What's New In Python 3.5

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

* Anyone can add text to this document. Do not spend very much time on the wording of your changes, because your text will probably get rewritten to some degree.

* The maintainer will go through Misc/NEWS periodically and add changes; it's therefore more important to add your changes to Misc/NEWS than to this file.

* This is not a complete list of every single change; completeness is the purpose of Misc/NEWS. Some changes I consider too small or esoteric to include. If such a change is added to the text, I'll just remove it. (This is another reason you shouldn't spend too much time on writing your addition.)

* If you want to draw your new text to the attention of the maintainer, add 'XXX' to the beginning of the paragraph or section.

* It's OK to just add a fragmentary note about a change. For example: "XXX Describe the transmogrify() function added to the socket module." The maintainer will research the change and write the necessary text.

* You can comment out your additions if you like, but it's not necessary (especially when a final release is some months away).

* Credit the author of a patch or bugfix. Just the name is sufficient; the e-mail address isn't necessary.

• It's helpful to add the bug/patch number as a comment:

XXX Describe the transmogrify() function added to the socket module. (Contributed by P.Y. Developer in 12345.)

This saves the maintainer the effort of going through the Mercurial log when researching a change.

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.

Summary -- Release highlights

New syntax features:

• PEP 492 <whatsnew-pep-492>, coroutines with async and await syntax.
• PEP 465 <whatsnew-pep-465>, a new matrix multiplication operator: a @ b.
• PEP 448 <whatsnew-pep-448>, additional unpacking generalizations.

New library modules:

• typing: PEP 484 -- Type Hints <whatsnew-pep-484>.
• zipapp: PEP 441 Improving Python ZIP Application Support <whatsnew-zipapp>.

New built-in features:

• bytes % args, bytearray % args: PEP 461 <whatsnew-pep-461> -- Adding % formatting to bytes and bytearray.
• New bytes.hex, bytearray.hex and memoryview.hex methods. (Contributed by Arnon Yaari in 9951.)
• memoryview now supports tuple indexing (including multi-dimensional). (Contributed by Antoine Pitrou in 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 24450.)
• A new RecursionError exception is now raised when maximum recursion depth is reached. (Contributed by Georg Brandl in 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 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 <whatsnew-pep-488>.)
• 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 <whatsnew-pep-489>.)

Significant improvements in the standard library:

• collections.OrderedDict is now implemented in C <whatsnew-ordereddict>, which makes it 4 to 100 times faster.
• The ssl module gained support for Memory BIO <whatsnew-sslmemorybio>, which decouples SSL protocol handling from network IO.
• The new os.scandir function provides a better and significantly faster way <whatsnew-pep-471> of directory traversal.
• functools.lru_cache has been mostly reimplemented in C <whatsnew-lrucache>, yielding much better performance.
• The new subprocess.run function provides a streamlined way to run subprocesses <whatsnew-subprocess>.
• The traceback module has been significantly enhanced <whatsnew-traceback> 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 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 22796.)

Windows improvements:

• A new installer for Windows has replaced the old MSI. See using-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

492 greatly improves support for asynchronous programming in Python by adding awaitable objects <awaitable>, coroutine functions <coroutine function>, asynchronous iteration <asynchronous iterable>, and asynchronous context managers <asynchronous context manager>.

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 ~object.__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 <asyncio-event-loop>.

[!NOTE]

3.5.2 Starting with CPython 3.5.2, __aiter__ can directly return asynchronous iterators <asynchronous iterator>. Returning an awaitable object will result in a PendingDeprecationWarning.

See more details in the async-iterators documentation section.

PEP 465 - A dedicated infix operator for matrix multiplication

465 adds the @ infix operator for matrix multiplication. Currently, no builtin Python types implement the new operator, however, it can be implemented by defining ~object.__matmul__, ~object.__rmatmul__, and ~object.__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.])

PEP 448 - Additional Unpacking Generalizations

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 <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 exprlists and dict):

>>> *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}

PEP 461 - percent formatting support for bytes and bytearray

461 adds support for the % interpolation operator <bytes-formatting> 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.

PEP 484 - Type Hints

Function annotation syntax has been a Python feature since version 3.0 (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.

484 introduces a provisional module <provisional API> 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 ~object.__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 ~typing.Any which is consistent with (i.e. assignable to and from) all types.

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

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() <os.DirEntry.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)

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

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: ~os.fchdir, ~os.fchmod, ~os.fchown, ~os.fdatasync, ~os.fstat, ~os.fstatvfs, ~os.fsync, ~os.ftruncate, ~os.mkfifo, ~os.mknod, ~os.open, ~os.posix_fadvise, ~os.posix_fallocate, ~os.pread, ~os.pwrite, ~os.read, ~os.readv, ~os.sendfile, ~os.wait3, ~os.wait4, ~os.wait, ~os.waitid, ~os.waitpid, ~os.write, ~os.writev;
• special cases: os.close and os.dup2 now ignore ~errno.EINTR errors; the syscall is not retried (see the PEP for the rationale);
• select functions: devpoll.poll() <select.devpoll.poll>, epoll.poll() <select.epoll.poll>, kqueue.control() <select.kqueue.control>, poll.poll() <select.poll.poll>, ~select.select;
• methods of the ~socket.socket class: ~socket.socket.accept, ~socket.socket.connect (except for non-blocking sockets), ~socket.socket.recv, ~socket.socket.recvfrom, ~socket.socket.recvmsg, ~socket.socket.send, ~socket.socket.sendall, ~socket.socket.sendmsg, ~socket.socket.sendto;
• signal.sigtimedwait and signal.sigwaitinfo;
• time.sleep.

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.

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.

PEP 485: A function for testing approximate equality

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

PEP 486: Make the Python Launcher aware of virtual environments

486 makes the Windows launcher (see 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.

PEP 488: Elimination of PYO files

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.

PEP 489: Multi-phase extension module initialization

489 updates extension module initialization to take advantage of the two step module loading mechanism introduced by 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.

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 19676 and 22286.)
• The -b option now affects comparisons of bytes with int. (Contributed by Serhiy Storchaka in 23681.)
• New Kazakh kz1048 and Tajik koi8_t codecs <standard-encodings>. (Contributed by Serhiy Storchaka in 22682 and 22681.)
• Property docstrings are now writable. This is especially useful for collections.namedtuple docstrings. (Contributed by Berker Peksag in 24064.)
• Circular imports involving relative imports are now supported. (Contributed by Brett Cannon and Antoine Pitrou in 17636.)

New Modules

typing

The new typing provisional <provisional API> module provides standard definitions and tools for function type annotations. See Type Hints <whatsnew-pep-484> for more information.

zipapp

The new zipapp module (specified in 441) provides an API and command line tool for creating executable Python Zip Applications, which were introduced in Python 2.6 in 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 23491.

Improved Modules

argparse

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

asyncio

Since the asyncio module is provisional <provisional API>, 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() <asyncio.loop.set_debug> and loop.get_debug() <asyncio.loop.get_debug> methods. (Contributed by Victor Stinner.)
• The proactor event loop now supports SSL. (Contributed by Antoine Pitrou and Victor Stinner in 22560.)
• A new loop.is_closed() <asyncio.loop.is_closed> method to check if the event loop is closed. (Contributed by Victor Stinner in 21326.)
• A new loop.create_task() <asyncio.loop.create_task> to conveniently create and schedule a new ~asyncio.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() <asyncio.WriteTransport.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 ~asyncio.ensure_future. (Contributed by Yury Selivanov.)
• New loop.set_task_factory() <asyncio.loop.set_task_factory> and loop.get_task_factory() <asyncio.loop.get_task_factory> methods to customize the task factory that loop.create_task() <asyncio.loop.create_task> method uses. (Contributed by Yury Selivanov.)
• New Queue.join() <asyncio.Queue.join> and Queue.task_done() <asyncio.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 ~asyncio.ensure_future function and all functions that use it, such as loop.run_until_complete() <asyncio.loop.run_until_complete>, now accept all kinds of awaitable objects <awaitable>. (Contributed by Yury Selivanov.)
• New ~asyncio.run_coroutine_threadsafe function to submit coroutines to event loops from other threads. (Contributed by Vincent Michel.)
• New Transport.is_closing() <asyncio.BaseTransport.is_closing> method to check if the transport is closing or closed. (Contributed by Yury Selivanov.)
• The loop.create_server() <asyncio.loop.create_server> method can now accept a list of hosts. (Contributed by Yann Sionneau.)

Updates in 3.5.2:

• New loop.create_future() <asyncio.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() <asyncio.loop.get_exception_handler> method to get the current exception handler. (Contributed by Yury Selivanov.)
• New StreamReader.readuntil() <asyncio.StreamReader.readuntil> method to read data from the stream until a separator bytes sequence appears. (Contributed by Mark Korenberg.)
• The loop.create_connection() <asyncio.loop.create_connection> and loop.create_server() <asyncio.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) <asyncio.loop.sock_connect> no longer requires the address to be resolved prior to the call. (Contributed by A. Jesse Jiryu Davis.)

bz2

The BZ2Decompressor.decompress <bz2.BZ2Decompressor.decompress> method now accepts an optional max_length argument to limit the maximum size of decompressed data. (Contributed by Nikolaus Rath in 15955.)

cgi

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

cmath

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

code

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

collections

!OrderedDict.items, !OrderedDict.keys, and !OrderedDict.values views now support reversed iteration. (Contributed by Serhiy Storchaka in 19505.)

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

Docstrings produced by ~collections.namedtuple can now be updated:

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

(Contributed by Berker Peksag in 24064.)

The ~collections.UserString class now implements the ~object.__getnewargs__, ~object.__rmod__, ~str.casefold, ~str.format_map, ~str.isprintable, and ~str.maketrans methods to match the corresponding methods of str. (Contributed by Joe Jevnik in 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 23086.)

A new ~collections.abc.Generator abstract base class. (Contributed by Stefan Behnel in 24018.)

New ~collections.abc.Awaitable, ~collections.abc.Coroutine, ~collections.abc.AsyncIterator, and ~collections.abc.AsyncIterable abstract base classes. (Contributed by Yury Selivanov in 24184.)

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

compileall

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

Another new option, -r, allows controlling the maximum recursion level for subdirectories. (Contributed by Claudiu Popa in 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 ~compileall.compile_dir, ~compileall.compile_file, and ~compileall.compile_path can now accept an integer value indicating the level of output suppression. (Contributed by Thomas Kluyver in 21338.)

concurrent.futures

The Executor.map() <concurrent.futures.Executor.map> method now accepts a chunksize argument to allow batching of tasks to improve performance when ~concurrent.futures.ProcessPoolExecutor is used. (Contributed by Dan O'Reilly in 11271.)

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

configparser

configparser now provides a way to customize the conversion of values by specifying a dictionary of converters in the ~configparser.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 18159.)

contextlib

The new ~contextlib.redirect_stderr context manager (similar to ~contextlib.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 22389.)

csv

The ~csv.csvwriter.writerow method now supports arbitrary iterables, not just sequences. (Contributed by Serhiy Storchaka in 23171.)

curses

The new ~curses.update_lines_cols function updates the ~curses.LINES and ~curses.COLS module variables. This is useful for detecting manual screen resizing. (Contributed by Arnon Yaari in 4254.)

dbm

dumb.open <dbm.dumb.open> always creates a new database when the flag has the value "n". (Contributed by Claudiu Popa in 18039.)

difflib

The charset of HTML documents generated by HtmlDiff.make_file() <difflib.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 2052.)

The ~difflib.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 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 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 16314.)

doctest

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

email

A new policy option Policy.mangle_from_ <email.policy.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 ~email.policy.compat32 and False for all other policies. (Contributed by Milan Oberkirch in 20098.)

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

A new policy option EmailPolicy.utf8 <email.policy.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 6532 and used with an SMTP server that supports the 6531 SMTPUTF8 extension. (Contributed by R. David Murray in 24211.)

The mime.text.MIMEText <email.mime.text.MIMEText> constructor now accepts a charset.Charset <email.charset.Charset> instance. (Contributed by Claude Paroz and Berker Peksag in 16324.)

enum

The ~enum.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 21706.)

faulthandler

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

functools

glob

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

gzip

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

heapq

Element comparison in ~heapq.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 13742.)

http

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

http.client

HTTPConnection.getresponse() <http.client.HTTPConnection.getresponse> now raises a ~http.client.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 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 ~imaplib.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 4972.)

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

imghdr

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

importlib

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

The abc.InspectLoader.source_to_code() <importlib.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 21156.)

The new util.module_from_spec() <importlib.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 20383.)

inspect

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

A new BoundArguments.apply_defaults() <inspect.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 24190.)

A new class method Signature.from_callable() <inspect.Signature.from_callable> makes subclassing of ~inspect.Signature easier. (Contributed by Yury Selivanov and Eric Snow in 17373.)

The ~inspect.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 20691.)

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

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

io

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

ipaddress

Both the ~ipaddress.IPv4Network and ~ipaddress.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 16531.)

A new ~ipaddress.IPv4Address.reverse_pointer attribute for the ~ipaddress.IPv4Address and ~ipaddress.IPv6Address 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 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 21650.)

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

linecache

A new ~linecache.lazycache function can be used to capture information about a non-file-based module to permit getting its lines later via ~linecache.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 17911.)

locale

A new ~locale.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 13918.)

logging

All logging methods (~logging.Logger ~logging.Logger.log, ~logging.Logger.exception, ~logging.Logger.critical, ~logging.Logger.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 20537.)

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

The handlers.QueueListener <logging.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() <lzma.LZMADecompressor.decompress> method now accepts an optional max_length argument to limit the maximum size of decompressed data. (Contributed by Martin Panter in 15955.)

math

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

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

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

multiprocessing

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

operator

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

New ~operator.matmul and ~operator.imatmul functions to perform matrix multiplication. (Contributed by Benjamin Peterson in 21176.)

os

The new ~os.scandir function returning an iterator of ~os.DirEntry objects has been added. If possible, ~os.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 22524.)

On Windows, a new stat_result.st_file_attributes <os.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 21719.)

The ~os.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 22181.)

New ~os.get_blocking and ~os.set_blocking functions allow getting and setting a file descriptor's blocking mode (~os.O_NONBLOCK.) (Contributed by Victor Stinner in 22054.)

The ~os.truncate and ~os.ftruncate functions are now supported on Windows. (Contributed by Steve Dower in 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 10395.)

pathlib

The new Path.samefile() <pathlib.Path.samefile> method can be used to check whether the path points to the same file as another path, which can be either another ~pathlib.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 19775.)

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

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

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

New Path.write_text() <pathlib.Path.write_text>, Path.read_text() <pathlib.Path.read_text>, Path.write_bytes() <pathlib.Path.write_bytes>, Path.read_bytes() <pathlib.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 20218.)

pickle

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

poplib

A new POP3.utf8() <poplib.POP3.utf8> command enables 6856 (Internationalized Email) support, if a POP server supports it. (Contributed by Milan OberKirch in 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 9179.)

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

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

The re.error <re.PatternError> exceptions have new attributes, ~re.PatternError.msg, ~re.PatternError.pattern, ~re.PatternError.pos, ~re.PatternError.lineno, and ~re.PatternError.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 22578.)

readline

A new ~readline.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 22940.)

selectors

The new ~selectors.DevpollSelector supports efficient /dev/poll polling on Solaris. (Contributed by Giampaolo Rodola' in 18931.)

shutil

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

The ~shutil.make_archive function now supports the xztar format. (Contributed by Serhiy Storchaka in 5411.)

signal

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

Various SIG* constants in the signal module have been converted into Enums <enum>. This allows meaningful names to be printed during debugging, instead of integer "magic numbers". (Contributed by Giampaolo Rodola' in 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 19662.)

The !SMTPServer class now advertises the 8BITMIME extension (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 21795.)

The !SMTPServer class now also supports the SMTPUTF8 extension (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 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 14758.)

smtplib

A new SMTP.auth() <smtplib.SMTP.auth> method provides a convenient way to implement custom authentication mechanisms. (Contributed by Milan Oberkirch in 15014.)

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

Both the SMTP.sendmail() <smtplib.SMTP.sendmail> and SMTP.send_message() <smtplib.SMTP.send_message> methods now support 6531 (SMTPUTF8). (Contributed by Milan Oberkirch and R. David Murray in 22027.)

sndhdr

The !what and !whathdr functions now return a ~collections.namedtuple. (Contributed by Claudiu Popa in 18615.)

socket

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

A new socket.sendfile() <socket.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() <socket.socket.send>. (Contributed by Giampaolo Rodola' in 17552.)

The socket.sendall() <socket.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 23853.)

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

ssl

Memory BIO Support

(Contributed by Geert Jansen in 21965.)

The new ~ssl.SSLObject class has been added to provide SSL protocol support for cases when the network I/O capabilities of ~ssl.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 ~ssl.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 ~ssl.SSLSocket's readiness model ("select/poll") is inefficient.

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

Application-Layer Protocol Negotiation Support

(Contributed by Benjamin Peterson in 20188.)

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

The new SSLContext.set_alpn_protocols() <ssl.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() <ssl.SSLSocket.selected_alpn_protocol> returns the protocol that was selected during the TLS handshake. The ~ssl.HAS_ALPN flag indicates whether ALPN support is present.

Other Changes

There is a new SSLSocket.version() <ssl.SSLSocket.version> method to query the actual protocol version in use. (Contributed by Antoine Pitrou in 20421.)

The ~ssl.SSLSocket class now implements a !SSLSocket.sendfile method. (Contributed by Giampaolo Rodola' in 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 20951.)

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

New !SSLObject.shared_ciphers and SSLSocket.shared_ciphers() <ssl.SSLSocket.shared_ciphers> methods return the list of ciphers sent by the client during the handshake. (Contributed by Benjamin Peterson in 23186.)

The SSLSocket.do_handshake() <ssl.SSLSocket.do_handshake>, SSLSocket.read() <ssl.SSLSocket.read>, !SSLSocket.shutdown, and SSLSocket.write() <ssl.SSLSocket.write> methods of the ~ssl.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 23853.)

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

sqlite3

The ~sqlite3.Row class now fully supports the sequence protocol, in particular reversed iteration and slice indexing. (Contributed by Claudiu Popa in 10203; by Lucas Sinclair, Jessica McKellar, and Serhiy Storchaka in 13583.)

subprocess

The new ~subprocess.run function has been added. It runs the specified command and returns a ~subprocess.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 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 <coroutine> 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 <provisional API>, and are intended for debugging purposes only. (Contributed by Yury Selivanov in 24017.)

A new ~sys.is_finalizing function can be used to check if the Python interpreter is shutting down <interpreter shutdown>. (Contributed by Antoine Pitrou in 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 23437.)

tarfile

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

The TarFile.extractall() <tarfile.TarFile.extractall> and TarFile.extract() <tarfile.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 23193.)

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

threading

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

time

The ~time.monotonic function is now always available. (Contributed by Victor Stinner in 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 18983.)

The ~timeit.timeit function has a new globals parameter for specifying the namespace in which the code will be running. (Contributed by Ben Roberts in 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 20035.)

traceback

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

New lightweight classes: ~traceback.TracebackException, ~traceback.StackSummary, and ~traceback.FrameSummary. (Contributed by Robert Collins in 17911.)

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

types

A new ~types.coroutine function to transform generator <generator iterator> and generator-like <collections.abc.Generator> objects into awaitables <awaitable>. (Contributed by Yury Selivanov in 24017.)

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

unicodedata

The unicodedata module now uses data from Unicode 8.0.0.

unittest

The TestLoader.loadTestsFromModule() <unittest.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 16662.)

Unittest discovery errors now are exposed in the TestLoader.errors <unittest.TestLoader.errors> attribute of the ~unittest.TestLoader instance. (Contributed by Robert Collins in 19746.)

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

unittest.mock

The ~unittest.mock.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 21238.)
• A new Mock.assert_not_called() <unittest.mock.Mock.assert_not_called> method to check if the mock object was called. (Contributed by Kushal Das in 21262.)

The ~unittest.mock.MagicMock class now supports ~object.__truediv__, ~object.__divmod__ and ~object.__matmul__ operators. (Contributed by Johannes Baiter in 20968, and Håkan Lövdahl in 23581 and 23568.)

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

urllib

A new request.HTTPPasswordMgrWithPriorAuth <urllib.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 19494 and Akshit Khurana in 7159.)

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

The request.urlopen() <urllib.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 22366.)

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

wsgiref

The headers argument of the headers.Headers <wsgiref.headers.Headers> class constructor is now optional. (Contributed by Pablo Torres Navarrete and SilentGhost in 5800.)

xmlrpc

The client.ServerProxy <xmlrpc.client.ServerProxy> class now supports the context manager protocol. (Contributed by Claudiu Popa in 20627.)

The client.ServerProxy <xmlrpc.client.ServerProxy> constructor now accepts an optional ssl.SSLContext instance. (Contributed by Alex Gaynor in 22960.)

xml.sax

SAX parsers now support a character stream of the xmlreader.InputSource <xml.sax.xmlreader.InputSource> object. (Contributed by Serhiy Storchaka in 2175.)

~xml.sax.parseString now accepts a str instance. (Contributed by Serhiy Storchaka in 10590.)

zipfile

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

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

Other module-level changes

Many functions in the mmap, !ossaudiodev, socket, ssl, and codecs modules now accept writable bytes-like objects <bytes-like object>. (Contributed by Serhiy Storchaka in 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 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 21233.)

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

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

Many operations on io.BytesIO are now 50% to 100% faster. (Contributed by Serhiy Storchaka in 15381 and David Wilson in 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 20416 and 23344.)

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

Regular expressions are now parsed up to 10% faster. (Contributed by Serhiy Storchaka in 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 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 22540.)

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

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

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

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

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

Build and C API Changes

New calloc functions were added:

• PyMem_RawCalloc,
• PyMem_Calloc,
• PyObject_Calloc.

(Contributed by Victor Stinner in 21233.)

New encoding/decoding helper functions:

• Py_DecodeLocale (replaced _Py_char2wchar()),
• Py_EncodeLocale (replaced _Py_wchar2char()).

(Contributed by Victor Stinner in 18395.)

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

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

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

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

New PyNumber_MatrixMultiply and PyNumber_InPlaceMatrixMultiply functions to perform matrix multiplication. (Contributed by Benjamin Peterson in 21176. See also 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 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 <whatsnew-pep-479> 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 ~asyncio.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 ~http.cookies.Morsel.key, ~http.cookies.Morsel.value and ~http.cookies.Morsel.coded_value of http.cookies.Morsel objects is deprecated. Use the ~http.cookies.Morsel.set method instead. In addition, the undocumented LegalChars parameter of ~http.cookies.Morsel.set is deprecated, and is now ignored.

Passing a format string as keyword argument format_string to the ~string.Formatter.format method of the string.Formatter class has been deprecated. (Contributed by Serhiy Storchaka in 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 1322.)

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

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

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

~inspect.getargvalues and ~inspect.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 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 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 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 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 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

• 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 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 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 21205.)
• The deprecated "strict" mode and argument of ~html.parser.HTMLParser, !HTMLParser.error, and the !HTMLParserError exception have been removed. (Contributed by Ezio Melotti in 15114.) The convert_charrefs argument of ~html.parser.HTMLParser is now True by default. (Contributed by Berker Peksag in 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 16518.)
• If the current directory is set to a directory that no longer exists then FileNotFoundError will no longer be raised and instead ~importlib.machinery.FileFinder.find_spec will return None without caching None in sys.path_importer_cache, which is different than the typical case (22834).
• HTTP status code and messages from http.client and http.server were refactored into a common ~http.HTTPStatus enum. The values in http.client and http.server remain available for backwards compatibility. (Contributed by Demian Brecht in 21793.)
• When an import loader defines ~importlib.abc.Loader.exec_module it is now expected to also define ~importlib.abc.Loader.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 ~importlib.abc.Loader.create_module to return None. (Contributed by Brett Cannon in 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 22818.)
• The http.cookies.Morsel dict-like interface has been made self consistent: morsel comparison now takes the ~http.cookies.Morsel.key and ~http.cookies.Morsel.value into account, ~http.cookies.Morsel.copy now results in a ~http.cookies.Morsel instance rather than a dict, and ~http.cookies.Morsel.update will now raise an exception if any of the keys in the update dictionary are invalid. In addition, the undocumented LegalChars parameter of ~http.cookies.Morsel.set is deprecated and is now ignored. (Contributed by Demian Brecht in 2211.)
• 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 ~socket.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 5280. Additionally, the return value is always an int. (Contributed by Akira Li in 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 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 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 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 22453.)
• Because the lack of the ~type.__module__ attribute breaks pickling and introspection, a deprecation warning is now raised for builtin types without the ~type.__module__ attribute. This will be an AttributeError in the future. (Contributed by Serhiy Storchaka in 20204.)
• As part of the 492 implementation, the tp_reserved slot of PyTypeObject was replaced with a ~PyTypeObject.tp_as_async slot. Refer to coro-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 23404.)

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 23404.)