i nspect.signature()](https://docs.python.org/3.6/library/inspect.html#inspect. signature "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()](https://docs.python.org/3.6/li brary/logging.handlers.html#logging.handlers.WatchedFileHandler.reopenIfNeeded "logging.handlers.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](https://docs.python.org/3.6/library/multiprocessing.html #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](https://docs.python.org/3.6/library/exceptions.html#Reso urceWarning "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](https://docs.python.org/3.6/library/pickle.html #pickle-protocols) older than protocol version 4. Protocol version 4 already supports this case. (Contributed by Serhiy Storchaka in issue 24164.)

pickletools¶

[pickletools.dis()](https://docs.python.org/3.6/library/pickletools.html#pic kletools.dis "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](https://docs.python.org/3.6 /reference/datamodel.html#object.index "object.index" ) as group indices. (Contributed by Jeroen Demeyer and Xiang Zhang in issue 27177.)

readline¶

Added [set_auto_history()](https://docs.python.org/3.6/library/readline.html #readline.set_auto_history "readline.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](https://docs.python.org/3.6/library/shlex.html#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](https://docs.python.org/3.6/library/sqlite3.html# sqlite3.Cursor.lastrowid "sqlite3.Cursor.lastrowid" ) now supports the REPLACE statement. (Contributed by Alex LordThorsen in issue 16864.)

socket¶

The [ioctl()](https://docs.python.org/3.6/library/socket.html#socket.socket. ioctl "socket.socket.ioctl" ) function now supports the [SIO_LOOPBACK_FAST_PA TH](https://docs.python.org/3.6/library/socket.html#socket.SIO_LOOPBACK_FAST_ PATH "socket.SIO_LOOPBACK_FAST_PATH" ) control code. (Contributed by Daniel Stokes in issue 26536.)

The [getsockopt()](https://docs.python.org/3.6/library/socket.html#socket.so cket.getsockopt "socket.socket.getsockopt" ) constants SO_DOMAIN, SO_PROTOCOL, SO_PEERSEC, and SO_PASSSEC are now supported. (Contributed by Christian Heimes in issue 26907.)

The [setsockopt()](https://docs.python.org/3.6/library/socket.html#socket.so cket.setsockopt "socket.socket.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()](https://docs.python.org/3.6/library/socket.h tml#socket.socket.sendmsg_afalg "socket.socket.sendmsg_afalg" ) were added. (Contributed by Christian Heimes in issue 27744 with support from Victor Stinner.)

socketserver¶

Servers based on the [socketserver](https://docs.python.org/3.6/library/socketserver.html#module- socketserver "socketserver: A framework for network servers." ) module, including those defined in [http.server](https://docs.python.org/3.6/library/http.server.html#module- http.server "http.server: HTTP server and request handlers." ), [xmlrpc.server](https://docs.python.org/3.6/library/xmlrpc.server.html #module-xmlrpc.server "xmlrpc.server: Basic XML-RPC server implementations." ) and [wsgiref.simple_server](https://docs.python.org/3.6/library/wsgiref.html #module-wsgiref.simple_server "wsgiref.simple_server: A simple WSGI HTTP server." ), now support the context manager protocol. (Contributed by Aviv Palivoda in issue 26404.)

The wfile attribute of [StreamRequestHandler](https://docs.python.org/3.6/ library/socketserver.html#socketserver.StreamRequestHandler "socketserver.StreamRequestHandler" ) classes now implements the io.Buffered IOBase writable interface. In particular, calling [write()](h ttps://docs.python.org/3.6/library/io.html#io.BufferedIOBase.write "io.BufferedIOBase.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()](https://docs.python.org/3.6/library/ssl.html#ssl.SSL Context.get_ciphers "ssl.SSLContext.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()](https://docs.python.org/3.6/library/statistics.html# statistics.harmonic_mean "statistics.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](https://docs.python.org/3.6/library/subprocess.html#subpr ocess.Popen "subprocess.Popen" ) destructor now emits a [ResourceWarning](ht tps://docs.python.org/3.6/library/exceptions.html#ResourceWarning "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](https://docs.python.org/3.6/library/subprocess.html#s ubprocess.Popen "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()](https://docs.python.org/3.6/library/sy s.html#sys.getfilesystemencodeerrors "sys.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()](https://docs.python .org/3.6/library/sys.html#sys.getwindowsversion "sys.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()](https://docs.python.org/3.6/library/timeit.html# timeit.Timer.autorange "timeit.Timer.autorange" ) convenience method has been added to call [Timer.timeit()](https://docs.python.org/3.6/library/timeit.ht ml#timeit.Timer.timeit "timeit.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](https://docs.python.org/3.6/library/tracemalloc.html #module-tracemalloc "tracemalloc: Trace memory allocations." ) module now supports tracing memory allocations in multiple different address spaces.

The new [DomainFilter](https://docs.python.org/3.6/library/tracemalloc.html# tracemalloc.DomainFilter "tracemalloc.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](https://docs.python.org/3.6/library/typing.html#module- typing "typing: Support for type hints (see PEP 484)." ) 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](https://docs.python.org/3.6/library/typing.html# typing.ContextManager "typing.ContextManager" ) class has been added for representing [contextlib.AbstractContextManager](https://docs.python.org/3.6 /library/contextlib.html#contextlib.AbstractContextManager "contextlib.AbstractContextManager" ). (Contributed by Brett Cannon in issue 25609.)

The [typing.Collection](https://docs.python.org/3.6/library/typing.html#typi ng.Collection "typing.Collection" ) class has been added for representing [co llections.abc.Collection](https://docs.python.org/3.6/library/collections.abc .html#collections.abc.Collection "collections.abc.Collection" ). (Contributed by Ivan Levkivskyi in issue 27598.)

The [typing.ClassVar](https://docs.python.org/3.6/library/typing.html#typing .ClassVar "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](https://docs.python.org/3.6/library/typing.html#typing .TYPE_CHECKING "typing.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](https://docs.python.org/3.6/library/unicodedata.html #module-unicodedata "unicodedata: Access the Unicode Database." ) module now uses data from Unicode 9.0.0. (Contributed by Benjamin Peterson.)

unittest.mock¶

The [Mock](https://docs.python.org/3.6/library/unittest.mock.html#unittest.m ock.Mock "unittest.mock.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](https://docs.python.org/3.6/library/urllib.robotparser.htm l#urllib.robotparser.RobotFileParser "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_explic it()](https://docs.python.org/3.6/library/warnings.html#warnings.warn_explici t "warnings.warn_explicit" ) function: the destroyed object which emitted a [ ResourceWarning](https://docs.python.org/3.6/library/exceptions.html#Resource Warning "ResourceWarning" ). A source attribute has also been added to warnings.WarningMessage (contributed by Victor Stinner in issue 26568 and issue 26567).

When a [ResourceWarning](https://docs.python.org/3.6/library/exceptions.html #ResourceWarning "ResourceWarning" ) warning is logged, the [tracemalloc](https://docs.python.org/3.6/library/tracemalloc.html#module- tracemalloc "tracemalloc: Trace memory allocations." ) 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](https://docs.python.org/3.6/library/tracemalloc.html#module- tracemalloc "tracemalloc: Trace memory allocations." ) 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](https://docs.python.org/3.6/library/winsoun d.html#winsound.MessageBeep "winsound.MessageBeep" ), and [PlaySound](https: //docs.python.org/3.6/library/winsound.html#winsound.PlaySound "winsound.PlaySound" ) (issue 27982).

xmlrpc.client¶

The [xmlrpc.client](https://docs.python.org/3.6/library/xmlrpc.client.html #module-xmlrpc.client "xmlrpc.client: XML-RPC client access." ) 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()](https://docs.python.org/3.6/library/zipfile.html #zipfile.ZipInfo.from_file "zipfile.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()](https://docs.python.org/3.6/library/zipfile.html#zipfil e.ZipFile.open "zipfile.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 asyncio.Future class now has an optimized C implementation. (Contributed by Yury Selivanov and INADA Naoki in issue 26081.)
The asyncio.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](https://doc s.python.org/3.6/library/exceptions.html#DeprecationWarning "DeprecationWarning" ).

Deprecated Python behavior¶

Raising the [StopIteration](https://docs.python.org/3.6/library/exceptions.h tml#StopIteration "StopIteration" ) exception inside a generator will now generate a [DeprecationWarning](https://docs.python.org/3.6/library/exceptio ns.html#DeprecationWarning "DeprecationWarning" ), and will trigger a Runtim eError in Python 3.7. See [PEP 479: Change StopIteration handling inside generators](https://docs.python.org/3.6/whatsnew/3.5.html#whatsnew- pep-479) for details.

The [__aiter__()](https://docs.python.org/3.6/reference/datamodel.html#objec t.aiter "object.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](https://doc s.python.org/3.6/library/exceptions.html#DeprecationWarning "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](https://docs.python.org/3.6/library/exceptions.html#Dep recationWarning "DeprecationWarning" ). Although this will eventually become a [SyntaxError](https://docs.python.org/3.6/library/exceptions.html#SyntaxErro r "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](https://docs.python.org/3.6/library/exceptions.htm l#ImportWarning "ImportWarning" ). (Contributed by Rose Ames in issue 25791.)

Deprecated Python modules, functions and methods¶

asynchat¶

The [asynchat](https://docs.python.org/3.6/library/asynchat.html#module- asynchat "asynchat: Support for asynchronous command/response protocols." ) has been deprecated in favor of asyncio. (Contributed by Mariatta in issue 25002.)

asyncore¶

The [asyncore](https://docs.python.org/3.6/library/asyncore.html#module- asyncore "asyncore: A base class for developing asynchronous socket handling services." ) has been deprecated in favor of asyncio. (Contributed by Mariatta in issue 25002.)

dbm¶

Unlike other [dbm](https://docs.python.org/3.6/library/dbm.html#module-dbm "dbm: Interfaces to various Unix "database" formats." ) 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()](https://docs.python .org/3.6/library/importlib.html#importlib.machinery.SourceFileLoader.load_modu le "importlib.machinery.SourceFileLoader.load_module" ) and [importlib.machin ery.SourcelessFileLoader.load_module()](https://docs.python.org/3.6/library/i mportlib.html#importlib.machinery.SourcelessFileLoader.load_module "importlib.machinery.SourcelessFileLoader.load_module" ) methods are now deprecated. They were the only remaining implementations of [importlib.abc.Lo ader.load_module()](https://docs.python.org/3.6/library/importlib.html#import lib.abc.Loader.load_module "importlib.abc.Loader.load_module" ) in [importlib](https://docs.python.org/3.6/library/importlib.html#module- importlib "importlib: The implementation of the import machinery." ) that had not been deprecated in previous versions of Python in favour of [importlib.ab c.Loader.exec_module()](https://docs.python.org/3.6/library/importlib.html#im portlib.abc.Loader.exec_module "importlib.abc.Loader.exec_module" ).

The [importlib.machinery.WindowsRegistryFinder](https://docs.python.org/3.6/ library/importlib.html#importlib.machinery.WindowsRegistryFinder "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 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](https://docs.python.org/3.6/library/http.client.html#module- http.client "http.client: HTTP and HTTPS protocol client (requires sockets)." ), [imaplib](https://docs.python.org/3.6/library/imaplib.html #module-imaplib "imaplib: IMAP4 protocol client (requires sockets)." ), 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](https://docs.python.org/3.6/library/tkinter.tix.html #module-tkinter.tix "tkinter.tix: Tk Extension Widgets for Tkinter" ) module is now deprecated. tkinter users should use [tkinter.ttk](https://docs.python.org/3.6/library/tkinter.ttk.html #module-tkinter.ttk "tkinter.ttk: Tk themed widget set" ) 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:


    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.)

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What's New In Python 3.6¶

Summary - Release highlights¶

New Features¶

PEP 498: Formatted string literals¶

PEP 526: Syntax for variable annotations¶

PEP 515: Underscores in Numeric Literals¶

PEP 525: Asynchronous Generators¶

PEP 530: Asynchronous Comprehensions¶

PEP 487: Simpler customization of class creation¶

PEP 487: Descriptor Protocol Enhancements¶

PEP 519: Adding a file system path protocol¶

PEP 495: Local Time Disambiguation¶

PEP 529: 更改windows下文件系统编码格式为UTF-8¶

PEP 528: 更改windows控制台编码为UTF-8¶

PEP 520: 保存类属性定义顺序¶

PEP 468: 保存关键字参数顺序¶

新的 字典dict类型的实现¶

PEP 523: Adding a frame evaluation API to CPython¶

PYTHONMALLOC environment variable¶

DTrace and SystemTap probing support¶

Other Language Changes¶

New Modules¶

secrets¶

Improved Modules¶

array¶

ast¶

asyncio¶

binascii¶

cmath¶

collections¶

concurrent.futures¶

contextlib¶

datetime¶

decimal¶

distutils¶

email¶

encodings¶

enum¶

faulthandler¶

fileinput¶

hashlib¶

http.client¶

idlelib and IDLE¶

importlib¶

inspect¶

json¶

logging¶

math¶

multiprocessing¶

os¶

pathlib¶

pdb¶

pickle¶

pickletools¶

pydoc¶

random¶

re¶

readline¶

rlcompleter¶

shlex¶

site¶

sqlite3¶

socket¶

socketserver¶

ssl¶

statistics¶

struct¶

subprocess¶

sys¶

telnetlib¶

time¶

timeit¶

tkinter¶

traceback¶

新的字典dict类型的实现¶