# 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, bpo-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.)

Added `ssl.SSLContext.post_handshake_auth` to enable and `ssl.SSLSocket.verify_client_post_handshake()` to initiate TLS 1.3 post-handshake authentication. (Contributed by Christian Heimes in gh-78851.)

### 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

`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 `distutils.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.)

### xml

- As mitigation against DTD and external entity retrieval, the `xml.dom.minidom` and `xml.sax` modules no longer process external entities by default. (Contributed by Christian Heimes in gh-61441.)

### 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, implementers 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. Implementers 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.)

Added 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

`xml.dom.minidom` and `xml.sax` modules no longer process external entities by default. See also gh-61441.

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.)

## Notable changes in Python 3.6.14

A security fix alters the `ftplib.FTP` behavior to not trust the IPv4 address sent from the remote server when setting up a passive data channel. We reuse the ftp server IP address instead. For unusual code requiring the old behavior, set a `trust_server_pasv_ipv4_address` attribute on your FTP instance to `True`. (See gh-87451)

The presence of newline or tab characters in parts of a URL allows for some forms of attacks. Following the WHATWG specification that updates RFC 3986, ASCII newline `\n`, `\r` and tab `\t` characters are stripped from the URL by the parser `urllib.parse()` preventing such attacks. The removal characters are controlled by a new module level variable `urllib.parse._UNSAFE_URL_BYTES_TO_REMOVE`. (See gh-88048)