What’s New In Python 3.5 — Python 3.8.1 documentation

# What’s New In Python 3.5

Editors

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

This article explains the new features in Python 3.5, compared to 3.4. Python 3.5 was released on September 13, 2015. See the changelog for a full list of changes.

See also

PEP 478 - Python 3.5 Release Schedule

## Summary – Release highlights

New syntax features:

- PEP 492, coroutines with async and await syntax.

- PEP 465, a new matrix multiplication operator: `a @ b`.

- PEP 448, additional unpacking generalizations.

New library modules:

- `typing`: PEP 484 – Type Hints.

- `zipapp`: PEP 441 Improving Python ZIP Application Support.

New built-in features:

- `bytes % args`, `bytearray % args`: PEP 461 – Adding `%` formatting to bytes and bytearray.

- New `bytes.hex()`, `bytearray.hex()` and `memoryview.hex()` methods. (Contributed by Arnon Yaari in bpo-9951.)

- `memoryview` now supports tuple indexing (including multi-dimensional). (Contributed by Antoine Pitrou in bpo-23632.)

- Generators have a new `gi_yieldfrom` attribute, which returns the object being iterated by `yield from` expressions. (Contributed by Benno Leslie and Yury Selivanov in bpo-24450.)

- A new `RecursionError` exception is now raised when maximum recursion depth is reached. (Contributed by Georg Brandl in bpo-19235.)

CPython implementation improvements:

- When the `LC_TYPE` locale is the POSIX locale (`C` locale), `sys.stdin` and `sys.stdout` now use the `surrogateescape` error handler, instead of the `strict` error handler. (Contributed by Victor Stinner in bpo-19977.)

- `.pyo` files are no longer used and have been replaced by a more flexible scheme that includes the optimization level explicitly in `.pyc` name. (See PEP 488 overview.)

- Builtin and extension modules are now initialized in a multi-phase process, which is similar to how Python modules are loaded. (See PEP 489 overview.)

Significant improvements in the standard library:

- `collections.OrderedDict` is now implemented in C, which makes it 4 to 100 times faster.

- The `ssl` module gained support for Memory BIO, which decouples SSL protocol handling from network IO.

- The new `os.scandir()` function provides a better and significantly faster way of directory traversal.

- `functools.lru_cache()` has been mostly reimplemented in C, yielding much better performance.

- The new `subprocess.run()` function provides a streamlined way to run subprocesses.

- The `traceback` module has been significantly enhanced for improved performance and developer convenience.

Security improvements:

- SSLv3 is now disabled throughout the standard library. It can still be enabled by instantiating a `ssl.SSLContext` manually. (See bpo-22638 for more details; this change was backported to CPython 3.4 and 2.7.)

- HTTP cookie parsing is now stricter, in order to protect against potential injection attacks. (Contributed by Antoine Pitrou in bpo-22796.)

Windows improvements:

- A new installer for Windows has replaced the old MSI. See Using Python on Windows for more information.

- Windows builds now use Microsoft Visual C++ 14.0, and extension modules should use the same.

Please read on for a comprehensive list of user-facing changes, including many other smaller improvements, CPython optimizations, deprecations, and potential porting issues.

## New Features

### PEP 492 - Coroutines with async and await syntax

PEP 492 greatly improves support for asynchronous programming in Python by adding awaitable objects, coroutine functions, asynchronous iteration, and asynchronous context managers.

Coroutine functions are declared using the new `async def` syntax:

    >>> async def coro():
    ...     return 'spam'

Inside a coroutine function, the new `await` expression can be used to suspend coroutine execution until the result is available. Any object can be awaited, as long as it implements the awaitable protocol by defining the `__await__()` method.

PEP 492 also adds `async for` statement for convenient iteration over asynchronous iterables.

An example of a rudimentary HTTP client written using the new syntax:

    import asyncio

    async def http_get(domain):
        reader, writer = await asyncio.open_connection(domain, 80)

        writer.write(b'\r\n'.join([
            b'GET / HTTP/1.1',
            b'Host: %b' % domain.encode('latin-1'),
            b'Connection: close',
            b'', b''
        ]))

        async for line in reader:
            print('>>>', line)

        writer.close()

    loop = asyncio.get_event_loop()
    try:
        loop.run_until_complete(http_get('example.com'))
    finally:
        loop.close()

Similarly to asynchronous iteration, there is a new syntax for asynchronous context managers. The following script:

    import asyncio

    async def coro(name, lock):
        print('coro {}: waiting for lock'.format(name))
        async with lock:
            print('coro {}: holding the lock'.format(name))
            await asyncio.sleep(1)
            print('coro {}: releasing the lock'.format(name))

    loop = asyncio.get_event_loop()
    lock = asyncio.Lock()
    coros = asyncio.gather(coro(1, lock), coro(2, lock))
    try:
        loop.run_until_complete(coros)
    finally:
        loop.close()

will output:

    coro 2: waiting for lock
    coro 2: holding the lock
    coro 1: waiting for lock
    coro 2: releasing the lock
    coro 1: holding the lock
    coro 1: releasing the lock

Note that both `async for` and `async with` can only be used inside a coroutine function declared with `async def`.

Coroutine functions are intended to be run inside a compatible event loop, such as the asyncio loop.

Note

Changed in version 3.5.2: Starting with CPython 3.5.2, `__aiter__` can directly return asynchronous iterators. Returning an awaitable object will result in a `PendingDeprecationWarning`.

See more details in the Asynchronous Iterators documentation section.

See also

PEP 492 – Coroutines with async and await syntax

  PEP written and implemented by Yury Selivanov.

### PEP 465 - A dedicated infix operator for matrix multiplication

PEP 465 adds the `@` infix operator for matrix multiplication. Currently, no builtin Python types implement the new operator, however, it can be implemented by defining `__matmul__()`, `__rmatmul__()`, and `__imatmul__()` for regular, reflected, and in-place matrix multiplication. The semantics of these methods is similar to that of methods defining other infix arithmetic operators.

Matrix multiplication is a notably common operation in many fields of mathematics, science, engineering, and the addition of `@` allows writing cleaner code:

    S = (H @ beta - r).T @ inv(H @ V @ H.T) @ (H @ beta - r)

instead of:

    S = dot((dot(H, beta) - r).T,
            dot(inv(dot(dot(H, V), H.T)), dot(H, beta) - r))

NumPy 1.10 has support for the new operator:

    >>> import numpy

    >>> x = numpy.ones(3)
    >>> x
    array([ 1., 1., 1.])

    >>> m = numpy.eye(3)
    >>> m
    array([[ 1., 0., 0.],
           [ 0., 1., 0.],
           [ 0., 0., 1.]])

    >>> x @ m
    array([ 1., 1., 1.])

See also

PEP 465 – A dedicated infix operator for matrix multiplication

  PEP written by Nathaniel J. Smith; implemented by Benjamin Peterson.

### PEP 448 - Additional Unpacking Generalizations

PEP 448 extends the allowed uses of the `*` iterable unpacking operator and `**` dictionary unpacking operator. It is now possible to use an arbitrary number of unpackings in function calls:

    >>> print(*[1], *[2], 3, *[4, 5])
    1 2 3 4 5

    >>> def fn(a, b, c, d):
    ...     print(a, b, c, d)
    ...

    >>> fn(**{'a': 1, 'c': 3}, **{'b': 2, 'd': 4})
    1 2 3 4

Similarly, tuple, list, set, and dictionary displays allow multiple unpackings (see Expression lists and Dictionary displays):

    >>> *range(4), 4
    (0, 1, 2, 3, 4)

    >>> [*range(4), 4]
    [0, 1, 2, 3, 4]

    >>> {*range(4), 4, *(5, 6, 7)}
    {0, 1, 2, 3, 4, 5, 6, 7}

    >>> {'x': 1, **{'y': 2}}
    {'x': 1, 'y': 2}

See also

PEP 448 – Additional Unpacking Generalizations

  PEP written by Joshua Landau; implemented by Neil Girdhar, Thomas Wouters, and Joshua Landau.

### PEP 461 - percent formatting support for bytes and bytearray

PEP 461 adds support for the `%` interpolation operator to `bytes` and `bytearray`.

While interpolation is usually thought of as a string operation, there are cases where interpolation on `bytes` or `bytearrays` makes sense, and the work needed to make up for this missing functionality detracts from the overall readability of the code. This issue is particularly important when dealing with wire format protocols, which are often a mixture of binary and ASCII compatible text.

Examples:

    >>> b'Hello %b!' % b'World'
    b'Hello World!'

    >>> b'x=%i y=%f' % (1, 2.5)
    b'x=1 y=2.500000'

Unicode is not allowed for `%b`, but it is accepted by `%a` (equivalent of `repr(obj).encode('ascii', 'backslashreplace')`):

    >>> b'Hello %b!' % 'World'
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    TypeError: %b requires bytes, or an object that implements __bytes__, not 'str'

    >>> b'price: %a' % '10€'
    b"price: '10\\u20ac'"

Note that `%s` and `%r` conversion types, although supported, should only be used in codebases that need compatibility with Python 2.

See also

PEP 461 – Adding % formatting to bytes and bytearray

  PEP written by Ethan Furman; implemented by Neil Schemenauer and Ethan Furman.

### PEP 484 - Type Hints

Function annotation syntax has been a Python feature since version 3.0 (PEP 3107), however the semantics of annotations has been left undefined.

Experience has shown that the majority of function annotation uses were to provide type hints to function parameters and return values. It became evident that it would be beneficial for Python users, if the standard library included the base definitions and tools for type annotations.

PEP 484 introduces a provisional module to provide these standard definitions and tools, along with some conventions for situations where annotations are not available.

For example, here is a simple function whose argument and return type are declared in the annotations:

    def greeting(name: str) -> str:
        return 'Hello ' + name

While these annotations are available at runtime through the usual `__annotations__` attribute, no automatic type checking happens at runtime. Instead, it is assumed that a separate off-line type checker (e.g. mypy) will be used for on-demand source code analysis.

The type system supports unions, generic types, and a special type named `Any` which is consistent with (i.e. assignable to and from) all types.

See also

- `typing` module documentation

- PEP 484 – Type Hints

    PEP written by Guido van Rossum, Jukka Lehtosalo, and Łukasz Langa; implemented by Guido van Rossum.

- PEP 483 – The Theory of Type Hints

    PEP written by Guido van Rossum

### PEP 471 - os.scandir() function – a better and faster directory iterator

PEP 471 adds a new directory iteration function, `os.scandir()`, to the standard library. Additionally, `os.walk()` is now implemented using `scandir`, which makes it 3 to 5 times faster on POSIX systems and 7 to 20 times faster on Windows systems. This is largely achieved by greatly reducing the number of calls to `os.stat()` required to walk a directory tree.

Additionally, `scandir` returns an iterator, as opposed to returning a list of file names, which improves memory efficiency when iterating over very large directories.

The following example shows a simple use of `os.scandir()` to display all the files (excluding directories) in the given path that don’t start with `'.'`. The `entry.is_file()` call will generally not make an additional system call:

    for entry in os.scandir(path):
        if not entry.name.startswith('.') and entry.is_file():
            print(entry.name)

See also

PEP 471 – os.scandir() function – a better and faster directory iterator

  PEP written and implemented by Ben Hoyt with the help of Victor Stinner.

### PEP 475: Retry system calls failing with EINTR

An `errno.EINTR` error code is returned whenever a system call, that is waiting for I/O, is interrupted by a signal. Previously, Python would raise `InterruptedError` in such cases. This meant that, when writing a Python application, the developer had two choices:

1. Ignore the `InterruptedError`.

2. Handle the `InterruptedError` and attempt to restart the interrupted system call at every call site.

The first option makes an application fail intermittently. The second option adds a large amount of boilerplate that makes the code nearly unreadable. Compare:

    print("Hello World")

and:

    while True:
        try:
            print("Hello World")
            break
        except InterruptedError:
            continue

PEP 475 implements automatic retry of system calls on `EINTR`. This removes the burden of dealing with `EINTR` or `InterruptedError` in user code in most situations and makes Python programs, including the standard library, more robust. Note that the system call is only retried if the signal handler does not raise an exception.

Below is a list of functions which are now retried when interrupted by a signal:

- `open()` and `io.open()`;

- functions of the `faulthandler` module;

- `os` functions: `fchdir()`, `fchmod()`, `fchown()`, `fdatasync()`, `fstat()`, `fstatvfs()`, `fsync()`, `ftruncate()`, `mkfifo()`, `mknod()`, `open()`, `posix_fadvise()`, `posix_fallocate()`, `pread()`, `pwrite()`, `read()`, `readv()`, `sendfile()`, `wait3()`, `wait4()`, `wait()`, `waitid()`, `waitpid()`, `write()`, `writev()`;

- special cases: `os.close()` and `os.dup2()` now ignore `EINTR` errors; the syscall is not retried (see the PEP for the rationale);

- `select` functions: `devpoll.poll()`, `epoll.poll()`, `kqueue.control()`, `poll.poll()`, `select()`;

- methods of the `socket` class: `accept()`, `connect()` (except for non-blocking sockets), `recv()`, `recvfrom()`, `recvmsg()`, `send()`, `sendall()`, `sendmsg()`, `sendto()`;

- `signal.sigtimedwait()` and `signal.sigwaitinfo()`;

- `time.sleep()`.

See also

PEP 475 – Retry system calls failing with EINTR

  PEP and implementation written by Charles-François Natali and Victor Stinner, with the help of Antoine Pitrou (the French connection).

### PEP 479: Change StopIteration handling inside generators

The interaction of generators and `StopIteration` in Python 3.4 and earlier was sometimes surprising, and could conceal obscure bugs. Previously, `StopIteration` raised accidentally inside a generator function was interpreted as the end of the iteration by the loop construct driving the generator.

PEP 479 changes the behavior of generators: when a `StopIteration` exception is raised inside a generator, it is replaced with a `RuntimeError` before it exits the generator frame. The main goal of this change is to ease debugging in the situation where an unguarded `next()` call raises `StopIteration` and causes the iteration controlled by the generator to terminate silently. This is particularly pernicious in combination with the `yield from` construct.

This is a backwards incompatible change, so to enable the new behavior, a __future__ import is necessary:

    >>> from __future__ import generator_stop

    >>> def gen():
    ...     next(iter([]))
    ...     yield
    ...
    >>> next(gen())
    Traceback (most recent call last):
      File "<stdin>", line 2, in gen
    StopIteration

    The above exception was the direct cause of the following exception:

    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    RuntimeError: generator raised StopIteration

Without a `__future__` import, a `PendingDeprecationWarning` will be raised whenever a `StopIteration` exception is raised inside a generator.

See also

PEP 479 – Change StopIteration handling inside generators

  PEP written by Chris Angelico and Guido van Rossum. Implemented by Chris Angelico, Yury Selivanov and Nick Coghlan.

### PEP 485: A function for testing approximate equality

PEP 485 adds the `math.isclose()` and `cmath.isclose()` functions which tell whether two values are approximately equal or “close” to each other. Whether or not two values are considered close is determined according to given absolute and relative tolerances. Relative tolerance is the maximum allowed difference between `isclose` arguments, relative to the larger absolute value:

    >>> import math
    >>> a = 5.0
    >>> b = 4.99998
    >>> math.isclose(a, b, rel_tol=1e-5)
    True
    >>> math.isclose(a, b, rel_tol=1e-6)
    False

It is also possible to compare two values using absolute tolerance, which must be a non-negative value:

    >>> import math
    >>> a = 5.0
    >>> b = 4.99998
    >>> math.isclose(a, b, abs_tol=0.00003)
    True
    >>> math.isclose(a, b, abs_tol=0.00001)
    False

See also

PEP 485 – A function for testing approximate equality

  PEP written by Christopher Barker; implemented by Chris Barker and Tal Einat.

### PEP 486: Make the Python Launcher aware of virtual environments

PEP 486 makes the Windows launcher (see PEP 397) aware of an active virtual environment. When the default interpreter would be used and the `VIRTUAL_ENV` environment variable is set, the interpreter in the virtual environment will be used.

See also

PEP 486 – Make the Python Launcher aware of virtual environments

  PEP written and implemented by Paul Moore.

### PEP 488: Elimination of PYO files

PEP 488 does away with the concept of `.pyo` files. This means that `.pyc` files represent both unoptimized and optimized bytecode. To prevent the need to constantly regenerate bytecode files, `.pyc` files now have an optional `opt-` tag in their name when the bytecode is optimized. This has the side-effect of no more bytecode file name clashes when running under either `-O` or `-OO`. Consequently, bytecode files generated from `-O`, and `-OO` may now exist simultaneously. `importlib.util.cache_from_source()` has an updated API to help with this change.

See also

PEP 488 – Elimination of PYO files

  PEP written and implemented by Brett Cannon.

### PEP 489: Multi-phase extension module initialization

PEP 489 updates extension module initialization to take advantage of the two step module loading mechanism introduced by PEP 451 in Python 3.4.

This change brings the import semantics of extension modules that opt-in to using the new mechanism much closer to those of Python source and bytecode modules, including the ability to use any valid identifier as a module name, rather than being restricted to ASCII.

See also

PEP 489 – Multi-phase extension module initialization

  PEP written by Petr Viktorin, Stefan Behnel, and Nick Coghlan; implemented by Petr Viktorin.

## Other Language Changes

Some smaller changes made to the core Python language are:

- Added the `"namereplace"` error handlers. The `"backslashreplace"` error handlers now work with decoding and translating. (Contributed by Serhiy Storchaka in bpo-19676 and bpo-22286.)

- The `-b` option now affects comparisons of `bytes` with `int`. (Contributed by Serhiy Storchaka in bpo-23681.)

- New Kazakh `kz1048` and Tajik `koi8_t` codecs. (Contributed by Serhiy Storchaka in bpo-22682 and bpo-22681.)

- Property docstrings are now writable. This is especially useful for `collections.namedtuple()` docstrings. (Contributed by Berker Peksag in bpo-24064.)

- Circular imports involving relative imports are now supported. (Contributed by Brett Cannon and Antoine Pitrou in bpo-17636.)

## New Modules

### typing

The new `typing` provisional module provides standard definitions and tools for function type annotations. See Type Hints for more information.

### zipapp

The new `zipapp` module (specified in PEP 441) provides an API and command line tool for creating executable Python Zip Applications, which were introduced in Python 2.6 in bpo-1739468, but which were not well publicized, either at the time or since.

With the new module, bundling your application is as simple as putting all the files, including a `__main__.py` file, into a directory `myapp` and running:

    $ python -m zipapp myapp
    $ python myapp.pyz

The module implementation has been contributed by Paul Moore in bpo-23491.

See also

PEP 441 – Improving Python ZIP Application Support

## Improved Modules

### argparse

The `ArgumentParser` class now allows disabling abbreviated usage of long options by setting allow_abbrev to `False`. (Contributed by Jonathan Paugh, Steven Bethard, paul j3 and Daniel Eriksson in bpo-14910.)

### asyncio

Since the `asyncio` module is provisional, all changes introduced in Python 3.5 have also been backported to Python 3.4.x.

Notable changes in the `asyncio` module since Python 3.4.0:

- New debugging APIs: `loop.set_debug()` and `loop.get_debug()` methods. (Contributed by Victor Stinner.)

- The proactor event loop now supports SSL. (Contributed by Antoine Pitrou and Victor Stinner in bpo-22560.)

- A new `loop.is_closed()` method to check if the event loop is closed. (Contributed by Victor Stinner in bpo-21326.)

- A new `loop.create_task()` to conveniently create and schedule a new `Task` for a coroutine. The `create_task` method is also used by all asyncio functions that wrap coroutines into tasks, such as `asyncio.wait()`, `asyncio.gather()`, etc. (Contributed by Victor Stinner.)

- A new `transport.get_write_buffer_limits()` method to inquire for high- and low- water limits of the flow control. (Contributed by Victor Stinner.)

- The `async()` function is deprecated in favor of `ensure_future()`. (Contributed by Yury Selivanov.)

- New `loop.set_task_factory()` and `loop.get_task_factory()` methods to customize the task factory that `loop.create_task()` method uses. (Contributed by Yury Selivanov.)

- New `Queue.join()` and `Queue.task_done()` queue methods. (Contributed by Victor Stinner.)

- The `JoinableQueue` class was removed, in favor of the `asyncio.Queue` class. (Contributed by Victor Stinner.)

Updates in 3.5.1:

- 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.)

Updates in 3.5.2:

- 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.)

- New `loop.get_exception_handler()` method to get the current exception handler. (Contributed by Yury Selivanov.)

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

- The `loop.create_connection()` and `loop.create_server()` methods are optimized to avoid calling the system `getaddrinfo` function if the address is already resolved. (Contributed by A. Jesse Jiryu Davis.)

- The `loop.sock_connect(sock, address)` no longer requires the address to be resolved prior to the call. (Contributed by A. Jesse Jiryu Davis.)

### bz2

The `BZ2Decompressor.decompress` method now accepts an optional max_length argument to limit the maximum size of decompressed data. (Contributed by Nikolaus Rath in bpo-15955.)

### cgi

The `FieldStorage` class now supports the context manager protocol. (Contributed by Berker Peksag in bpo-20289.)

### cmath

A new function `isclose()` provides a way to test for approximate equality. (Contributed by Chris Barker and Tal Einat in bpo-24270.)

### code

The `InteractiveInterpreter.showtraceback()` method now prints the full chained traceback, just like the interactive interpreter. (Contributed by Claudiu Popa in bpo-17442.)

### collections

The `OrderedDict` class is now implemented in C, which makes it 4 to 100 times faster. (Contributed by Eric Snow in bpo-16991.)

`OrderedDict.items()`, `OrderedDict.keys()`, `OrderedDict.values()` views now support `reversed()` iteration. (Contributed by Serhiy Storchaka in bpo-19505.)

The `deque` class now defines `index()`, `insert()`, and `copy()`, and supports the `+` and `*` operators. This allows deques to be recognized as a `MutableSequence` and improves their substitutability for lists. (Contributed by Raymond Hettinger in bpo-23704.)

Docstrings produced by `namedtuple()` can now be updated:

    Point = namedtuple('Point', ['x', 'y'])
    Point.__doc__ += ': Cartesian coodinate'
    Point.x.__doc__ = 'abscissa'
    Point.y.__doc__ = 'ordinate'

(Contributed by Berker Peksag in bpo-24064.)

The `UserString` class now implements the `__getnewargs__()`, `__rmod__()`, `casefold()`, `format_map()`, `isprintable()`, and `maketrans()` methods to match the corresponding methods of `str`. (Contributed by Joe Jevnik in bpo-22189.)

### collections.abc

The `Sequence.index()` method now accepts start and stop arguments to match the corresponding methods of `tuple`, `list`, etc. (Contributed by Devin Jeanpierre in bpo-23086.)

A new `Generator` abstract base class. (Contributed by Stefan Behnel in bpo-24018.)

New `Awaitable`, `Coroutine`, `AsyncIterator`, and `AsyncIterable` abstract base classes. (Contributed by Yury Selivanov in bpo-24184.)

For earlier Python versions, a backport of the new ABCs is available in an external PyPI package.

### compileall

A new `compileall` option, `-j N`, allows running N workers simultaneously to perform parallel bytecode compilation. The `compile_dir()` function has a corresponding `workers` parameter. (Contributed by Claudiu Popa in bpo-16104.)

Another new option, `-r`, allows controlling the maximum recursion level for subdirectories. (Contributed by Claudiu Popa in bpo-19628.)

The `-q` command line option can now be specified more than once, in which case all output, including errors, will be suppressed. The corresponding `quiet` parameter in `compile_dir()`, `compile_file()`, and `compile_path()` can now accept an integer value indicating the level of output suppression. (Contributed by Thomas Kluyver in bpo-21338.)

### concurrent.futures

The `Executor.map()` method now accepts a chunksize argument to allow batching of tasks to improve performance when `ProcessPoolExecutor()` is used. (Contributed by Dan O’Reilly in bpo-11271.)

The number of workers in the `ThreadPoolExecutor` constructor is optional now. The default value is 5 times the number of CPUs. (Contributed by Claudiu Popa in bpo-21527.)

### configparser

`configparser` now provides a way to customize the conversion of values by specifying a dictionary of converters in the `ConfigParser` constructor, or by defining them as methods in `ConfigParser` subclasses. Converters defined in a parser instance are inherited by its section proxies.

Example:

    >>> import configparser
    >>> conv = {}
    >>> conv['list'] = lambda v: [e.strip() for e in v.split() if e.strip()]
    >>> cfg = configparser.ConfigParser(converters=conv)
    >>> cfg.read_string("""
    ... [s]
    ... list = a b c d e f g
    ... """)
    >>> cfg.get('s', 'list')
    'a b c d e f g'
    >>> cfg.getlist('s', 'list')
    ['a', 'b', 'c', 'd', 'e', 'f', 'g']
    >>> section = cfg['s']
    >>> section.getlist('list')
    ['a', 'b', 'c', 'd', 'e', 'f', 'g']

(Contributed by Łukasz Langa in bpo-18159.)

### contextlib

The new `redirect_stderr()` context manager (similar to `redirect_stdout()`) makes it easier for utility scripts to handle inflexible APIs that write their output to `sys.stderr` and don’t provide any options to redirect it:

    >>> import contextlib, io, logging
    >>> f = io.StringIO()
    >>> with contextlib.redirect_stderr(f):
    ...     logging.warning('warning')
    ...
    >>> f.getvalue()
    'WARNING:root:warning\n'

(Contributed by Berker Peksag in bpo-22389.)

### csv

The `writerow()` method now supports arbitrary iterables, not just sequences. (Contributed by Serhiy Storchaka in bpo-23171.)

### curses

The new `update_lines_cols()` function updates the `LINES` and `COLS` environment variables. This is useful for detecting manual screen resizing. (Contributed by Arnon Yaari in bpo-4254.)

### dbm

`dumb.open` always creates a new database when the flag has the value `"n"`. (Contributed by Claudiu Popa in bpo-18039.)

### difflib

The charset of HTML documents generated by `HtmlDiff.make_file()` can now be customized by using a new charset keyword-only argument. The default charset of HTML document changed from `"ISO-8859-1"` to `"utf-8"`. (Contributed by Berker Peksag in bpo-2052.)

The `diff_bytes()` function can now compare lists of byte strings. This fixes a regression from Python 2. (Contributed by Terry J. Reedy and Greg Ward in bpo-17445.)

### distutils

Both the `build` and `build_ext` commands now accept a `-j` option to enable parallel building of extension modules. (Contributed by Antoine Pitrou in bpo-5309.)

The `distutils` module now supports `xz` compression, and can be enabled by passing `xztar` as an argument to `bdist --format`. (Contributed by Serhiy Storchaka in bpo-16314.)

### doctest

The `DocTestSuite()` function returns an empty `unittest.TestSuite` if module contains no docstrings, instead of raising `ValueError`. (Contributed by Glenn Jones in bpo-15916.)

### email

A new policy option `Policy.mangle_from_` controls whether or not lines that start with `"From "` in email bodies are prefixed with a `">"` character by generators. The default is `True` for `compat32` and `False` for all other policies. (Contributed by Milan Oberkirch in bpo-20098.)

A new `Message.get_content_disposition()` method provides easy access to a canonical value for the Content-Disposition header. (Contributed by Abhilash Raj in bpo-21083.)

A new policy option `EmailPolicy.utf8` can be set to `True` to encode email headers using the UTF-8 charset instead of using encoded words. This allows `Messages` to be formatted according to RFC 6532 and used with an SMTP server that supports the RFC 6531 `SMTPUTF8` extension. (Contributed by R. David Murray in bpo-24211.)

The `mime.text.MIMEText` constructor now accepts a `charset.Charset` instance. (Contributed by Claude Paroz and Berker Peksag in bpo-16324.)

### enum

The `Enum` callable has a new parameter start to specify the initial number of enum values if only names are provided:

    >>> Animal = enum.Enum('Animal', 'cat dog', start=10)
    >>> Animal.cat
    <Animal.cat: 10>
    >>> Animal.dog
    <Animal.dog: 11>

(Contributed by Ethan Furman in bpo-21706.)

### faulthandler

The `enable()`, `register()`, `dump_traceback()` and `dump_traceback_later()` functions now accept file descriptors in addition to file-like objects. (Contributed by Wei Wu in bpo-23566.)

### functools

Most of the `lru_cache()` machinery is now implemented in C, making it significantly faster. (Contributed by Matt Joiner, Alexey Kachayev, and Serhiy Storchaka in bpo-14373.)

### glob

The `iglob()` and `glob()` functions now support recursive search in subdirectories, using the `"**"` pattern. (Contributed by Serhiy Storchaka in bpo-13968.)

### gzip

The mode argument of the `GzipFile` constructor now accepts `"x"` to request exclusive creation. (Contributed by Tim Heaney in bpo-19222.)

### heapq

Element comparison in `merge()` can now be customized by passing a key function in a new optional key keyword argument, and a new optional reverse keyword argument can be used to reverse element comparison:

    >>> import heapq
    >>> a = ['9', '777', '55555']
    >>> b = ['88', '6666']
    >>> list(heapq.merge(a, b, key=len))
    ['9', '88', '777', '6666', '55555']
    >>> list(heapq.merge(reversed(a), reversed(b), key=len, reverse=True))
    ['55555', '6666', '777', '88', '9']

(Contributed by Raymond Hettinger in bpo-13742.)

### http

A new `HTTPStatus` enum that defines a set of HTTP status codes, reason phrases and long descriptions written in English. (Contributed by Demian Brecht in bpo-21793.)

### http.client

`HTTPConnection.getresponse()` now raises a `RemoteDisconnected` exception when a remote server connection is closed unexpectedly. Additionally, if a `ConnectionError` (of which `RemoteDisconnected` is a subclass) is raised, the client socket is now closed automatically, and will reconnect on the next request:

    import http.client
    conn = http.client.HTTPConnection('www.python.org')
    for retries in range(3):
        try:
            conn.request('GET', '/')
            resp = conn.getresponse()
        except http.client.RemoteDisconnected:
            pass

(Contributed by Martin Panter in bpo-3566.)

### idlelib and IDLE

Since idlelib implements the IDLE shell and editor and is not intended for import by other programs, it gets improvements with every release. See `Lib/idlelib/NEWS.txt` for a cumulative list of changes since 3.4.0, as well as changes made in future 3.5.x releases. This file is also available from the IDLE Help ‣ About IDLE dialog.

### imaplib

The `IMAP4` class now supports the context manager protocol. When used in a `with` statement, the IMAP4 `LOGOUT` command will be called automatically at the end of the block. (Contributed by Tarek Ziadé and Serhiy Storchaka in bpo-4972.)

The `imaplib` module now supports RFC 5161 (ENABLE Extension) and RFC 6855 (UTF-8 Support) via the `IMAP4.enable()` method. A new `IMAP4.utf8_enabled` attribute tracks whether or not RFC 6855 support is enabled. (Contributed by Milan Oberkirch, R. David Murray, and Maciej Szulik in bpo-21800.)

The `imaplib` module now automatically encodes non-ASCII string usernames and passwords using UTF-8, as recommended by the RFCs. (Contributed by Milan Oberkirch in bpo-21800.)

### imghdr

The `what()` function now recognizes the OpenEXR format (contributed by Martin Vignali and Claudiu Popa in bpo-20295), and the WebP format (contributed by Fabrice Aneche and Claudiu Popa in bpo-20197.)

### importlib

The `util.LazyLoader` class allows for lazy loading of modules in applications where startup time is important. (Contributed by Brett Cannon in bpo-17621.)

The `abc.InspectLoader.source_to_code()` method is now a static method. This makes it easier to initialize a module object with code compiled from a string by running `exec(code, module.__dict__)`. (Contributed by Brett Cannon in bpo-21156.)

The new `util.module_from_spec()` function is now the preferred way to create a new module. As opposed to creating a `types.ModuleType` instance directly, this new function will set the various import-controlled attributes based on the passed-in spec object. (Contributed by Brett Cannon in bpo-20383.)

### inspect

Both the `Signature` and `Parameter` classes are now picklable and hashable. (Contributed by Yury Selivanov in bpo-20726 and bpo-20334.)

A new `BoundArguments.apply_defaults()` method provides a way to set default values for missing arguments:

    >>> def foo(a, b='ham', *args): pass
    >>> ba = inspect.signature(foo).bind('spam')
    >>> ba.apply_defaults()
    >>> ba.arguments
    OrderedDict([('a', 'spam'), ('b', 'ham'), ('args', ())])

(Contributed by Yury Selivanov in bpo-24190.)

A new class method `Signature.from_callable()` makes subclassing of `Signature` easier. (Contributed by Yury Selivanov and Eric Snow in bpo-17373.)

The `signature()` function now accepts a follow_wrapped optional keyword argument, which, when set to `False`, disables automatic following of `__wrapped__` links. (Contributed by Yury Selivanov in bpo-20691.)

A set of new functions to inspect coroutine functions and coroutine objects has been added: `iscoroutine()`, `iscoroutinefunction()`, `isawaitable()`, `getcoroutinelocals()`, and `getcoroutinestate()`. (Contributed by Yury Selivanov in bpo-24017 and bpo-24400.)

The `stack()`, `trace()`, `getouterframes()`, and `getinnerframes()` functions now return a list of named tuples. (Contributed by Daniel Shahaf in bpo-16808.)

### io

A new `BufferedIOBase.readinto1()` method, that uses at most one call to the underlying raw stream’s `RawIOBase.read()` or `RawIOBase.readinto()` methods. (Contributed by Nikolaus Rath in bpo-20578.)

### ipaddress

Both the `IPv4Network` and `IPv6Network` classes now accept an `(address, netmask)` tuple argument, so as to easily construct network objects from existing addresses:

    >>> import ipaddress
    >>> ipaddress.IPv4Network(('127.0.0.0', 8))
    IPv4Network('127.0.0.0/8')
    >>> ipaddress.IPv4Network(('127.0.0.0', '255.0.0.0'))
    IPv4Network('127.0.0.0/8')

(Contributed by Peter Moody and Antoine Pitrou in bpo-16531.)

A new `reverse_pointer` attribute for the `IPv4Network` and `IPv6Network` classes returns the name of the reverse DNS PTR record:

    >>> import ipaddress
    >>> addr = ipaddress.IPv4Address('127.0.0.1')
    >>> addr.reverse_pointer
    '1.0.0.127.in-addr.arpa'
    >>> addr6 = ipaddress.IPv6Address('::1')
    >>> addr6.reverse_pointer
    '1.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.ip6.arpa'

(Contributed by Leon Weber in bpo-20480.)

### json

The `json.tool` command line interface now preserves the order of keys in JSON objects passed in input. The new `--sort-keys` option can be used to sort the keys alphabetically. (Contributed by Berker Peksag in bpo-21650.)

JSON decoder now raises `JSONDecodeError` instead of `ValueError` to provide better context information about the error. (Contributed by Serhiy Storchaka in bpo-19361.)

### linecache

A new `lazycache()` function can be used to capture information about a non-file-based module to permit getting its lines later via `getline()`. This avoids doing I/O until a line is actually needed, without having to carry the module globals around indefinitely. (Contributed by Robert Collins in bpo-17911.)

### locale

A new `delocalize()` function can be used to convert a string into a normalized number string, taking the `LC_NUMERIC` settings into account:

    >>> import locale
    >>> locale.setlocale(locale.LC_NUMERIC, 'de_DE.UTF-8')
    'de_DE.UTF-8'
    >>> locale.delocalize('1.234,56')
    '1234.56'
    >>> locale.setlocale(locale.LC_NUMERIC, 'en_US.UTF-8')
    'en_US.UTF-8'
    >>> locale.delocalize('1,234.56')
    '1234.56'

(Contributed by Cédric Krier in bpo-13918.)

### logging

All logging methods (`Logger` `log()`, `exception()`, `critical()`, `debug()`, etc.), now accept exception instances as an exc_info argument, in addition to boolean values and exception tuples:

    >>> import logging
    >>> try:
    ...     1/0
    ... except ZeroDivisionError as ex:
    ...     logging.error('exception', exc_info=ex)
    ERROR:root:exception

(Contributed by Yury Selivanov in bpo-20537.)

The `handlers.HTTPHandler` class now accepts an optional `ssl.SSLContext` instance to configure SSL settings used in an HTTP connection. (Contributed by Alex Gaynor in bpo-22788.)

The `handlers.QueueListener` class now takes a respect_handler_level keyword argument which, if set to `True`, will pass messages to handlers taking handler levels into account. (Contributed by Vinay Sajip.)

### lzma

The `LZMADecompressor.decompress()` method now accepts an optional max_length argument to limit the maximum size of decompressed data. (Contributed by Martin Panter in bpo-15955.)

### math

Two new constants have been added to the `math` module: `inf` and `nan`. (Contributed by Mark Dickinson in bpo-23185.)

A new function `isclose()` provides a way to test for approximate equality. (Contributed by Chris Barker and Tal Einat in bpo-24270.)

A new `gcd()` function has been added. The `fractions.gcd()` function is now deprecated. (Contributed by Mark Dickinson and Serhiy Storchaka in bpo-22486.)

### multiprocessing

`sharedctypes.synchronized()` objects now support the context manager protocol. (Contributed by Charles-François Natali in bpo-21565.)

### operator

`attrgetter()`, `itemgetter()`, and `methodcaller()` objects now support pickling. (Contributed by Josh Rosenberg and Serhiy Storchaka in bpo-22955.)

New `matmul()` and `imatmul()` functions to perform matrix multiplication. (Contributed by Benjamin Peterson in bpo-21176.)

### os

The new `scandir()` function returning an iterator of `DirEntry` objects has been added. If possible, `scandir()` extracts file attributes while scanning a directory, removing the need to perform subsequent system calls to determine file type or attributes, which may significantly improve performance. (Contributed by Ben Hoyt with the help of Victor Stinner in bpo-22524.)

On Windows, a new `stat_result.st_file_attributes` attribute is now available. It corresponds to the `dwFileAttributes` member of the `BY_HANDLE_FILE_INFORMATION` structure returned by `GetFileInformationByHandle()`. (Contributed by Ben Hoyt in bpo-21719.)

The `urandom()` function now uses the `getrandom()` syscall on Linux 3.17 or newer, and `getentropy()` on OpenBSD 5.6 and newer, removing the need to use `/dev/urandom` and avoiding failures due to potential file descriptor exhaustion. (Contributed by Victor Stinner in bpo-22181.)

New `get_blocking()` and `set_blocking()` functions allow getting and setting a file descriptor’s blocking mode (`O_NONBLOCK`.) (Contributed by Victor Stinner in bpo-22054.)

The `truncate()` and `ftruncate()` functions are now supported on Windows. (Contributed by Steve Dower in bpo-23668.)

There is a new `os.path.commonpath()` function returning the longest common sub-path of each passed pathname. Unlike the `os.path.commonprefix()` function, it always returns a valid path:

    >>> os.path.commonprefix(['/usr/lib', '/usr/local/lib'])
    '/usr/l'

    >>> os.path.commonpath(['/usr/lib', '/usr/local/lib'])
    '/usr'

(Contributed by Rafik Draoui and Serhiy Storchaka in bpo-10395.)

### pathlib

The new `Path.samefile()` method can be used to check whether the path points to the same file as another path, which can be either another `Path` object, or a string:

    >>> import pathlib
    >>> p1 = pathlib.Path('/etc/hosts')
    >>> p2 = pathlib.Path('/etc/../etc/hosts')
    >>> p1.samefile(p2)
    True

(Contributed by Vajrasky Kok and Antoine Pitrou in bpo-19775.)

The `Path.mkdir()` method now accepts a new optional exist_ok argument to match `mkdir -p` and `os.makedirs()` functionality. (Contributed by Berker Peksag in bpo-21539.)

There is a new `Path.expanduser()` method to expand `~` and `~user` prefixes. (Contributed by Serhiy Storchaka and Claudiu Popa in bpo-19776.)

A new `Path.home()` class method can be used to get a `Path` instance representing the user’s home directory. (Contributed by Victor Salgado and Mayank Tripathi in bpo-19777.)

New `Path.write_text()`, `Path.read_text()`, `Path.write_bytes()`, `Path.read_bytes()` methods to simplify read/write operations on files.

The following code snippet will create or rewrite existing file `~/spam42`:

    >>> import pathlib
    >>> p = pathlib.Path('~/spam42')
    >>> p.expanduser().write_text('ham')
    3

(Contributed by Christopher Welborn in bpo-20218.)

### pickle

Nested objects, such as unbound methods or nested classes, can now be pickled using pickle protocols older than protocol version 4. Protocol version 4 already supports these cases. (Contributed by Serhiy Storchaka in bpo-23611.)

### poplib

A new `POP3.utf8()` command enables RFC 6856 (Internationalized Email) support, if a POP server supports it. (Contributed by Milan OberKirch in bpo-21804.)

### re

References and conditional references to groups with fixed length are now allowed in lookbehind assertions:

    >>> import re
    >>> pat = re.compile(r'(a|b).(?<=\1)c')
    >>> pat.match('aac')
    <_sre.SRE_Match object; span=(0, 3), match='aac'>
    >>> pat.match('bbc')
    <_sre.SRE_Match object; span=(0, 3), match='bbc'>

(Contributed by Serhiy Storchaka in bpo-9179.)

The number of capturing groups in regular expressions is no longer limited to 100. (Contributed by Serhiy Storchaka in bpo-22437.)

The `sub()` and `subn()` functions now replace unmatched groups with empty strings instead of raising an exception. (Contributed by Serhiy Storchaka in bpo-1519638.)

The `re.error` exceptions have new attributes, `msg`, `pattern`, `pos`, `lineno`, and `colno`, that provide better context information about the error:

    >>> re.compile("""
    ...     (?x)
    ...     .++
    ... """)
    Traceback (most recent call last):
       ...
    sre_constants.error: multiple repeat at position 16 (line 3, column 7)

(Contributed by Serhiy Storchaka in bpo-22578.)

### readline

A new `append_history_file()` function can be used to append the specified number of trailing elements in history to the given file. (Contributed by Bruno Cauet in bpo-22940.)

### selectors

The new `DevpollSelector` supports efficient `/dev/poll` polling on Solaris. (Contributed by Giampaolo Rodola’ in bpo-18931.)

### shutil

The `move()` function now accepts a copy_function argument, allowing, for example, the `copy()` function to be used instead of the default `copy2()` if there is a need to ignore file metadata when moving. (Contributed by Claudiu Popa in bpo-19840.)

The `make_archive()` function now supports the xztar format. (Contributed by Serhiy Storchaka in bpo-5411.)

### signal

On Windows, the `set_wakeup_fd()` function now also supports socket handles. (Contributed by Victor Stinner in bpo-22018.)

Various `SIG*` constants in the `signal` module have been converted into `Enums`. This allows meaningful names to be printed during debugging, instead of integer “magic numbers”. (Contributed by Giampaolo Rodola’ in bpo-21076.)

### smtpd

Both the `SMTPServer` and `SMTPChannel` classes now accept a decode_data keyword argument to determine if the `DATA` portion of the SMTP transaction is decoded using the `"utf-8"` codec or is instead provided to the `SMTPServer.process_message()` method as a byte string. The default is `True` for backward compatibility reasons, but will change to `False` in Python 3.6. If decode_data is set to `False`, the `process_message` method must be prepared to accept keyword arguments. (Contributed by Maciej Szulik in bpo-19662.)

The `SMTPServer` class now advertises the `8BITMIME` extension (RFC 6152) if decode_data has been set `True`. If the client specifies `BODY=8BITMIME` on the `MAIL` command, it is passed to `SMTPServer.process_message()` via the mail_options keyword. (Contributed by Milan Oberkirch and R. David Murray in bpo-21795.)

The `SMTPServer` class now also supports the `SMTPUTF8` extension (RFC 6531: Internationalized Email). If the client specified `SMTPUTF8 BODY=8BITMIME` on the `MAIL` command, they are passed to `SMTPServer.process_message()` via the mail_options keyword. It is the responsibility of the `process_message` method to correctly handle the `SMTPUTF8` data. (Contributed by Milan Oberkirch in bpo-21725.)

It is now possible to provide, directly or via name resolution, IPv6 addresses in the `SMTPServer` constructor, and have it successfully connect. (Contributed by Milan Oberkirch in bpo-14758.)

### smtplib

A new `SMTP.auth()` method provides a convenient way to implement custom authentication mechanisms. (Contributed by Milan Oberkirch in bpo-15014.)

The `SMTP.set_debuglevel()` method now accepts an additional debuglevel (2), which enables timestamps in debug messages. (Contributed by Gavin Chappell and Maciej Szulik in bpo-16914.)

Both the `SMTP.sendmail()` and `SMTP.send_message()` methods now support RFC 6531 (SMTPUTF8). (Contributed by Milan Oberkirch and R. David Murray in bpo-22027.)

### sndhdr

The `what()` and `whathdr()` functions now return a `namedtuple()`. (Contributed by Claudiu Popa in bpo-18615.)

### socket

Functions with timeouts now use a monotonic clock, instead of a system clock. (Contributed by Victor Stinner in bpo-22043.)

A new `socket.sendfile()` method allows sending a file over a socket by using the high-performance `os.sendfile()` function on UNIX, resulting in uploads being from 2 to 3 times faster than when using plain `socket.send()`. (Contributed by Giampaolo Rodola’ in bpo-17552.)

The `socket.sendall()` method no longer resets the socket timeout every time bytes are received or sent. The socket timeout is now the maximum total duration to send all data. (Contributed by Victor Stinner in bpo-23853.)

The backlog argument of the `socket.listen()` method is now optional. By default it is set to `SOMAXCONN` or to `128`, whichever is less. (Contributed by Charles-François Natali in bpo-21455.)

### ssl

#### Memory BIO Support

(Contributed by Geert Jansen in bpo-21965.)

The new `SSLObject` class has been added to provide SSL protocol support for cases when the network I/O capabilities of `SSLSocket` are not necessary or are suboptimal. `SSLObject` represents an SSL protocol instance, but does not implement any network I/O methods, and instead provides a memory buffer interface. The new `MemoryBIO` class can be used to pass data between Python and an SSL protocol instance.

The memory BIO SSL support is primarily intended to be used in frameworks implementing asynchronous I/O for which `SSLSocket`’s readiness model (“select/poll”) is inefficient.

A new `SSLContext.wrap_bio()` method can be used to create a new `SSLObject` instance.

#### Application-Layer Protocol Negotiation Support

(Contributed by Benjamin Peterson in bpo-20188.)

Where OpenSSL support is present, the `ssl` module now implements the Application-Layer Protocol Negotiation TLS extension as described in RFC 7301.

The new `SSLContext.set_alpn_protocols()` can be used to specify which protocols a socket should advertise during the TLS handshake.

The new `SSLSocket.selected_alpn_protocol()` returns the protocol that was selected during the TLS handshake. The `HAS_ALPN` flag indicates whether ALPN support is present.

#### Other Changes

There is a new `SSLSocket.version()` method to query the actual protocol version in use. (Contributed by Antoine Pitrou in bpo-20421.)

The `SSLSocket` class now implements a `SSLSocket.sendfile()` method. (Contributed by Giampaolo Rodola’ in bpo-17552.)

The `SSLSocket.send()` method now raises either the `ssl.SSLWantReadError` or `ssl.SSLWantWriteError` exception on a non-blocking socket if the operation would block. Previously, it would return `0`. (Contributed by Nikolaus Rath in bpo-20951.)

The `cert_time_to_seconds()` function now interprets the input time as UTC and not as local time, per RFC 5280. Additionally, the return value is always an `int`. (Contributed by Akira Li in bpo-19940.)

New `SSLObject.shared_ciphers()` and `SSLSocket.shared_ciphers()` methods return the list of ciphers sent by the client during the handshake. (Contributed by Benjamin Peterson in bpo-23186.)

The `SSLSocket.do_handshake()`, `SSLSocket.read()`, `SSLSocket.shutdown()`, and `SSLSocket.write()` methods of the `SSLSocket` class no longer reset the socket timeout every time bytes are received or sent. The socket timeout is now the maximum total duration of the method. (Contributed by Victor Stinner in bpo-23853.)

The `match_hostname()` function now supports matching of IP addresses. (Contributed by Antoine Pitrou in bpo-23239.)

### sqlite3

The `Row` class now fully supports the sequence protocol, in particular `reversed()` iteration and slice indexing. (Contributed by Claudiu Popa in bpo-10203; by Lucas Sinclair, Jessica McKellar, and Serhiy Storchaka in bpo-13583.)

### subprocess

The new `run()` function has been added. It runs the specified command and returns a `CompletedProcess` object, which describes a finished process. The new API is more consistent and is the recommended approach to invoking subprocesses in Python code that does not need to maintain compatibility with earlier Python versions. (Contributed by Thomas Kluyver in bpo-23342.)

Examples:

    >>> subprocess.run(["ls", "-l"])  # doesn't capture output
    CompletedProcess(args=['ls', '-l'], returncode=0)

    >>> subprocess.run("exit 1", shell=True, check=True)
    Traceback (most recent call last):
      ...
    subprocess.CalledProcessError: Command 'exit 1' returned non-zero exit status 1

    >>> subprocess.run(["ls", "-l", "/dev/null"], stdout=subprocess.PIPE)
    CompletedProcess(args=['ls', '-l', '/dev/null'], returncode=0,
    stdout=b'crw-rw-rw- 1 root root 1, 3 Jan 23 16:23 /dev/null\n')

### sys

A new `set_coroutine_wrapper()` function allows setting a global hook that will be called whenever a coroutine object is created by an `async def` function. A corresponding `get_coroutine_wrapper()` can be used to obtain a currently set wrapper. Both functions are provisional, and are intended for debugging purposes only. (Contributed by Yury Selivanov in bpo-24017.)

A new `is_finalizing()` function can be used to check if the Python interpreter is shutting down. (Contributed by Antoine Pitrou in bpo-22696.)

### sysconfig

The name of the user scripts directory on Windows now includes the first two components of the Python version. (Contributed by Paul Moore in bpo-23437.)

### tarfile

The mode argument of the `open()` function now accepts `"x"` to request exclusive creation. (Contributed by Berker Peksag in bpo-21717.)

The `TarFile.extractall()` and `TarFile.extract()` methods now take a keyword argument numeric_owner. If set to `True`, the extracted files and directories will be owned by the numeric `uid` and `gid` from the tarfile. If set to `False` (the default, and the behavior in versions prior to 3.5), they will be owned by the named user and group in the tarfile. (Contributed by Michael Vogt and Eric Smith in bpo-23193.)

The `TarFile.list()` now accepts an optional members keyword argument that can be set to a subset of the list returned by `TarFile.getmembers()`. (Contributed by Serhiy Storchaka in bpo-21549.)

### threading

Both the `Lock.acquire()` and `RLock.acquire()` methods now use a monotonic clock for timeout management. (Contributed by Victor Stinner in bpo-22043.)

### time

The `monotonic()` function is now always available. (Contributed by Victor Stinner in bpo-22043.)

### timeit

A new command line option `-u` or `--unit=U` can be used to specify the time unit for the timer output. Supported options are `usec`, `msec`, or `sec`. (Contributed by Julian Gindi in bpo-18983.)

The `timeit()` function has a new globals parameter for specifying the namespace in which the code will be running. (Contributed by Ben Roberts in bpo-2527.)

### tkinter

The `tkinter._fix` module used for setting up the Tcl/Tk environment on Windows has been replaced by a private function in the `_tkinter` module which makes no permanent changes to environment variables. (Contributed by Zachary Ware in bpo-20035.)

### traceback

New `walk_stack()` and `walk_tb()` functions to conveniently traverse frame and traceback objects. (Contributed by Robert Collins in bpo-17911.)

New lightweight classes: `TracebackException`, `StackSummary`, and `FrameSummary`. (Contributed by Robert Collins in bpo-17911.)

Both the `print_tb()` and `print_stack()` functions now support negative values for the limit argument. (Contributed by Dmitry Kazakov in bpo-22619.)

### types

A new `coroutine()` function to transform generator and `generator-like` objects into awaitables. (Contributed by Yury Selivanov in bpo-24017.)

A new type called `CoroutineType`, which is used for coroutine objects created by `async def` functions. (Contributed by Yury Selivanov in bpo-24400.)

### unicodedata

The `unicodedata` module now uses data from Unicode 8.0.0.

### unittest

The `TestLoader.loadTestsFromModule()` method now accepts a keyword-only argument pattern which is passed to `load_tests` as the third argument. Found packages are now checked for `load_tests` regardless of whether their path matches pattern, because it is impossible for a package name to match the default pattern. (Contributed by Robert Collins and Barry A. Warsaw in bpo-16662.)

Unittest discovery errors now are exposed in the `TestLoader.errors` attribute of the `TestLoader` instance. (Contributed by Robert Collins in bpo-19746.)

A new command line option `--locals` to show local variables in tracebacks. (Contributed by Robert Collins in bpo-22936.)

### unittest.mock

The `Mock` class has the following improvements:

- The class constructor has a new unsafe parameter, which causes mock objects to raise `AttributeError` on attribute names starting with `"assert"`. (Contributed by Kushal Das in bpo-21238.)

- A new `Mock.assert_not_called()` method to check if the mock object was called. (Contributed by Kushal Das in bpo-21262.)

The `MagicMock` class now supports `__truediv__()`, `__divmod__()` and `__matmul__()` operators. (Contributed by Johannes Baiter in bpo-20968, and Håkan Lövdahl in bpo-23581 and bpo-23568.)

It is no longer necessary to explicitly pass `create=True` to the `patch()` function when patching builtin names. (Contributed by Kushal Das in bpo-17660.)

### urllib

A new `request.HTTPPasswordMgrWithPriorAuth` class allows HTTP Basic Authentication credentials to be managed so as to eliminate unnecessary `401` response handling, or to unconditionally send credentials on the first request in order to communicate with servers that return a `404` response instead of a `401` if the `Authorization` header is not sent. (Contributed by Matej Cepl in bpo-19494 and Akshit Khurana in bpo-7159.)

A new quote_via argument for the `parse.urlencode()` function provides a way to control the encoding of query parts if needed. (Contributed by Samwyse and Arnon Yaari in bpo-13866.)

The `request.urlopen()` function accepts an `ssl.SSLContext` object as a context argument, which will be used for the HTTPS connection. (Contributed by Alex Gaynor in bpo-22366.)

The `parse.urljoin()` was updated to use the RFC 3986 semantics for the resolution of relative URLs, rather than RFC 1808 and RFC 2396. (Contributed by Demian Brecht and Senthil Kumaran in bpo-22118.)

### wsgiref

The headers argument of the `headers.Headers` class constructor is now optional. (Contributed by Pablo Torres Navarrete and SilentGhost in bpo-5800.)

### xmlrpc

The `client.ServerProxy` class now supports the context manager protocol. (Contributed by Claudiu Popa in bpo-20627.)

The `client.ServerProxy` constructor now accepts an optional `ssl.SSLContext` instance. (Contributed by Alex Gaynor in bpo-22960.)

### xml.sax

SAX parsers now support a character stream of the `xmlreader.InputSource` object. (Contributed by Serhiy Storchaka in bpo-2175.)

`parseString()` now accepts a `str` instance. (Contributed by Serhiy Storchaka in bpo-10590.)

### zipfile

ZIP output can now be written to unseekable streams. (Contributed by Serhiy Storchaka in bpo-23252.)

The mode argument of `ZipFile.open()` method now accepts `"x"` to request exclusive creation. (Contributed by Serhiy Storchaka in bpo-21717.)

## Other module-level changes

Many functions in the `mmap`, `ossaudiodev`, `socket`, `ssl`, and `codecs` modules now accept writable bytes-like objects. (Contributed by Serhiy Storchaka in bpo-23001.)

## Optimizations

The `os.walk()` function has been sped up by 3 to 5 times on POSIX systems, and by 7 to 20 times on Windows. This was done using the new `os.scandir()` function, which exposes file information from the underlying `readdir` or `FindFirstFile`/`FindNextFile` system calls. (Contributed by Ben Hoyt with help from Victor Stinner in bpo-23605.)

Construction of `bytes(int)` (filled by zero bytes) is faster and uses less memory for large objects. `calloc()` is used instead of `malloc()` to allocate memory for these objects. (Contributed by Victor Stinner in bpo-21233.)

Some operations on `ipaddress` `IPv4Network` and `IPv6Network` have been massively sped up, such as `subnets()`, `supernet()`, `summarize_address_range()`, `collapse_addresses()`. The speed up can range from 3 to 15 times. (Contributed by Antoine Pitrou, Michel Albert, and Markus in bpo-21486, bpo-21487, bpo-20826, bpo-23266.)

Pickling of `ipaddress` objects was optimized to produce significantly smaller output. (Contributed by Serhiy Storchaka in bpo-23133.)

Many operations on `io.BytesIO` are now 50% to 100% faster. (Contributed by Serhiy Storchaka in bpo-15381 and David Wilson in bpo-22003.)

The `marshal.dumps()` function is now faster: 65–85% with versions 3 and 4, 20–25% with versions 0 to 2 on typical data, and up to 5 times in best cases. (Contributed by Serhiy Storchaka in bpo-20416 and bpo-23344.)

The UTF-32 encoder is now 3 to 7 times faster. (Contributed by Serhiy Storchaka in bpo-15027.)

Regular expressions are now parsed up to 10% faster. (Contributed by Serhiy Storchaka in bpo-19380.)

The `json.dumps()` function was optimized to run with `ensure_ascii=False` as fast as with `ensure_ascii=True`. (Contributed by Naoki Inada in bpo-23206.)

The `PyObject_IsInstance()` and `PyObject_IsSubclass()` functions have been sped up in the common case that the second argument has `type` as its metaclass. (Contributed Georg Brandl by in bpo-22540.)

Method caching was slightly improved, yielding up to 5% performance improvement in some benchmarks. (Contributed by Antoine Pitrou in bpo-22847.)

Objects from the `random` module now use 50% less memory on 64-bit builds. (Contributed by Serhiy Storchaka in bpo-23488.)

The `property()` getter calls are up to 25% faster. (Contributed by Joe Jevnik in bpo-23910.)

Instantiation of `fractions.Fraction` is now up to 30% faster. (Contributed by Stefan Behnel in bpo-22464.)

String methods `find()`, `rfind()`, `split()`, `partition()` and the `in` string operator are now significantly faster for searching 1-character substrings. (Contributed by Serhiy Storchaka in bpo-23573.)

## Build and C API Changes

New `calloc` functions were added:

- `PyMem_RawCalloc()`,

- `PyMem_Calloc()`,

- `PyObject_Calloc()`.

(Contributed by Victor Stinner in bpo-21233.)

New encoding/decoding helper functions:

- `Py_DecodeLocale()` (replaced `_Py_char2wchar()`),

- `Py_EncodeLocale()` (replaced `_Py_wchar2char()`).

(Contributed by Victor Stinner in bpo-18395.)

A new `PyCodec_NameReplaceErrors()` function to replace the unicode encode error with `\N{...}` escapes. (Contributed by Serhiy Storchaka in bpo-19676.)

A new `PyErr_FormatV()` function similar to `PyErr_Format()`, but accepts a `va_list` argument. (Contributed by Antoine Pitrou in bpo-18711.)

A new `PyExc_RecursionError` exception. (Contributed by Georg Brandl in bpo-19235.)

New `PyModule_FromDefAndSpec()`, `PyModule_FromDefAndSpec2()`, and `PyModule_ExecDef()` functions introduced by PEP 489 – multi-phase extension module initialization. (Contributed by Petr Viktorin in bpo-24268.)

New `PyNumber_MatrixMultiply()` and `PyNumber_InPlaceMatrixMultiply()` functions to perform matrix multiplication. (Contributed by Benjamin Peterson in bpo-21176. See also PEP 465 for details.)

The `PyTypeObject.tp_finalize` slot is now part of the stable ABI.

Windows builds now require Microsoft Visual C++ 14.0, which is available as part of Visual Studio 2015.

Extension modules now include a platform information tag in their filename on some platforms (the tag is optional, and CPython will import extensions without it, although if the tag is present and mismatched, the extension won’t be loaded):

- On Linux, extension module filenames end with `.cpython-<major><minor>m-<architecture>-<os>.pyd`:

  - `<major>` is the major number of the Python version; for Python 3.5 this is `3`.

  - `<minor>` is the minor number of the Python version; for Python 3.5 this is `5`.

  - `<architecture>` is the hardware architecture the extension module was built to run on. It’s most commonly either `i386` for 32-bit Intel platforms or `x86_64` for 64-bit Intel (and AMD) platforms.

  - `<os>` is always `linux-gnu`, except for extensions built to talk to the 32-bit ABI on 64-bit platforms, in which case it is `linux-gnu32` (and `<architecture>` will be `x86_64`).

- On Windows, extension module filenames end with `<debug>.cp<major><minor>-<platform>.pyd`:

  - `<major>` is the major number of the Python version; for Python 3.5 this is `3`.

  - `<minor>` is the minor number of the Python version; for Python 3.5 this is `5`.

  - `<platform>` is the platform the extension module was built for, either `win32` for Win32, `win_amd64` for Win64, `win_ia64` for Windows Itanium 64, and `win_arm` for Windows on ARM.

  - If built in debug mode, `<debug>` will be `_d`, otherwise it will be blank.

- On OS X platforms, extension module filenames now end with `-darwin.so`.

- On all other platforms, extension module filenames are the same as they were with Python 3.4.

## 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.

### Deprecated Python Behavior

Raising the `StopIteration` exception inside a generator will now generate a silent `PendingDeprecationWarning`, which will become a non-silent deprecation warning in Python 3.6 and will trigger a `RuntimeError` in Python 3.7. See PEP 479: Change StopIteration handling inside generators for details.

### Unsupported Operating Systems

Windows XP is no longer supported by Microsoft, thus, per PEP 11, CPython 3.5 is no longer officially supported on this OS.

### Deprecated Python modules, functions and methods

The `formatter` module has now graduated to full deprecation and is still slated for removal in Python 3.6.

The `asyncio.async()` function is deprecated in favor of `ensure_future()`.

The `smtpd` module has in the past always decoded the DATA portion of email messages using the `utf-8` codec. This can now be controlled by the new decode_data keyword to `SMTPServer`. The default value is `True`, but this default is deprecated. Specify the decode_data keyword with an appropriate value to avoid the deprecation warning.

Directly assigning values to the `key`, `value` and `coded_value` of `http.cookies.Morsel` objects is deprecated. Use the `set()` method instead. In addition, the undocumented LegalChars parameter of `set()` is deprecated, and is now ignored.

Passing a format string as keyword argument format_string to the `format()` method of the `string.Formatter` class has been deprecated. (Contributed by Serhiy Storchaka in bpo-23671.)

The `platform.dist()` and `platform.linux_distribution()` functions are now deprecated. Linux distributions use too many different ways of describing themselves, so the functionality is left to a package. (Contributed by Vajrasky Kok and Berker Peksag in bpo-1322.)

The previously undocumented `from_function` and `from_builtin` methods of `inspect.Signature` are deprecated. Use the new `Signature.from_callable()` method instead. (Contributed by Yury Selivanov in bpo-24248.)

The `inspect.getargspec()` function is deprecated and scheduled to be removed in Python 3.6. (See bpo-20438 for details.)

The `inspect` `getfullargspec()`, `getcallargs()`, and `formatargspec()` functions are deprecated in favor of the `inspect.signature()` API. (Contributed by Yury Selivanov in bpo-20438.)

`getargvalues()` and `formatargvalues()` functions were inadvertently marked as deprecated with the release of Python 3.5.0.

Use of `re.LOCALE` flag with str patterns or `re.ASCII` is now deprecated. (Contributed by Serhiy Storchaka in bpo-22407.)

Use of unrecognized special sequences consisting of `'\'` and an ASCII letter in regular expression patterns and replacement patterns now raises a deprecation warning and will be forbidden in Python 3.6. (Contributed by Serhiy Storchaka in bpo-23622.)

The undocumented and unofficial use_load_tests default argument of the `unittest.TestLoader.loadTestsFromModule()` method now is deprecated and ignored. (Contributed by Robert Collins and Barry A. Warsaw in bpo-16662.)

## Removed

### API and Feature Removals

The following obsolete and previously deprecated APIs and features have been removed:

- The `__version__` attribute has been dropped from the email package. The email code hasn’t been shipped separately from the stdlib for a long time, and the `__version__` string was not updated in the last few releases.

- The internal `Netrc` class in the `ftplib` module was deprecated in 3.4, and has now been removed. (Contributed by Matt Chaput in bpo-6623.)

- The concept of `.pyo` files has been removed.

- The JoinableQueue class in the provisional `asyncio` module was deprecated in 3.4.4 and is now removed. (Contributed by A. Jesse Jiryu Davis in bpo-23464.)

## Porting to Python 3.5

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

### Changes in Python behavior

- Due to an oversight, earlier Python versions erroneously accepted the following syntax:

      f(1 for x in [1], *args)
      f(1 for x in [1], **kwargs)

  Python 3.5 now correctly raises a `SyntaxError`, as generator expressions must be put in parentheses if not a sole argument to a function.

### Changes in the Python API

- PEP 475: System calls are now retried when interrupted by a signal instead of raising `InterruptedError` if the Python signal handler does not raise an exception.

- Before Python 3.5, a `datetime.time` object was considered to be false if it represented midnight in UTC. This behavior was considered obscure and error-prone and has been removed in Python 3.5. See bpo-13936 for full details.

- The `ssl.SSLSocket.send()` method now raises either `ssl.SSLWantReadError` or `ssl.SSLWantWriteError` on a non-blocking socket if the operation would block. Previously, it would return `0`. (Contributed by Nikolaus Rath in bpo-20951.)

- The `__name__` attribute of generators is now set from the function name, instead of being set from the code name. Use `gen.gi_code.co_name` to retrieve the code name. Generators also have a new `__qualname__` attribute, the qualified name, which is now used for the representation of a generator (`repr(gen)`). (Contributed by Victor Stinner in bpo-21205.)

- The deprecated “strict” mode and argument of `HTMLParser`, `HTMLParser.error()`, and the `HTMLParserError` exception have been removed. (Contributed by Ezio Melotti in bpo-15114.) The convert_charrefs argument of `HTMLParser` is now `True` by default. (Contributed by Berker Peksag in bpo-21047.)

- Although it is not formally part of the API, it is worth noting for porting purposes (ie: fixing tests) that error messages that were previously of the form “‘sometype’ does not support the buffer protocol” are now of the form “a bytes-like object is required, not ‘sometype’”. (Contributed by Ezio Melotti in bpo-16518.)

- If the current directory is set to a directory that no longer exists then `FileNotFoundError` will no longer be raised and instead `find_spec()` will return `None` without caching `None` in `sys.path_importer_cache`, which is different than the typical case (bpo-22834).

- HTTP status code and messages from `http.client` and `http.server` were refactored into a common `HTTPStatus` enum. The values in `http.client` and `http.server` remain available for backwards compatibility. (Contributed by Demian Brecht in bpo-21793.)

- When an import loader defines `importlib.machinery.Loader.exec_module()` it is now expected to also define `create_module()` (raises a `DeprecationWarning` now, will be an error in Python 3.6). If the loader inherits from `importlib.abc.Loader` then there is nothing to do, else simply define `create_module()` to return `None`. (Contributed by Brett Cannon in bpo-23014.)

- The `re.split()` function always ignored empty pattern matches, so the `"x*"` pattern worked the same as `"x+"`, and the `"\b"` pattern never worked. Now `re.split()` raises a warning if the pattern could match an empty string. For compatibility, use patterns that never match an empty string (e.g. `"x+"` instead of `"x*"`). Patterns that could only match an empty string (such as `"\b"`) now raise an error. (Contributed by Serhiy Storchaka in bpo-22818.)

- The `http.cookies.Morsel` dict-like interface has been made self consistent: morsel comparison now takes the `key` and `value` into account, `copy()` now results in a `Morsel` instance rather than a `dict`, and `update()` will now raise an exception if any of the keys in the update dictionary are invalid. In addition, the undocumented LegalChars parameter of `set()` is deprecated and is now ignored. (Contributed by Demian Brecht in bpo-2211.)

- PEP 488 has removed `.pyo` files from Python and introduced the optional `opt-` tag in `.pyc` file names. The `importlib.util.cache_from_source()` has gained an optimization parameter to help control the `opt-` tag. Because of this, the debug_override parameter of the function is now deprecated. .pyo files are also no longer supported as a file argument to the Python interpreter and thus serve no purpose when distributed on their own (i.e. sourceless code distribution). Due to the fact that the magic number for bytecode has changed in Python 3.5, all old .pyo files from previous versions of Python are invalid regardless of this PEP.

- The `socket` module now exports the `CAN_RAW_FD_FRAMES` constant on linux 3.6 and greater.

- The `ssl.cert_time_to_seconds()` function now interprets the input time as UTC and not as local time, per RFC 5280. Additionally, the return value is always an `int`. (Contributed by Akira Li in bpo-19940.)

- The `pygettext.py` Tool now uses the standard +NNNN format for timezones in the POT-Creation-Date header.

- The `smtplib` module now uses `sys.stderr` instead of the previous module-level `stderr` variable for debug output. If your (test) program depends on patching the module-level variable to capture the debug output, you will need to update it to capture sys.stderr instead.

- The `str.startswith()` and `str.endswith()` methods no longer return `True` when finding the empty string and the indexes are completely out of range. (Contributed by Serhiy Storchaka in bpo-24284.)

- The `inspect.getdoc()` function now returns documentation strings inherited from base classes. Documentation strings no longer need to be duplicated if the inherited documentation is appropriate. To suppress an inherited string, an empty string must be specified (or the documentation may be filled in). This change affects the output of the `pydoc` module and the `help()` function. (Contributed by Serhiy Storchaka in bpo-15582.)

- Nested `functools.partial()` calls are now flattened. If you were relying on the previous behavior, you can now either add an attribute to a `functools.partial()` object or you can create a subclass of `functools.partial()`. (Contributed by Alexander Belopolsky in bpo-7830.)

### Changes in the C API

- The undocumented `format` member of the (non-public) `PyMemoryViewObject` structure has been removed. All extensions relying on the relevant parts in `memoryobject.h` must be rebuilt.

- The `PyMemAllocator` structure was renamed to `PyMemAllocatorEx` and a new `calloc` field was added.

- Removed non-documented macro `PyObject_REPR` which leaked references. Use format character `%R` in `PyUnicode_FromFormat()`-like functions to format the `repr()` of the object. (Contributed by Serhiy Storchaka in bpo-22453.)

- Because the lack of the `__module__` attribute breaks pickling and introspection, a deprecation warning is now raised for builtin types without the `__module__` attribute. This would be an AttributeError in the future. (Contributed by Serhiy Storchaka in bpo-20204.)

- As part of the PEP 492 implementation, the `tp_reserved` slot of `PyTypeObject` was replaced with a `tp_as_async` slot. Refer to Coroutine Objects for new types, structures and functions.

## Notable changes in Python 3.5.4

### New `make regen-all` build target

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

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

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

(Contributed by Victor Stinner in bpo-23404.)

New in version 3.5.4.

### 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 3.5.4.