What's New In Python 3.8

* 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 Git log when researching a change.

Editor
Raymond Hettinger

This article explains the new features in Python 3.8, compared to 3.7. Python 3.8 was released on October 14, 2019. For full details, see the changelog <changelog>.

Summary -- Release highlights

New Features

Assignment expressions

There is new syntax := that assigns values to variables as part of a larger expression. It is affectionately known as "the walrus operator" due to its resemblance to the eyes and tusks of a walrus.

In this example, the assignment expression helps avoid calling len twice:

if (n := len(a)) > 10:
    print(f"List is too long ({n} elements, expected <= 10)")

A similar benefit arises during regular expression matching where match objects are needed twice, once to test whether a match occurred and another to extract a subgroup:

discount = 0.0
if (mo := re.search(r'(\d+)% discount', advertisement)):
    discount = float(mo.group(1)) / 100.0

The operator is also useful with while-loops that compute a value to test loop termination and then need that same value again in the body of the loop:

# Loop over fixed length blocks
while (block := f.read(256)) != '':
    process(block)

Another motivating use case arises in list comprehensions where a value computed in a filtering condition is also needed in the expression body:

[clean_name.title() for name in names
 if (clean_name := normalize('NFC', name)) in allowed_names]

Try to limit use of the walrus operator to clean cases that reduce complexity and improve readability.

See 572 for a full description.

(Contributed by Emily Morehouse in 35224.)

Positional-only parameters

There is a new function parameter syntax / to indicate that some function parameters must be specified positionally and cannot be used as keyword arguments. This is the same notation shown by help() for C functions annotated with Larry Hastings' Argument Clinic tool.

In the following example, parameters a and b are positional-only, while c or d can be positional or keyword, and e or f are required to be keywords:

def f(a, b, /, c, d, *, e, f):
    print(a, b, c, d, e, f)

The following is a valid call:

f(10, 20, 30, d=40, e=50, f=60)

However, these are invalid calls:

f(10, b=20, c=30, d=40, e=50, f=60)   # b cannot be a keyword argument
f(10, 20, 30, 40, 50, f=60)           # e must be a keyword argument

One use case for this notation is that it allows pure Python functions to fully emulate behaviors of existing C coded functions. For example, the built-in divmod function does not accept keyword arguments:

def divmod(a, b, /):
    "Emulate the built in divmod() function"
    return (a // b, a % b)

Another use case is to preclude keyword arguments when the parameter name is not helpful. For example, the builtin len function has the signature len(obj, /). This precludes awkward calls such as:

len(obj='hello')  # The "obj" keyword argument impairs readability

A further benefit of marking a parameter as positional-only is that it allows the parameter name to be changed in the future without risk of breaking client code. For example, in the statistics module, the parameter name dist may be changed in the future. This was made possible with the following function specification:

def quantiles(dist, /, *, n=4, method='exclusive'):
    ...

Since the parameters to the left of / are not exposed as possible keywords, the parameters names remain available for use in **kwargs:

>>> def f(a, b, /, **kwargs):
...     print(a, b, kwargs)
...
>>> f(10, 20, a=1, b=2, c=3)         # a and b are used in two ways
10 20 {'a': 1, 'b': 2, 'c': 3}

This greatly simplifies the implementation of functions and methods that need to accept arbitrary keyword arguments. For example, here is an excerpt from code in the collections module:

class Counter(dict):

    def __init__(self, iterable=None, /, **kwds):
        # Note "iterable" is a possible keyword argument

See 570 for a full description.

(Contributed by Pablo Galindo in 36540.)

Parallel filesystem cache for compiled bytecode files

The new PYTHONPYCACHEPREFIX setting (also available as -X pycache_prefix) configures the implicit bytecode cache to use a separate parallel filesystem tree, rather than the default __pycache__ subdirectories within each source directory.

The location of the cache is reported in sys.pycache_prefix (None indicates the default location in __pycache__ subdirectories).

(Contributed by Carl Meyer in 33499.)

Debug build uses the same ABI as release build

The ABI of Python debug builds <debug-build> is now compatible with Python release builds. On Unix, when Python is built in debug mode, it is now possible to load C extensions built in release mode and C extensions built using the stable ABI. The inverse is not true, as debug builds expose additional symbols not available in release builds.

Defining the Py_DEBUG macro no longer implies the Py_TRACE_REFS macro, which introduces the only ABI incompatibility. The Py_TRACE_REFS macro, which adds the sys.getobjects function and the PYTHONDUMPREFS environment variable, can be set using the new ./configure --with-trace-refs <--with-trace-refs> build option. (Contributed by Victor Stinner in 36465.)

On Unix, C extensions are no longer linked to libpython except on Android and Cygwin. It is now possible for a statically linked Python to load a C extension built using a shared library Python. (Contributed by Victor Stinner in 21536.)

On Unix, when Python is built in debug mode, import now also looks for C extensions compiled in release mode and for C extensions compiled with the stable ABI. (Contributed by Victor Stinner in 36722.)

To embed Python into an application, a new --embed option must be passed to python3-config --libs --embed to get -lpython3.8 (link the application to libpython). To support both 3.8 and older, try python3-config --libs --embed first and fallback to python3-config --libs (without --embed) if the previous command fails.

Add a pkg-config python-3.8-embed module to embed Python into an application: pkg-config python-3.8-embed --libs includes -lpython3.8. To support both 3.8 and older, try pkg-config python-X.Y-embed --libs first and fallback to pkg-config python-X.Y --libs (without --embed) if the previous command fails (replace X.Y with the Python version).

On the other hand, pkg-config python3.8 --libs no longer contains -lpython3.8. C extensions must not be linked to libpython (except on Android and Cygwin, whose cases are handled by the script); this change is backward incompatible on purpose. (Contributed by Victor Stinner in 36721.)

f-strings support = for self-documenting expressions and debugging

Added an = specifier to f-strings. An f-string such as f'{expr=}' will expand to the text of the expression, an equal sign, then the representation of the evaluated expression. For example:

>>> import datetime as dt >>> user = 'eric_idle' >>> member_since = dt.date(1975, 7, 31) >>> f'{user=} {member_since=}' "user='eric_idle' member_since=datetime.date(1975, 7, 31)"

The usual f-string format specifiers <f-strings> allow more control over how the result of the expression is displayed:

>>> delta = dt.date.today() - member_since
>>> f'{user=!s}  {delta.days=:,d}'
'user=eric_idle  delta.days=16,075'

The = specifier will display the whole expression so that calculations can be shown:

>>> print(f'{theta=}  {cos(radians(theta))=:.3f}')
theta=30  cos(radians(theta))=0.866

(Contributed by Eric V. Smith and Larry Hastings in 36817.)

PEP 578: Python Runtime Audit Hooks

The PEP adds an Audit Hook and Verified Open Hook. Both are available from Python and native code, allowing applications and frameworks written in pure Python code to take advantage of extra notifications, while also allowing embedders or system administrators to deploy builds of Python where auditing is always enabled.

See 578 for full details.

PEP 587: Python Initialization Configuration

The 587 adds a new C API to configure the Python Initialization providing finer control on the whole configuration and better error reporting.

New structures:

• PyConfig
• PyPreConfig
• PyStatus
• PyWideStringList

New functions:

• PyConfig_Clear
• PyConfig_InitIsolatedConfig
• PyConfig_InitPythonConfig
• PyConfig_Read
• PyConfig_SetArgv
• PyConfig_SetBytesArgv
• PyConfig_SetBytesString
• PyConfig_SetString
• PyPreConfig_InitIsolatedConfig
• PyPreConfig_InitPythonConfig
• PyStatus_Error
• PyStatus_Exception
• PyStatus_Exit
• PyStatus_IsError
• PyStatus_IsExit
• PyStatus_NoMemory
• PyStatus_Ok
• PyWideStringList_Append
• PyWideStringList_Insert
• Py_BytesMain
• Py_ExitStatusException
• Py_InitializeFromConfig
• Py_PreInitialize
• Py_PreInitializeFromArgs
• Py_PreInitializeFromBytesArgs
• Py_RunMain

This PEP also adds _PyRuntimeState.preconfig (PyPreConfig type) and PyInterpreterState.config (PyConfig type) fields to these internal structures. PyInterpreterState.config becomes the new reference configuration, replacing global configuration variables and other private variables.

See Python Initialization Configuration <init-config> for the documentation.

See 587 for a full description.

(Contributed by Victor Stinner in 36763.)

PEP 590: Vectorcall: a fast calling protocol for CPython

vectorcall is added to the Python/C API. It is meant to formalize existing optimizations which were already done for various classes. Any static type <static-types> implementing a callable can use this protocol.

This is currently provisional. The aim is to make it fully public in Python 3.9.

See 590 for a full description.

(Contributed by Jeroen Demeyer, Mark Shannon and Petr Viktorin in 36974.)

Pickle protocol 5 with out-of-band data buffers

When pickle is used to transfer large data between Python processes in order to take advantage of multi-core or multi-machine processing, it is important to optimize the transfer by reducing memory copies, and possibly by applying custom techniques such as data-dependent compression.

The pickle protocol 5 introduces support for out-of-band buffers where 3118-compatible data can be transmitted separately from the main pickle stream, at the discretion of the communication layer.

See 574 for a full description.

(Contributed by Antoine Pitrou in 36785.)

Other Language Changes

• A continue statement was illegal in the finally clause due to a problem with the implementation. In Python 3.8 this restriction was lifted. (Contributed by Serhiy Storchaka in 32489.)

• The bool, int, and fractions.Fraction types now have an ~int.as_integer_ratio method like that found in float and decimal.Decimal. This minor API extension makes it possible to write numerator, denominator = x.as_integer_ratio() and have it work across multiple numeric types. (Contributed by Lisa Roach in 33073 and Raymond Hettinger in 37819.)

• Constructors of int, float and complex will now use the ~object.__index__ special method, if available and the corresponding method ~object.__int__, ~object.__float__ or ~object.__complex__ is not available. (Contributed by Serhiy Storchaka in 20092.)

• Added support of \\N\\{{name}\\} escapes in regular expressions <re>:

  >>> notice = 'Copyright © 2019'
  >>> copyright_year_pattern = re.compile(r'\N{copyright sign}\s*(\d{4})')
  >>> int(copyright_year_pattern.search(notice).group(1))
  2019
  

  (Contributed by Jonathan Eunice and Serhiy Storchaka in 30688.)

• Dict and dictviews are now iterable in reversed insertion order using reversed. (Contributed by Rémi Lapeyre in 33462.)

• The syntax allowed for keyword names in function calls was further restricted. In particular, f((keyword)=arg) is no longer allowed. It was never intended to permit more than a bare name on the left-hand side of a keyword argument assignment term. (Contributed by Benjamin Peterson in 34641.)

• Generalized iterable unpacking in yield and return statements no longer requires enclosing parentheses. This brings the yield and return syntax into better agreement with normal assignment syntax:

  >>> def parse(family):
  ...     lastname, *members = family.split()
  ...     return lastname.upper(), *members
  ...
  >>> parse('simpsons homer marge bart lisa maggie')
  ('SIMPSONS', 'homer', 'marge', 'bart', 'lisa', 'maggie')
  

  (Contributed by David Cuthbert and Jordan Chapman in 32117.)

• When a comma is missed in code such as [(10, 20) (30, 40)], the compiler displays a SyntaxWarning with a helpful suggestion. This improves on just having a TypeError indicating that the first tuple was not callable. (Contributed by Serhiy Storchaka in 15248.)

• Arithmetic operations between subclasses of datetime.date or datetime.datetime and datetime.timedelta objects now return an instance of the subclass, rather than the base class. This also affects the return type of operations whose implementation (directly or indirectly) uses datetime.timedelta arithmetic, such as ~datetime.datetime.astimezone. (Contributed by Paul Ganssle in 32417.)

• When the Python interpreter is interrupted by Ctrl-C (SIGINT) and the resulting KeyboardInterrupt exception is not caught, the Python process now exits via a SIGINT signal or with the correct exit code such that the calling process can detect that it died due to a Ctrl-C. Shells on POSIX and Windows use this to properly terminate scripts in interactive sessions. (Contributed by Google via Gregory P. Smith in 1054041.)

• Some advanced styles of programming require updating the types.CodeType object for an existing function. Since code objects are immutable, a new code object needs to be created, one that is modeled on the existing code object. With 19 parameters, this was somewhat tedious. Now, the new replace() method makes it possible to create a clone with a few altered parameters.

  Here's an example that alters the statistics.mean function to prevent the data parameter from being used as a keyword argument:

  >>> from statistics import mean
  >>> mean(data=[10, 20, 90])
  40
  >>> mean.__code__ = mean.__code__.replace(co_posonlyargcount=1)
  >>> mean(data=[10, 20, 90])
  Traceback (most recent call last):
    ...
  TypeError: mean() got some positional-only arguments passed as keyword arguments: 'data'
  

  (Contributed by Victor Stinner in 37032.)

• For integers, the three-argument form of the pow function now permits the exponent to be negative in the case where the base is relatively prime to the modulus. It then computes a modular inverse to the base when the exponent is -1, and a suitable power of that inverse for other negative exponents. For example, to compute the modular multiplicative inverse of 38 modulo 137, write:

  >>> pow(38, -1, 137)
  119
  >>> 119 * 38 % 137
  1
  

  Modular inverses arise in the solution of linear Diophantine equations. For example, to find integer solutions for 4258𝑥 + 147𝑦 = 369, first rewrite as 4258𝑥 ≡ 369 (mod 147) then solve:

  >>> x = 369 * pow(4258, -1, 147) % 147 >>> y = (4258 * x - 369) // -147 >>> 4258 * x + 147 * y 369

  (Contributed by Mark Dickinson in 36027.)

• Dict comprehensions have been synced-up with dict literals so that the key is computed first and the value second:

  >>> # Dict comprehension
  >>> cast = {input('role? '): input('actor? ') for i in range(2)}
  role? King Arthur
  actor? Chapman
  role? Black Knight
  actor? Cleese
  
  >>> # Dict literal
  >>> cast = {input('role? '): input('actor? ')}
  role? Sir Robin
  actor? Eric Idle
  

  The guaranteed execution order is helpful with assignment expressions because variables assigned in the key expression will be available in the value expression:

  >>> names = ['Martin von Löwis', 'Łukasz Langa', 'Walter Dörwald']
  >>> {(n := normalize('NFC', name)).casefold() : n for name in names}
  {'martin von löwis': 'Martin von Löwis',
   'łukasz langa': 'Łukasz Langa',
   'walter dörwald': 'Walter Dörwald'}
  

  (Contributed by Jörn Heissler in 35224.)

• The object.__reduce__ method can now return a tuple from two to six elements long. Formerly, five was the limit. The new, optional sixth element is a callable with a (obj, state) signature. This allows the direct control over the state-updating behavior of a specific object. If not None, this callable will have priority over the object's ~object.__setstate__ method. (Contributed by Pierre Glaser and Olivier Grisel in 35900.)

New Modules

• The new importlib.metadata module provides (provisional) support for reading metadata from third-party packages. For example, it can extract an installed package's version number, list of entry points, and more:

  >>> # Note following example requires that the popular "requests"
  >>> # package has been installed.
  >>>
  >>> from importlib.metadata import version, requires, files
  >>> version('requests')
  '2.22.0'
  >>> list(requires('requests'))
  ['chardet (<3.1.0,>=3.0.2)']
  >>> list(files('requests'))[:5]
  [PackagePath('requests-2.22.0.dist-info/INSTALLER'),
   PackagePath('requests-2.22.0.dist-info/LICENSE'),
   PackagePath('requests-2.22.0.dist-info/METADATA'),
   PackagePath('requests-2.22.0.dist-info/RECORD'),
   PackagePath('requests-2.22.0.dist-info/WHEEL')]
  

  (Contributed by Barry Warsaw and Jason R. Coombs in 34632.)

Improved Modules

ast

AST nodes now have end_lineno and end_col_offset attributes, which give the precise location of the end of the node. (This only applies to nodes that have lineno and col_offset attributes.)

New function ast.get_source_segment returns the source code for a specific AST node.

(Contributed by Ivan Levkivskyi in 33416.)

The ast.parse function has some new flags:

• type_comments=True causes it to return the text of 484 and 526 type comments associated with certain AST nodes;
• mode='func_type' can be used to parse 484 "signature type comments" (returned for function definition AST nodes);
• feature_version=(3, N) allows specifying an earlier Python 3 version. For example, feature_version=(3, 4) will treat async and await as non-reserved words.

(Contributed by Guido van Rossum in 35766.)

asyncio

asyncio.run has graduated from the provisional to stable API. This function can be used to execute a coroutine and return the result while automatically managing the event loop. For example:

import asyncio

async def main():
    await asyncio.sleep(0)
    return 42

asyncio.run(main())

This is roughly equivalent to:

import asyncio

async def main():
    await asyncio.sleep(0)
    return 42

loop = asyncio.new_event_loop()
asyncio.set_event_loop(loop)
try:
    loop.run_until_complete(main())
finally:
    asyncio.set_event_loop(None)
    loop.close()

The actual implementation is significantly more complex. Thus, asyncio.run should be the preferred way of running asyncio programs.

(Contributed by Yury Selivanov in 32314.)

Running python -m asyncio launches a natively async REPL. This allows rapid experimentation with code that has a top-level await. There is no longer a need to directly call asyncio.run() which would spawn a new event loop on every invocation:

$ python -m asyncio
asyncio REPL 3.8.0
Use "await" directly instead of "asyncio.run()".
Type "help", "copyright", "credits" or "license" for more information.
>>> import asyncio
>>> await asyncio.sleep(10, result='hello')
hello

(Contributed by Yury Selivanov in 37028.)

The exception asyncio.CancelledError now inherits from BaseException rather than Exception and no longer inherits from concurrent.futures.CancelledError. (Contributed by Yury Selivanov in 32528.)

On Windows, the default event loop is now ~asyncio.ProactorEventLoop. (Contributed by Victor Stinner in 34687.)

~asyncio.ProactorEventLoop now also supports UDP. (Contributed by Adam Meily and Andrew Svetlov in 29883.)

~asyncio.ProactorEventLoop can now be interrupted by KeyboardInterrupt ("CTRL+C"). (Contributed by Vladimir Matveev in 23057.)

Added asyncio.Task.get_coro for getting the wrapped coroutine within an asyncio.Task. (Contributed by Alex Grönholm in 36999.)

Asyncio tasks can now be named, either by passing the name keyword argument to asyncio.create_task or the ~asyncio.loop.create_task event loop method, or by calling the ~asyncio.Task.set_name method on the task object. The task name is visible in the repr() output of asyncio.Task and can also be retrieved using the ~asyncio.Task.get_name method. (Contributed by Alex Grönholm in 34270.)

Added support for Happy Eyeballs to asyncio.loop.create_connection. To specify the behavior, two new parameters have been added: happy_eyeballs_delay and interleave. The Happy Eyeballs algorithm improves responsiveness in applications that support IPv4 and IPv6 by attempting to simultaneously connect using both. (Contributed by twisteroid ambassador in 33530.)

builtins

The compile built-in has been improved to accept the ast.PyCF_ALLOW_TOP_LEVEL_AWAIT flag. With this new flag passed, compile will allow top-level await, async for and async with constructs that are usually considered invalid syntax. Asynchronous code object marked with the CO_COROUTINE flag may then be returned. (Contributed by Matthias Bussonnier in 34616)

collections

The ~collections.somenamedtuple._asdict method for collections.namedtuple now returns a dict instead of a collections.OrderedDict. This works because regular dicts have guaranteed ordering since Python 3.7. If the extra features of OrderedDict are required, the suggested remediation is to cast the result to the desired type: OrderedDict(nt._asdict()). (Contributed by Raymond Hettinger in 35864.)

cProfile

The cProfile.Profile <profile.Profile> class can now be used as a context manager. Profile a block of code by running:

import cProfile

with cProfile.Profile() as profiler:
      # code to be profiled
      ...

(Contributed by Scott Sanderson in 29235.)

csv

The csv.DictReader now returns instances of dict instead of a collections.OrderedDict. The tool is now faster and uses less memory while still preserving the field order. (Contributed by Michael Selik in 34003.)

curses

Added a new variable holding structured version information for the underlying ncurses library: ~curses.ncurses_version. (Contributed by Serhiy Storchaka in 31680.)

ctypes

On Windows, ~ctypes.CDLL and subclasses now accept a winmode parameter to specify flags for the underlying LoadLibraryEx call. The default flags are set to only load DLL dependencies from trusted locations, including the path where the DLL is stored (if a full or partial path is used to load the initial DLL) and paths added by ~os.add_dll_directory. (Contributed by Steve Dower in 36085.)

datetime

Added new alternate constructors datetime.date.fromisocalendar and datetime.datetime.fromisocalendar, which construct ~datetime.date and ~datetime.datetime objects respectively from ISO year, week number, and weekday; these are the inverse of each class's isocalendar method. (Contributed by Paul Ganssle in 36004.)

functools

functools.lru_cache can now be used as a straight decorator rather than as a function returning a decorator. So both of these are now supported:

@lru_cache
def f(x):
    ...

@lru_cache(maxsize=256)
def f(x):
    ...

(Contributed by Raymond Hettinger in 36772.)

Added a new functools.cached_property decorator, for computed properties cached for the life of the instance. :

import functools
import statistics

class Dataset:
   def __init__(self, sequence_of_numbers):
      self.data = sequence_of_numbers

   @functools.cached_property
   def variance(self):
      return statistics.variance(self.data)

(Contributed by Carl Meyer in 21145)

Added a new functools.singledispatchmethod decorator that converts methods into generic functions <generic function> using single dispatch:

from functools import singledispatchmethod
from contextlib import suppress

class TaskManager:

    def __init__(self, tasks):
        self.tasks = list(tasks)

    @singledispatchmethod
    def discard(self, value):
        with suppress(ValueError):
            self.tasks.remove(value)

    @discard.register(list)
    def _(self, tasks):
        targets = set(tasks)
        self.tasks = [x for x in self.tasks if x not in targets]

(Contributed by Ethan Smith in 32380)

gc

~gc.get_objects can now receive an optional generation parameter indicating a generation to get objects from. (Contributed by Pablo Galindo in 36016.)

gettext

Added ~gettext.pgettext and its variants. (Contributed by Franz Glasner, Éric Araujo, and Cheryl Sabella in 2504.)

gzip

Added the mtime parameter to gzip.compress for reproducible output. (Contributed by Guo Ci Teo in 34898.)

A ~gzip.BadGzipFile exception is now raised instead of OSError for certain types of invalid or corrupt gzip files. (Contributed by Filip Gruszczyński, Michele Orrù, and Zackery Spytz in 6584.)

IDLE and idlelib

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

Add "Run Customized" to the Run menu to run a module with customized settings. Any command line arguments entered are added to sys.argv. They also re-appear in the box for the next customized run. One can also suppress the normal Shell main module restart. (Contributed by Cheryl Sabella, Terry Jan Reedy, and others in 5680 and 37627.)

Added optional line numbers for IDLE editor windows. Windows open without line numbers unless set otherwise in the General tab of the configuration dialog. Line numbers for an existing window are shown and hidden in the Options menu. (Contributed by Tal Einat and Saimadhav Heblikar in 17535.)

OS native encoding is now used for converting between Python strings and Tcl objects. This allows IDLE to work with emoji and other non-BMP characters. These characters can be displayed or copied and pasted to or from the clipboard. Converting strings from Tcl to Python and back now never fails. (Many people worked on this for eight years but the problem was finally solved by Serhiy Storchaka in 13153.)

New in 3.8.1:

Add option to toggle cursor blink off. (Contributed by Zackery Spytz in 4603.)

Escape key now closes IDLE completion windows. (Contributed by Johnny Najera in 38944.)

The changes above have been backported to 3.7 maintenance releases.

Add keywords to module name completion list. (Contributed by Terry J. Reedy in 37765.)

inspect

The inspect.getdoc function can now find docstrings for __slots__ if that attribute is a dict where the values are docstrings. This provides documentation options similar to what we already have for property, classmethod, and staticmethod:

class AudioClip:
    __slots__ = {'bit_rate': 'expressed in kilohertz to one decimal place',
                 'duration': 'in seconds, rounded up to an integer'}
    def __init__(self, bit_rate, duration):
        self.bit_rate = round(bit_rate / 1000.0, 1)
        self.duration = ceil(duration)

(Contributed by Raymond Hettinger in 36326.)

io

In development mode (-X env) and in debug build <debug-build>, the io.IOBase finalizer now logs the exception if the close() method fails. The exception is ignored silently by default in release build. (Contributed by Victor Stinner in 18748.)

itertools

The itertools.accumulate function added an option initial keyword argument to specify an initial value:

>>> from itertools import accumulate
>>> list(accumulate([10, 5, 30, 15], initial=1000))
[1000, 1010, 1015, 1045, 1060]

(Contributed by Lisa Roach in 34659.)

json.tool

Add option --json-lines to parse every input line as a separate JSON object. (Contributed by Weipeng Hong in 31553.)

logging

Added a force keyword argument to logging.basicConfig. When set to true, any existing handlers attached to the root logger are removed and closed before carrying out the configuration specified by the other arguments.

This solves a long-standing problem. Once a logger or basicConfig() had been called, subsequent calls to basicConfig() were silently ignored. This made it difficult to update, experiment with, or teach the various logging configuration options using the interactive prompt or a Jupyter notebook.

(Suggested by Raymond Hettinger, implemented by Donghee Na, and reviewed by Vinay Sajip in 33897.)

math

Added new function math.dist for computing Euclidean distance between two points. (Contributed by Raymond Hettinger in 33089.)

Expanded the math.hypot function to handle multiple dimensions. Formerly, it only supported the 2-D case. (Contributed by Raymond Hettinger in 33089.)

Added new function, math.prod, as analogous function to sum that returns the product of a 'start' value (default: 1) times an iterable of numbers:

>>> prior = 0.8
>>> likelihoods = [0.625, 0.84, 0.30]
>>> math.prod(likelihoods, start=prior)
0.126

(Contributed by Pablo Galindo in 35606.)

Added two new combinatoric functions math.perm and math.comb:

>>> math.perm(10, 3)    # Permutations of 10 things taken 3 at a time
720
>>> math.comb(10, 3)    # Combinations of 10 things taken 3 at a time
120

(Contributed by Yash Aggarwal, Keller Fuchs, Serhiy Storchaka, and Raymond Hettinger in 37128, 37178, and 35431.)

Added a new function math.isqrt for computing accurate integer square roots without conversion to floating point. The new function supports arbitrarily large integers. It is faster than floor(sqrt(n)) but slower than math.sqrt:

>>> r = 650320427
>>> s = r ** 2
>>> isqrt(s - 1)         # correct
650320426
>>> floor(sqrt(s - 1))   # incorrect
650320427

(Contributed by Mark Dickinson in 36887.)

The function math.factorial no longer accepts arguments that are not int-like. (Contributed by Pablo Galindo in 33083.)

mmap

The mmap.mmap class now has an ~mmap.mmap.madvise method to access the madvise() system call. (Contributed by Zackery Spytz in 32941.)

multiprocessing

Added new multiprocessing.shared_memory module. (Contributed by Davin Potts in 35813.)

On macOS, the spawn start method is now used by default. (Contributed by Victor Stinner in 33725.)

os

Added new function ~os.add_dll_directory on Windows for providing additional search paths for native dependencies when importing extension modules or loading DLLs using ctypes. (Contributed by Steve Dower in 36085.)

A new os.memfd_create function was added to wrap the memfd_create() syscall. (Contributed by Zackery Spytz and Christian Heimes in 26836.)

On Windows, much of the manual logic for handling reparse points (including symlinks and directory junctions) has been delegated to the operating system. Specifically, os.stat will now traverse anything supported by the operating system, while os.lstat will only open reparse points that identify as "name surrogates" while others are opened as for os.stat. In all cases, os.stat_result.st_mode will only have S_IFLNK set for symbolic links and not other kinds of reparse points. To identify other kinds of reparse point, check the new os.stat_result.st_reparse_tag attribute.

On Windows, os.readlink is now able to read directory junctions. Note that ~os.path.islink will return False for directory junctions, and so code that checks islink first will continue to treat junctions as directories, while code that handles errors from os.readlink may now treat junctions as links.

(Contributed by Steve Dower in 37834.)

os.path

os.path functions that return a boolean result like ~os.path.exists, ~os.path.lexists, ~os.path.isdir, ~os.path.isfile, ~os.path.islink, and ~os.path.ismount now return False instead of raising ValueError or its subclasses UnicodeEncodeError and UnicodeDecodeError for paths that contain characters or bytes unrepresentable at the OS level. (Contributed by Serhiy Storchaka in 33721.)

~os.path.expanduser on Windows now prefers the USERPROFILE environment variable and does not use HOME, which is not normally set for regular user accounts. (Contributed by Anthony Sottile in 36264.)

~os.path.isdir on Windows no longer returns True for a link to a non-existent directory.

~os.path.realpath on Windows now resolves reparse points, including symlinks and directory junctions.

(Contributed by Steve Dower in 37834.)

pathlib

pathlib.Path methods that return a boolean result like ~pathlib.Path.exists, ~pathlib.Path.is_dir, ~pathlib.Path.is_file, ~pathlib.Path.is_mount, ~pathlib.Path.is_symlink, ~pathlib.Path.is_block_device, ~pathlib.Path.is_char_device, ~pathlib.Path.is_fifo, ~pathlib.Path.is_socket now return False instead of raising ValueError or its subclass UnicodeEncodeError for paths that contain characters unrepresentable at the OS level. (Contributed by Serhiy Storchaka in 33721.)

Added !pathlib.Path.link_to which creates a hard link pointing to a path. (Contributed by Joannah Nanjekye in 26978) Note that link_to was deprecated in 3.10 and removed in 3.12 in favor of a hardlink_to method added in 3.10 which matches the semantics of the existing symlink_to method.

pickle

pickle extensions subclassing the C-optimized ~pickle.Pickler can now override the pickling logic of functions and classes by defining the special ~pickle.Pickler.reducer_override method. (Contributed by Pierre Glaser and Olivier Grisel in 35900.)

plistlib

Added new plistlib.UID and enabled support for reading and writing NSKeyedArchiver-encoded binary plists. (Contributed by Jon Janzen in 26707.)

pprint

The pprint module added a sort_dicts parameter to several functions. By default, those functions continue to sort dictionaries before rendering or printing. However, if sort_dicts is set to false, the dictionaries retain the order that keys were inserted. This can be useful for comparison to JSON inputs during debugging.

In addition, there is a convenience new function, pprint.pp that is like pprint.pprint but with sort_dicts defaulting to False:

>>> from pprint import pprint, pp
>>> d = dict(source='input.txt', operation='filter', destination='output.txt')
>>> pp(d, width=40)                  # Original order
{'source': 'input.txt',
 'operation': 'filter',
 'destination': 'output.txt'}
>>> pprint(d, width=40)              # Keys sorted alphabetically
{'destination': 'output.txt',
 'operation': 'filter',
 'source': 'input.txt'}

(Contributed by Rémi Lapeyre in 30670.)

py_compile

py_compile.compile now supports silent mode. (Contributed by Joannah Nanjekye in 22640.)

shlex

The new shlex.join function acts as the inverse of shlex.split. (Contributed by Bo Bayles in 32102.)

shutil

shutil.copytree now accepts a new dirs_exist_ok keyword argument. (Contributed by Josh Bronson in 20849.)

shutil.make_archive now defaults to the modern pax (POSIX.1-2001) format for new archives to improve portability and standards conformance, inherited from the corresponding change to the tarfile module. (Contributed by C.A.M. Gerlach in 30661.)

shutil.rmtree on Windows now removes directory junctions without recursively removing their contents first. (Contributed by Steve Dower in 37834.)

socket

Added ~socket.create_server and ~socket.has_dualstack_ipv6 convenience functions to automate the necessary tasks usually involved when creating a server socket, including accepting both IPv4 and IPv6 connections on the same socket. (Contributed by Giampaolo Rodolà in 17561.)

The socket.if_nameindex, socket.if_nametoindex, and socket.if_indextoname functions have been implemented on Windows. (Contributed by Zackery Spytz in 37007.)

ssl

Added ~ssl.SSLContext.post_handshake_auth to enable and ~ssl.SSLSocket.verify_client_post_handshake to initiate TLS 1.3 post-handshake authentication. (Contributed by Christian Heimes in 34670.)

statistics

Added statistics.fmean as a faster, floating-point variant of statistics.mean. (Contributed by Raymond Hettinger and Steven D'Aprano in 35904.)

Added statistics.geometric_mean (Contributed by Raymond Hettinger in 27181.)

Added statistics.multimode that returns a list of the most common values. (Contributed by Raymond Hettinger in 35892.)

Added statistics.quantiles that divides data or a distribution in to equiprobable intervals (e.g. quartiles, deciles, or percentiles). (Contributed by Raymond Hettinger in 36546.)

Added statistics.NormalDist, a tool for creating and manipulating normal distributions of a random variable. (Contributed by Raymond Hettinger in 36018.)

>>> temperature_feb = NormalDist.from_samples([4, 12, -3, 2, 7, 14])
>>> temperature_feb.mean
6.0
>>> temperature_feb.stdev
6.356099432828281

>>> temperature_feb.cdf(3)            # Chance of being under 3 degrees
0.3184678262814532
>>> # Relative chance of being 7 degrees versus 10 degrees
>>> temperature_feb.pdf(7) / temperature_feb.pdf(10)
1.2039930378537762

>>> el_niño = NormalDist(4, 2.5)
>>> temperature_feb += el_niño        # Add in a climate effect
>>> temperature_feb
NormalDist(mu=10.0, sigma=6.830080526611674)

>>> temperature_feb * (9/5) + 32      # Convert to Fahrenheit
NormalDist(mu=50.0, sigma=12.294144947901014)
>>> temperature_feb.samples(3)        # Generate random samples
[7.672102882379219, 12.000027119750287, 4.647488369766392]

sys

Add new sys.unraisablehook function which can be overridden to control how "unraisable exceptions" are handled. It is called when an exception has occurred but there is no way for Python to handle it. For example, when a destructor raises an exception or during garbage collection (gc.collect). (Contributed by Victor Stinner in 36829.)

tarfile

The tarfile module now defaults to the modern pax (POSIX.1-2001) format for new archives, instead of the previous GNU-specific one. This improves cross-platform portability with a consistent encoding (UTF-8) in a standardized and extensible format, and offers several other benefits. (Contributed by C.A.M. Gerlach in 36268.)

threading

Add a new threading.excepthook function which handles uncaught threading.Thread.run exception. It can be overridden to control how uncaught threading.Thread.run exceptions are handled. (Contributed by Victor Stinner in 1230540.)

Add a new threading.get_native_id function and a ~threading.Thread.native_id attribute to the threading.Thread class. These return the native integral Thread ID of the current thread assigned by the kernel. This feature is only available on certain platforms, see get_native_id <threading.get_native_id> for more information. (Contributed by Jake Tesler in 36084.)

tokenize

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

tkinter

Added methods !selection_from, !selection_present, !selection_range and !selection_to in the !tkinter.Spinbox class. (Contributed by Juliette Monsel in 34829.)

Added method !moveto in the !tkinter.Canvas class. (Contributed by Juliette Monsel in 23831.)

The !tkinter.PhotoImage class now has !transparency_get and !transparency_set methods. (Contributed by Zackery Spytz in 25451.)

time

Added new clock ~time.CLOCK_UPTIME_RAW for macOS 10.12. (Contributed by Joannah Nanjekye in 35702.)

typing

The typing module incorporates several new features:

• A dictionary type with per-key types. See 589 and typing.TypedDict. TypedDict uses only string keys. By default, every key is required to be present. Specify "total=False" to allow keys to be optional:

  class Location(TypedDict, total=False):
      lat_long: tuple
      grid_square: str
      xy_coordinate: tuple
  

• Literal types. See 586 and typing.Literal. Literal types indicate that a parameter or return value is constrained to one or more specific literal values:

  def get_status(port: int) -> Literal['connected', 'disconnected']:
      ...
  

• "Final" variables, functions, methods and classes. See 591, typing.Final and typing.final. The final qualifier instructs a static type checker to restrict subclassing, overriding, or reassignment:

  pi: Final[float] = 3.1415926536
  

• Protocol definitions. See 544, typing.Protocol and typing.runtime_checkable. Simple ABCs like typing.SupportsInt are now Protocol subclasses.

• New protocol class typing.SupportsIndex.

• New functions typing.get_origin and typing.get_args.

unicodedata

The unicodedata module has been upgraded to use the Unicode 12.1.0 release.

New function ~unicodedata.is_normalized can be used to verify a string is in a specific normal form, often much faster than by actually normalizing the string. (Contributed by Max Belanger, David Euresti, and Greg Price in 32285 and 37966).

unittest

Added ~unittest.mock.AsyncMock to support an asynchronous version of ~unittest.mock.Mock. Appropriate new assert functions for testing have been added as well. (Contributed by Lisa Roach in 26467).

Added ~unittest.addModuleCleanup and ~unittest.TestCase.addClassCleanup to unittest to support cleanups for ~unittest.setUpModule and ~unittest.TestCase.setUpClass. (Contributed by Lisa Roach in 24412.)

Several mock assert functions now also print a list of actual calls upon failure. (Contributed by Petter Strandmark in 35047.)

unittest module gained support for coroutines to be used as test cases with unittest.IsolatedAsyncioTestCase. (Contributed by Andrew Svetlov in 32972.)

Example:

import unittest


class TestRequest(unittest.IsolatedAsyncioTestCase):

    async def asyncSetUp(self):
        self.connection = await AsyncConnection()

    async def test_get(self):
        response = await self.connection.get("https://example.com")
        self.assertEqual(response.status_code, 200)

    async def asyncTearDown(self):
        await self.connection.close()


if __name__ == "__main__":
    unittest.main()

venv

venv now includes an Activate.ps1 script on all platforms for activating virtual environments under PowerShell Core 6.1. (Contributed by Brett Cannon in 32718.)

weakref

The proxy objects returned by weakref.proxy now support the matrix multiplication operators @ and @= in addition to the other numeric operators. (Contributed by Mark Dickinson in 36669.)

xml

As mitigation against DTD and external entity retrieval, the xml.dom.minidom and xml.sax modules no longer process external entities by default. (Contributed by Christian Heimes in 17239.)

The .find*() methods in the xml.etree.ElementTree module support wildcard searches like {*}tag which ignores the namespace and {namespace}* which returns all tags in the given namespace. (Contributed by Stefan Behnel in 28238.)

The xml.etree.ElementTree module provides a new function ~xml.etree.ElementTree.canonicalize that implements C14N 2.0. (Contributed by Stefan Behnel in 13611.)

The target object of xml.etree.ElementTree.XMLParser can receive namespace declaration events through the new callback methods start_ns() and end_ns(). Additionally, the xml.etree.ElementTree.TreeBuilder target can be configured to process events about comments and processing instructions to include them in the generated tree. (Contributed by Stefan Behnel in 36676 and 36673.)

xmlrpc

xmlrpc.client.ServerProxy now supports an optional headers keyword argument for a sequence of HTTP headers to be sent with each request. Among other things, this makes it possible to upgrade from default basic authentication to faster session authentication. (Contributed by Cédric Krier in 35153.)

Optimizations

• The subprocess module can now use the os.posix_spawn function in some cases for better performance. Currently, it is only used on macOS and Linux (using glibc 2.24 or newer) if all these conditions are met:

  • close_fds is false;
  • preexec_fn, pass_fds, cwd and start_new_session parameters are not set;
  • the executable path contains a directory.

  (Contributed by Joannah Nanjekye and Victor Stinner in 35537.)

• shutil.copyfile, shutil.copy, shutil.copy2, shutil.copytree and shutil.move use platform-specific "fast-copy" syscalls on Linux and macOS in order to copy the file more efficiently. "fast-copy" means that the copying operation occurs within the kernel, avoiding the use of userspace buffers in Python as in "outfd.write(infd.read())". On Windows shutil.copyfile uses a bigger default buffer size (1 MiB instead of 16 KiB) and a memoryview-based variant of shutil.copyfileobj is used. The speedup for copying a 512 MiB file within the same partition is about +26% on Linux, +50% on macOS and +40% on Windows. Also, much less CPU cycles are consumed. See shutil-platform-dependent-efficient-copy-operations section. (Contributed by Giampaolo Rodolà in 33671.)

• shutil.copytree uses os.scandir function and all copy functions depending from it use cached os.stat values. The speedup for copying a directory with 8000 files is around +9% on Linux, +20% on Windows and +30% on a Windows SMB share. Also the number of os.stat syscalls is reduced by 38% making shutil.copytree especially faster on network filesystems. (Contributed by Giampaolo Rodolà in 33695.)

• The default protocol in the pickle module is now Protocol 4, first introduced in Python 3.4. It offers better performance and smaller size compared to Protocol 3 available since Python 3.0.

• Removed one Py_ssize_t member from PyGC_Head. All GC tracked objects (e.g. tuple, list, dict) size is reduced 4 or 8 bytes. (Contributed by Inada Naoki in 33597.)

• uuid.UUID now uses __slots__ to reduce its memory footprint. (Contributed by Wouter Bolsterlee and Tal Einat in 30977)

• Improved performance of operator.itemgetter by 33%. Optimized argument handling and added a fast path for the common case of a single non-negative integer index into a tuple (which is the typical use case in the standard library). (Contributed by Raymond Hettinger in 35664.)

• Sped-up field lookups in collections.namedtuple. They are now more than two times faster, making them the fastest form of instance variable lookup in Python. (Contributed by Raymond Hettinger, Pablo Galindo, and Joe Jevnik, Serhiy Storchaka in 32492.)

• The list constructor does not overallocate the internal item buffer if the input iterable has a known length (the input implements __len__). This makes the created list 12% smaller on average. (Contributed by Raymond Hettinger and Pablo Galindo in 33234.)

• Doubled the speed of class variable writes. When a non-dunder attribute was updated, there was an unnecessary call to update slots. (Contributed by Stefan Behnel, Pablo Galindo Salgado, Raymond Hettinger, Neil Schemenauer, and Serhiy Storchaka in 36012.)

• Reduced an overhead of converting arguments passed to many builtin functions and methods. This sped up calling some simple builtin functions and methods up to 20--50%. (Contributed by Serhiy Storchaka in 23867, 35582 and 36127.)

• LOAD_GLOBAL instruction now uses new "per opcode cache" mechanism. It is about 40% faster now. (Contributed by Yury Selivanov and Inada Naoki in 26219.)

Build and C API Changes

• Default sys.abiflags became an empty string: the m flag for pymalloc became useless (builds with and without pymalloc are ABI compatible) and so has been removed. (Contributed by Victor Stinner in 36707.)

  Example of changes:

  • Only python3.8 program is installed, python3.8m program is gone.
  • Only python3.8-config script is installed, python3.8m-config script is gone.
  • The m flag has been removed from the suffix of dynamic library filenames: extension modules in the standard library as well as those produced and installed by third-party packages, like those downloaded from PyPI. On Linux, for example, the Python 3.7 suffix .cpython-37m-x86_64-linux-gnu.so became .cpython-38-x86_64-linux-gnu.so in Python 3.8.

• The header files have been reorganized to better separate the different kinds of APIs:

  • Include/*.h should be the portable public stable C API.
  • Include/cpython/*.h should be the unstable C API specific to CPython; public API, with some private API prefixed by _Py or _PY.
  • Include/internal/*.h is the private internal C API very specific to CPython. This API comes with no backward compatibility warranty and should not be used outside CPython. It is only exposed for very specific needs like debuggers and profiles which has to access to CPython internals without calling functions. This API is now installed by make install.

  (Contributed by Victor Stinner in 35134 and 35081, work initiated by Eric Snow in Python 3.7.)

• Some macros have been converted to static inline functions: parameter types and return type are well defined, they don't have issues specific to macros, variables have a local scopes. Examples:

  • Py_INCREF, Py_DECREF
  • Py_XINCREF, Py_XDECREF
  • !PyObject_INIT, !PyObject_INIT_VAR
  • Private functions: !_PyObject_GC_TRACK, !_PyObject_GC_UNTRACK, !_Py_Dealloc

  (Contributed by Victor Stinner in 35059.)

• The !PyByteArray_Init and !PyByteArray_Fini functions have been removed. They did nothing since Python 2.7.4 and Python 3.2.0, were excluded from the limited API (stable ABI), and were not documented. (Contributed by Victor Stinner in 35713.)

• The result of PyExceptionClass_Name is now of type const char * rather of char *. (Contributed by Serhiy Storchaka in 33818.)

• The duality of Modules/Setup.dist and Modules/Setup has been removed. Previously, when updating the CPython source tree, one had to manually copy Modules/Setup.dist (inside the source tree) to Modules/Setup (inside the build tree) in order to reflect any changes upstream. This was of a small benefit to packagers at the expense of a frequent annoyance to developers following CPython development, as forgetting to copy the file could produce build failures.

  Now the build system always reads from Modules/Setup inside the source tree. People who want to customize that file are encouraged to maintain their changes in a git fork of CPython or as patch files, as they would do for any other change to the source tree.

  (Contributed by Antoine Pitrou in 32430.)

• Functions that convert Python number to C integer like PyLong_AsLong and argument parsing functions like PyArg_ParseTuple with integer converting format units like 'i' will now use the ~object.__index__ special method instead of ~object.__int__, if available. The deprecation warning will be emitted for objects with the __int__() method but without the __index__() method (like ~decimal.Decimal and ~fractions.Fraction). PyNumber_Check will now return 1 for objects implementing __index__(). PyNumber_Long, PyNumber_Float and PyFloat_AsDouble also now use the __index__() method if available. (Contributed by Serhiy Storchaka in 36048 and 20092.)

• Heap-allocated type objects will now increase their reference count in PyObject_Init (and its parallel macro PyObject_INIT) instead of in PyType_GenericAlloc. Types that modify instance allocation or deallocation may need to be adjusted. (Contributed by Eddie Elizondo in 35810.)

• The new function !PyCode_NewWithPosOnlyArgs allows to create code objects like !PyCode_New, but with an extra posonlyargcount parameter for indicating the number of positional-only arguments. (Contributed by Pablo Galindo in 37221.)

• !Py_SetPath now sets sys.executable to the program full path (!Py_GetProgramFullPath) rather than to the program name (!Py_GetProgramName). (Contributed by Victor Stinner in 38234.)

Deprecated

• The distutils bdist_wininst command is now deprecated, use bdist_wheel (wheel packages) instead. (Contributed by Victor Stinner in 37481.)

• Deprecated methods getchildren() and getiterator() in the ~xml.etree.ElementTree module now emit a DeprecationWarning instead of PendingDeprecationWarning. They will be removed in Python 3.9. (Contributed by Serhiy Storchaka in 29209.)

• Passing an object that is not an instance of concurrent.futures.ThreadPoolExecutor to loop.set_default_executor() <asyncio.loop.set_default_executor> is deprecated and will be prohibited in Python 3.9. (Contributed by Elvis Pranskevichus in 34075.)

• The ~object.__getitem__ methods of xml.dom.pulldom.DOMEventStream, wsgiref.util.FileWrapper and fileinput.FileInput have been deprecated.

  Implementations of these methods have been ignoring their index parameter, and returning the next item instead. (Contributed by Berker Peksag in 9372.)

• The typing.NamedTuple class has deprecated the _field_types attribute in favor of the __annotations__ attribute which has the same information. (Contributed by Raymond Hettinger in 36320.)

• ast classes Num, Str, Bytes, NameConstant and Ellipsis are considered deprecated and will be removed in future Python versions. ~ast.Constant should be used instead. (Contributed by Serhiy Storchaka in 32892.)

• ast.NodeVisitor methods visit_Num(), visit_Str(), visit_Bytes(), visit_NameConstant() and visit_Ellipsis() are deprecated now and will not be called in future Python versions. Add the ~ast.NodeVisitor.visit_Constant method to handle all constant nodes. (Contributed by Serhiy Storchaka in 36917.)

• The !asyncio.coroutine decorator is deprecated and will be removed in version 3.10. Instead of @asyncio.coroutine, use async def instead. (Contributed by Andrew Svetlov in 36921.)

• In asyncio, the explicit passing of a loop argument has been deprecated and will be removed in version 3.10 for the following: asyncio.sleep, asyncio.gather, asyncio.shield, asyncio.wait_for, asyncio.wait, asyncio.as_completed, asyncio.Task, asyncio.Lock, asyncio.Event, asyncio.Condition, asyncio.Semaphore, asyncio.BoundedSemaphore, asyncio.Queue, asyncio.create_subprocess_exec, and asyncio.create_subprocess_shell.

• The explicit passing of coroutine objects to asyncio.wait has been deprecated and will be removed in version 3.11. (Contributed by Yury Selivanov in 34790.)

• The following functions and methods are deprecated in the gettext module: !lgettext, !ldgettext, !lngettext and !ldngettext. They return encoded bytes, and it's possible that you will get unexpected Unicode-related exceptions if there are encoding problems with the translated strings. It's much better to use alternatives which return Unicode strings in Python 3. These functions have been broken for a long time.

  Function !bind_textdomain_codeset, methods !NullTranslations.output_charset and !NullTranslations.set_output_charset, and the codeset parameter of functions ~gettext.translation and ~gettext.install are also deprecated, since they are only used for the l*gettext() functions. (Contributed by Serhiy Storchaka in 33710.)

• The !isAlive method of threading.Thread has been deprecated. (Contributed by Donghee Na in 35283.)

• Many builtin and extension functions that take integer arguments will now emit a deprecation warning for ~decimal.Decimals, ~fractions.Fractions and any other objects that can be converted to integers only with a loss (e.g. that have the ~object.__int__ method but do not have the ~object.__index__ method). In future version they will be errors. (Contributed by Serhiy Storchaka in 36048.)

• Deprecated passing the following arguments as keyword arguments:

  • func in functools.partialmethod, weakref.finalize, profile.Profile.runcall, !cProfile.Profile.runcall, bdb.Bdb.runcall, trace.Trace.runfunc and curses.wrapper.
  • function in unittest.TestCase.addCleanup.
  • fn in the ~concurrent.futures.Executor.submit method of concurrent.futures.ThreadPoolExecutor and concurrent.futures.ProcessPoolExecutor.
  • callback in contextlib.ExitStack.callback, !contextlib.AsyncExitStack.callback and contextlib.AsyncExitStack.push_async_callback.
  • c and typeid in the !create method of !multiprocessing.managers.Server and !multiprocessing.managers.SharedMemoryServer.
  • obj in weakref.finalize.

  In future releases of Python, they will be positional-only <positional-only_parameter>. (Contributed by Serhiy Storchaka in 36492.)

API and Feature Removals

The following features and APIs have been removed from Python 3.8:

• Starting with Python 3.3, importing ABCs from collections was deprecated, and importing should be done from collections.abc. Being able to import from collections was marked for removal in 3.8, but has been delayed to 3.9. (See 81134.)
• The !macpath module, deprecated in Python 3.7, has been removed. (Contributed by Victor Stinner in 35471.)
• The function !platform.popen has been removed, after having been deprecated since Python 3.3: use os.popen instead. (Contributed by Victor Stinner in 35345.)
• The function !time.clock has been removed, after having been deprecated since Python 3.3: use time.perf_counter or time.process_time instead, depending on your requirements, to have well-defined behavior. (Contributed by Matthias Bussonnier in 36895.)
• The pyvenv script has been removed in favor of python3.8 -m venv to help eliminate confusion as to what Python interpreter the pyvenv script is tied to. (Contributed by Brett Cannon in 25427.)
• parse_qs, parse_qsl, and escape are removed from the !cgi module. They are deprecated in Python 3.2 or older. They should be imported from the urllib.parse and html modules instead.
• filemode function is removed from the tarfile module. It is not documented and deprecated since Python 3.3.
• The ~xml.etree.ElementTree.XMLParser constructor no longer accepts the html argument. It never had an effect and was deprecated in Python 3.4. All other parameters are now keyword-only <keyword-only_parameter>. (Contributed by Serhiy Storchaka in 29209.)
• Removed the doctype() method of ~xml.etree.ElementTree.XMLParser. (Contributed by Serhiy Storchaka in 29209.)
• "unicode_internal" codec is removed. (Contributed by Inada Naoki in 36297.)
• The Cache and Statement objects of the sqlite3 module are not exposed to the user. (Contributed by Aviv Palivoda in 30262.)
• The bufsize keyword argument of fileinput.input and fileinput.FileInput which was ignored and deprecated since Python 3.6 has been removed. 36952 (Contributed by Matthias Bussonnier.)
• The functions !sys.set_coroutine_wrapper and !sys.get_coroutine_wrapper deprecated in Python 3.7 have been removed; 36933 (Contributed by Matthias Bussonnier.)

Porting to Python 3.8

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

Changes in Python behavior

• Yield expressions (both yield and yield from clauses) are now disallowed in comprehensions and generator expressions (aside from the iterable expression in the leftmost !for clause). (Contributed by Serhiy Storchaka in 10544.)
• The compiler now produces a SyntaxWarning when identity checks (is and is not) are used with certain types of literals (e.g. strings, numbers). These can often work by accident in CPython, but are not guaranteed by the language spec. The warning advises users to use equality tests (== and !=) instead. (Contributed by Serhiy Storchaka in 34850.)
• The CPython interpreter can swallow exceptions in some circumstances. In Python 3.8 this happens in fewer cases. In particular, exceptions raised when getting the attribute from the type dictionary are no longer ignored. (Contributed by Serhiy Storchaka in 35459.)
• Removed __str__ implementations from builtin types bool, int, float, complex and few classes from the standard library. They now inherit __str__() from object. As result, defining the __repr__() method in the subclass of these classes will affect their string representation. (Contributed by Serhiy Storchaka in 36793.)
• On AIX, sys.platform doesn't contain the major version anymore. It is always 'aix', instead of 'aix3' .. 'aix7'. Since older Python versions include the version number, so it is recommended to always use sys.platform.startswith('aix'). (Contributed by M. Felt in 36588.)
• !PyEval_AcquireLock and !PyEval_AcquireThread now terminate the current thread if called while the interpreter is finalizing, making them consistent with PyEval_RestoreThread, Py_END_ALLOW_THREADS, and PyGILState_Ensure. If this behavior is not desired, guard the call by checking !_Py_IsFinalizing or sys.is_finalizing. (Contributed by Joannah Nanjekye in 36475.)

Changes in the Python API

• The os.getcwdb function now uses the UTF-8 encoding on Windows, rather than the ANSI code page: see 529 for the rationale. The function is no longer deprecated on Windows. (Contributed by Victor Stinner in 37412.)
• subprocess.Popen can now use os.posix_spawn in some cases for better performance. On Windows Subsystem for Linux and QEMU User Emulation, the ~subprocess.Popen constructor using os.posix_spawn no longer raises an exception on errors like "missing program". Instead the child process fails with a non-zero ~subprocess.Popen.returncode. (Contributed by Joannah Nanjekye and Victor Stinner in 35537.)
• The preexec_fn argument of * subprocess.Popen is no longer compatible with subinterpreters. The use of the parameter in a subinterpreter now raises RuntimeError. (Contributed by Eric Snow in 34651, modified by Christian Heimes in 37951.)
• The imaplib.IMAP4.logout method no longer silently ignores arbitrary exceptions. (Contributed by Victor Stinner in 36348.)
• The function !platform.popen has been removed, after having been deprecated since Python 3.3: use os.popen instead. (Contributed by Victor Stinner in 35345.)
• The statistics.mode function no longer raises an exception when given multimodal data. Instead, it returns the first mode encountered in the input data. (Contributed by Raymond Hettinger in 35892.)
• The ~tkinter.ttk.Treeview.selection method of the tkinter.ttk.Treeview class no longer takes arguments. Using it with arguments for changing the selection was deprecated in Python 3.6. Use specialized methods like ~tkinter.ttk.Treeview.selection_set for changing the selection. (Contributed by Serhiy Storchaka in 31508.)
• The ~xml.dom.minidom.Node.writexml, ~xml.dom.minidom.Node.toxml and ~xml.dom.minidom.Node.toprettyxml methods of xml.dom.minidom and the ~xml.etree.ElementTree.ElementTree.write method of xml.etree.ElementTree now preserve the attribute order specified by the user. (Contributed by Diego Rojas and Raymond Hettinger in 34160.)
• A dbm.dumb database opened with flags 'r' is now read-only. dbm.dumb.open with flags 'r' and 'w' no longer creates a database if it does not exist. (Contributed by Serhiy Storchaka in 32749.)
• The doctype() method defined in a subclass of ~xml.etree.ElementTree.XMLParser will no longer be called and will emit a RuntimeWarning instead of a DeprecationWarning. Define the doctype() <xml.etree.ElementTree.TreeBuilder.doctype> method on a target for handling an XML doctype declaration. (Contributed by Serhiy Storchaka in 29209.)
• A RuntimeError is now raised when the custom metaclass doesn't provide the __classcell__ entry in the namespace passed to type.__new__. A DeprecationWarning was emitted in Python 3.6--3.7. (Contributed by Serhiy Storchaka in 23722.)
• The cProfile.Profile <profile.Profile> class can now be used as a context manager. (Contributed by Scott Sanderson in 29235.)
• shutil.copyfile, shutil.copy, shutil.copy2, shutil.copytree and shutil.move use platform-specific "fast-copy" syscalls (see shutil-platform-dependent-efficient-copy-operations section).
• shutil.copyfile default buffer size on Windows was changed from 16 KiB to 1 MiB.
• The PyGC_Head struct has changed completely. All code that touched the struct member should be rewritten. (See 33597.)
• The PyInterpreterState struct has been moved into the "internal" header files (specifically Include/internal/pycore_pystate.h). An opaque PyInterpreterState is still available as part of the public API (and stable ABI). The docs indicate that none of the struct's fields are public, so we hope no one has been using them. However, if you do rely on one or more of those private fields and have no alternative then please open a BPO issue. We'll work on helping you adjust (possibly including adding accessor functions to the public API). (See 35886.)
• The mmap.flush() <mmap.mmap.flush> method now returns None on success and raises an exception on error under all platforms. Previously, its behavior was platform-dependent: a nonzero value was returned on success; zero was returned on error under Windows. A zero value was returned on success; an exception was raised on error under Unix. (Contributed by Berker Peksag in 2122.)
• xml.dom.minidom and xml.sax modules no longer process external entities by default. (Contributed by Christian Heimes in 17239.)
• Deleting a key from a read-only dbm database (dbm.dumb, dbm.gnu or dbm.ndbm) raises !error (dbm.dumb.error, dbm.gnu.error or dbm.ndbm.error) instead of KeyError. (Contributed by Xiang Zhang in 33106.)
• Simplified AST for literals. All constants will be represented as ast.Constant instances. Instantiating old classes Num, Str, Bytes, NameConstant and Ellipsis will return an instance of Constant. (Contributed by Serhiy Storchaka in 32892.)
• ~os.path.expanduser on Windows now prefers the USERPROFILE environment variable and does not use HOME, which is not normally set for regular user accounts. (Contributed by Anthony Sottile in 36264.)
• The exception asyncio.CancelledError now inherits from BaseException rather than Exception and no longer inherits from concurrent.futures.CancelledError. (Contributed by Yury Selivanov in 32528.)
• The function asyncio.wait_for now correctly waits for cancellation when using an instance of asyncio.Task. Previously, upon reaching timeout, it was cancelled and immediately returned. (Contributed by Elvis Pranskevichus in 32751.)
• The function asyncio.BaseTransport.get_extra_info now returns a safe to use socket object when 'socket' is passed to the name parameter. (Contributed by Yury Selivanov in 37027.)
• asyncio.BufferedProtocol has graduated to the stable API.

Changes in the C API

• The PyCompilerFlags structure got a new cf_feature_version field. It should be initialized to PY_MINOR_VERSION. The field is ignored by default, and is used if and only if PyCF_ONLY_AST flag is set in cf_flags. (Contributed by Guido van Rossum in 35766.)

• The !PyEval_ReInitThreads function has been removed from the C API. It should not be called explicitly: use PyOS_AfterFork_Child instead. (Contributed by Victor Stinner in 36728.)

• On Unix, C extensions are no longer linked to libpython except on Android and Cygwin. When Python is embedded, libpython must not be loaded with RTLD_LOCAL, but RTLD_GLOBAL instead. Previously, using RTLD_LOCAL, it was already not possible to load C extensions which were not linked to libpython, like C extensions of the standard library built by the *shared* section of Modules/Setup. (Contributed by Victor Stinner in 21536.)

• Use of # variants of formats in parsing or building value (e.g. PyArg_ParseTuple, Py_BuildValue, PyObject_CallFunction, etc.) without PY_SSIZE_T_CLEAN defined raises DeprecationWarning now. It will be removed in 3.10 or 4.0. Read arg-parsing for detail. (Contributed by Inada Naoki in 36381.)

• Instances of heap-allocated types (such as those created with PyType_FromSpec) hold a reference to their type object. Increasing the reference count of these type objects has been moved from PyType_GenericAlloc to the more low-level functions, PyObject_Init and !PyObject_INIT. This makes types created through PyType_FromSpec behave like other classes in managed code.

  Statically allocated types <static-types> are not affected.

  For the vast majority of cases, there should be no side effect. However, types that manually increase the reference count after allocating an instance (perhaps to work around the bug) may now become immortal. To avoid this, these classes need to call Py_DECREF on the type object during instance deallocation.

  To correctly port these types into 3.8, please apply the following changes:

  • Remove Py_INCREF on the type object after allocating an instance - if any. This may happen after calling PyObject_New, PyObject_NewVar, PyObject_GC_New, PyObject_GC_NewVar, or any other custom allocator that uses PyObject_Init or !PyObject_INIT.

    Example:

    static foo_struct *
    foo_new(PyObject *type) {
        foo_struct *foo = PyObject_GC_New(foo_struct, (PyTypeObject *) type);
        if (foo == NULL)
            return NULL;
    #if PY_VERSION_HEX < 0x03080000
        // Workaround for Python issue 35810; no longer necessary in Python 3.8
        PY_INCREF(type)
    #endif
        return foo;
    }
    

  • Ensure that all custom tp_dealloc functions of heap-allocated types decrease the type's reference count.

    Example:

    static void
    foo_dealloc(foo_struct *instance) {
        PyObject *type = Py_TYPE(instance);
        PyObject_GC_Del(instance);
    #if PY_VERSION_HEX >= 0x03080000
        // This was not needed before Python 3.8 (Python issue 35810)
        Py_DECREF(type);
    #endif
    }
    

  (Contributed by Eddie Elizondo in 35810.)

• The Py_DEPRECATED() macro has been implemented for MSVC. The macro now must be placed before the symbol name.

  Example:

  Py_DEPRECATED(3.8) PyAPI_FUNC(int) Py_OldFunction(void);
  

  (Contributed by Zackery Spytz in 33407.)

• The interpreter does not pretend to support binary compatibility of extension types across feature releases, anymore. A PyTypeObject exported by a third-party extension module is supposed to have all the slots expected in the current Python version, including ~PyTypeObject.tp_finalize (Py_TPFLAGS_HAVE_FINALIZE is not checked anymore before reading ~PyTypeObject.tp_finalize).

  (Contributed by Antoine Pitrou in 32388.)

• The functions !PyNode_AddChild and !PyParser_AddToken now accept two additional int arguments end_lineno and end_col_offset.

• The libpython38.a file to allow MinGW tools to link directly against python38.dll is no longer included in the regular Windows distribution. If you require this file, it may be generated with the gendef and dlltool tools, which are part of the MinGW binutils package:

  gendef - python38.dll > tmp.def
  dlltool --dllname python38.dll --def tmp.def --output-lib libpython38.a
  

  The location of an installed pythonXY.dll will depend on the installation options and the version and language of Windows. See using-on-windows for more information. The resulting library should be placed in the same directory as pythonXY.lib, which is generally the libs directory under your Python installation.

  (Contributed by Steve Dower in 37351.)

CPython bytecode changes

• The interpreter loop has been simplified by moving the logic of unrolling the stack of blocks into the compiler. The compiler emits now explicit instructions for adjusting the stack of values and calling the cleaning-up code for break, continue and return.

  Removed opcodes !BREAK_LOOP, !CONTINUE_LOOP, !SETUP_LOOP and !SETUP_EXCEPT. Added new opcodes !ROT_FOUR, !BEGIN_FINALLY, !CALL_FINALLY and !POP_FINALLY. Changed the behavior of !END_FINALLY and !WITH_CLEANUP_START.

  (Contributed by Mark Shannon, Antoine Pitrou and Serhiy Storchaka in 17611.)

• Added new opcode END_ASYNC_FOR for handling exceptions raised when awaiting a next item in an async for loop. (Contributed by Serhiy Storchaka in 33041.)

• The MAP_ADD now expects the value as the first element in the stack and the key as the second element. This change was made so the key is always evaluated before the value in dictionary comprehensions, as proposed by 572. (Contributed by Jörn Heissler in 35224.)

Demos and Tools

Added a benchmark script for timing various ways to access variables: Tools/scripts/var_access_benchmark.py. (Contributed by Raymond Hettinger in 35884.)

Here's a summary of performance improvements since Python 3.3:

Python version                       3.3     3.4     3.5     3.6     3.7     3.8
--------------                       ---     ---     ---     ---     ---     ---

Variable and attribute read access:
    read_local                       4.0     7.1     7.1     5.4     5.1     3.9
    read_nonlocal                    5.3     7.1     8.1     5.8     5.4     4.4
    read_global                     13.3    15.5    19.0    14.3    13.6     7.6
    read_builtin                    20.0    21.1    21.6    18.5    19.0     7.5
    read_classvar_from_class        20.5    25.6    26.5    20.7    19.5    18.4
    read_classvar_from_instance     18.5    22.8    23.5    18.8    17.1    16.4
    read_instancevar                26.8    32.4    33.1    28.0    26.3    25.4
    read_instancevar_slots          23.7    27.8    31.3    20.8    20.8    20.2
    read_namedtuple                 68.5    73.8    57.5    45.0    46.8    18.4
    read_boundmethod                29.8    37.6    37.9    29.6    26.9    27.7

Variable and attribute write access:
    write_local                      4.6     8.7     9.3     5.5     5.3     4.3
    write_nonlocal                   7.3    10.5    11.1     5.6     5.5     4.7
    write_global                    15.9    19.7    21.2    18.0    18.0    15.8
    write_classvar                  81.9    92.9    96.0   104.6   102.1    39.2
    write_instancevar               36.4    44.6    45.8    40.0    38.9    35.5
    write_instancevar_slots         28.7    35.6    36.1    27.3    26.6    25.7

Data structure read access:
    read_list                       19.2    24.2    24.5    20.8    20.8    19.0
    read_deque                      19.9    24.7    25.5    20.2    20.6    19.8
    read_dict                       19.7    24.3    25.7    22.3    23.0    21.0
    read_strdict                    17.9    22.6    24.3    19.5    21.2    18.9

Data structure write access:
    write_list                      21.2    27.1    28.5    22.5    21.6    20.0
    write_deque                     23.8    28.7    30.1    22.7    21.8    23.5
    write_dict                      25.9    31.4    33.3    29.3    29.2    24.7
    write_strdict                   22.9    28.4    29.9    27.5    25.2    23.1

Stack (or queue) operations:
    list_append_pop                144.2    93.4   112.7    75.4    74.2    50.8
    deque_append_pop                30.4    43.5    57.0    49.4    49.2    42.5
    deque_append_popleft            30.8    43.7    57.3    49.7    49.7    42.8

Timing loop:
    loop_overhead                    0.3     0.5     0.6     0.4     0.3     0.3

The benchmarks were measured on an Intel® Core™ i7-4960HQ processor running the macOS 64-bit builds found at python.org. The benchmark script displays timings in nanoseconds.

Notable changes in Python 3.8.1

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

Notable changes in Python 3.8.2

Fixed a regression with the ignore callback of shutil.copytree. The argument types are now str and List[str] again. (Contributed by Manuel Barkhau and Giampaolo Rodola in 83571.)

Notable changes in Python 3.8.3

The constant values of future flags in the __future__ module are updated in order to prevent collision with compiler flags. Previously PyCF_ALLOW_TOP_LEVEL_AWAIT was clashing with CO_FUTURE_DIVISION. (Contributed by Batuhan Taskaya in 83743)

Notable changes in Python 3.8.8

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

Notable changes in Python 3.8.9

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

Notable changes in Python 3.8.10

macOS 11.0 (Big Sur) and Apple Silicon Mac support

As of 3.8.10, Python now supports building and running on macOS 11 (Big Sur) and on Apple Silicon Macs (based on the ARM64 architecture). A new universal build variant, universal2, is now available to natively support both ARM64 and Intel 64 in one set of executables. Note that support for "weaklinking", building binaries targeted for newer versions of macOS that will also run correctly on older versions by testing at runtime for missing features, is not included in this backport from Python 3.9; to support a range of macOS versions, continue to target for and build on the oldest version in the range.

(Originally contributed by Ronald Oussoren and Lawrence D'Anna in 85272, with fixes by FX Coudert and Eli Rykoff, and backported to 3.8 by Maxime Bélanger and Ned Deily)

Notable changes in Python 3.8.10

urllib.parse

The presence of newline or tab characters in parts of a URL allows for some forms of attacks. Following the WHATWG specification that updates 3986, ASCII newline \n, \r and tab \t characters are stripped from the URL by the parser in urllib.parse preventing such attacks. The removal characters are controlled by a new module level variable urllib.parse._UNSAFE_URL_BYTES_TO_REMOVE. (See 43882)

Notable changes in Python 3.8.12

Changes in the Python API

Starting with Python 3.8.12 the ipaddress module no longer accepts any leading zeros in IPv4 address strings. Leading zeros are ambiguous and interpreted as octal notation by some libraries. For example the legacy function socket.inet_aton treats leading zeros as octal notation. glibc implementation of modern ~socket.inet_pton does not accept any leading zeros.

(Originally contributed by Christian Heimes in 36384, and backported to 3.8 by Achraf Merzouki.)

Notable security feature in 3.8.14

Converting between int and str in bases other than 2 (binary), 4, 8 (octal), 16 (hexadecimal), or 32 such as base 10 (decimal) now raises a ValueError if the number of digits in string form is above a limit to avoid potential denial of service attacks due to the algorithmic complexity. This is a mitigation for 2020-10735. This limit can be configured or disabled by environment variable, command line flag, or sys APIs. See the integer string conversion length limitation <int_max_str_digits> documentation. The default limit is 4300 digits in string form.

Notable changes in 3.8.17

tarfile

• The extraction methods in tarfile, and shutil.unpack_archive, have a new a filter argument that allows limiting tar features than may be surprising or dangerous, such as creating files outside the destination directory. See tarfile-extraction-filter for details. In Python 3.12, use without the filter argument will show a DeprecationWarning. In Python 3.14, the default will switch to 'data'. (Contributed by Petr Viktorin in 706.)