What's New In Python 3.8
************************

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.


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 **PEP 572** for a full description.

(Contributed by Emily Morehouse in bpo-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 **PEP 570** for a full description.

(Contributed by Pablo Galindo in bpo-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 bpo-33499.)


Debug build uses the same ABI as release build
----------------------------------------------

Python now uses the same ABI whether it's built in release or debug
mode. 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.

Release builds and debug builds are now ABI compatible: 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" build option. (Contributed by Victor Stinner in
bpo-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 bpo-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 bpo-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 bpo-36721.)


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

Added an "=" specifier to *f-string*s. 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:

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

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

   >>> delta = 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 bpo-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 **PEP 578** for full details.


PEP 587: Python Initialization Configuration
--------------------------------------------

The **PEP 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 for the documentation.

See **PEP 587** for a full description.

(Contributed by Victor Stinner in bpo-36763.)


PEP 590: Vectorcall: a fast calling protocol for CPython
--------------------------------------------------------

The Vectorcall Protocol is added to the Python/C API. It is meant to
formalize existing optimizations which were already done for various
classes. Any static type implementing a callable can use this
protocol.

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

See **PEP 590** for a full description.

(Contributed by Jeroen Demeyer, Mark Shannon and Petr Viktorin in
bpo-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 **PEP 3118**-compatible data can be transmitted separately from
the main pickle stream, at the discretion of the communication layer.

See **PEP 574** for a full description.

(Contributed by Antoine Pitrou in bpo-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 bpo-32489.)

* The "bool", "int", and "fractions.Fraction" types now have an
  "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
  bpo-33073 and Raymond Hettinger in bpo-37819.)

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

* Added support of "\N{*name*}" escapes in "regular expressions":

     >>> 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 bpo-30688.)

* Dict and dictviews are now iterable in reversed insertion order
  using "reversed()". (Contributed by Rémi Lapeyre in bpo-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 bpo-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 bpo-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
  bpo-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
  "astimezone()". (Contributed by Paul Ganssle in bpo-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 bpo-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 bpo-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 bpo-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 bpo-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 "__setstate__()" method. (Contributed by Pierre
  Glaser and Olivier Grisel in bpo-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 bpo-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 bpo-33416.)

The "ast.parse()" function has some new flags:

* "type_comments=True" causes it to return the text of **PEP 484** and
  **PEP 526** type comments associated with certain AST nodes;

* "mode='func_type'" can be used to parse **PEP 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 bpo-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 bpo-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 bpo-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
bpo-32528.)

On Windows, the default event loop is now "ProactorEventLoop".
(Contributed by Victor Stinner in bpo-34687.)

"ProactorEventLoop" now also supports UDP. (Contributed by Adam Meily
and Andrew Svetlov in bpo-29883.)

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

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

Asyncio tasks can now be named, either by passing the "name" keyword
argument to "asyncio.create_task()" or the "create_task()" event loop
method, or by calling the "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 "get_name()" method. (Contributed by Alex
Grönholm in bpo-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 bpo-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 bpo-34616)


collections
-----------

The "_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 bpo-35864.)


cProfile
--------

The "cProfile.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 bpo-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 bpo-34003.)


curses
------

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


ctypes
------

On Windows, "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
"add_dll_directory()". (Contributed by Steve Dower in bpo-36085.)


datetime
--------

Added new alternate constructors "datetime.date.fromisocalendar()" and
"datetime.datetime.fromisocalendar()", which construct "date" and
"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 bpo-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 bpo-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 bpo-21145)

Added a new "functools.singledispatchmethod()" decorator that converts
methods into *generic functions* 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 bpo-32380)


gc
--

"get_objects()" can now receive an optional *generation* parameter
indicating a generation to get objects from. (Contributed by Pablo
Galindo in bpo-36016.)


gettext
-------

Added "pgettext()" and its variants. (Contributed by Franz Glasner,
Éric Araujo, and Cheryl Sabella in bpo-2504.)


gzip
----

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

A "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 bpo-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 bpo-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 bpo-5680 and
bpo-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 bpo-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 bpo-13153.)

New in 3.8.1:

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

Escape key now closes IDLE completion windows.  (Contributed by Johnny
Najera in bpo-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 bpo-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 bpo-36326.)


io
--

In development mode ("-X" "env") and in 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 bpo-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 bpo-34659.)


json.tool
---------

Add option "--json-lines" to parse every input line as a separate JSON
object. (Contributed by Weipeng Hong in bpo-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 bpo-33897.)


math
----

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

Expanded the "math.hypot()" function to handle multiple dimensions.
Formerly, it only supported the 2-D case. (Contributed by Raymond
Hettinger in bpo-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 bpo-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 bpo-37128, bpo-37178, and bpo-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 bpo-36887.)

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


mmap
----

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


multiprocessing
---------------

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

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


os
--

Added new function "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 bpo-36085.)

A new "os.memfd_create()" function was added to wrap the
"memfd_create()" syscall. (Contributed by Zackery Spytz and Christian
Heimes in bpo-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, "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 "stat_result.st_reparse_tag" attribute.

On Windows, "os.readlink()" is now able to read directory junctions.
Note that "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 bpo-37834.)


os.path
-------

"os.path" functions that return a boolean result like "exists()",
"lexists()", "isdir()", "isfile()", "islink()", and "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 bpo-33721.)

"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 bpo-36264.)

"isdir()" on Windows no longer returns "True" for a link to a non-
existent directory.

"realpath()" on Windows now resolves reparse points, including
symlinks and directory junctions.

(Contributed by Steve Dower in bpo-37834.)


pathlib
-------

"pathlib.Path" methods that return a boolean result like "exists()",
"is_dir()", "is_file()", "is_mount()", "is_symlink()",
"is_block_device()", "is_char_device()", "is_fifo()", "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 bpo-33721.)

Added "pathlib.Path.link_to()" which creates a hard link pointing to a
path. (Contributed by Joannah Nanjekye in bpo-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 "Pickler" can now
override the pickling logic of functions and classes by defining the
special "reducer_override()" method. (Contributed by Pierre Glaser and
Olivier Grisel in bpo-35900.)


plistlib
--------

Added new "plistlib.UID" and enabled support for reading and writing
NSKeyedArchiver-encoded binary plists. (Contributed by Jon Janzen in
bpo-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 bpo-30670.)


py_compile
----------

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


shlex
-----

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


shutil
------

"shutil.copytree()" now accepts a new "dirs_exist_ok" keyword
argument. (Contributed by Josh Bronson in bpo-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 bpo-30661.)

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


socket
------

Added "create_server()" and "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
bpo-17561.)

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


ssl
---

Added "post_handshake_auth" to enable and
"verify_client_post_handshake()" to initiate TLS 1.3 post-handshake
authentication. (Contributed by Christian Heimes in bpo-34670.)


statistics
----------

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

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

Added "statistics.multimode()" that returns a list of the most common
values. (Contributed by Raymond Hettinger in bpo-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 bpo-36546.)

Added "statistics.NormalDist", a tool for creating and manipulating
normal distributions of a random variable. (Contributed by Raymond
Hettinger in bpo-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
bpo-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 bpo-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 bpo-1230540.)

Add a new "threading.get_native_id()" function and a "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"
for more information. (Contributed by Jake Tesler in bpo-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 bpo-33899.)


tkinter
-------

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

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

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


time
----

Added new clock "CLOCK_UPTIME_RAW" for macOS 10.12. (Contributed by
Joannah Nanjekye in bpo-35702.)


typing
------

The "typing" module incorporates several new features:

* A dictionary type with per-key types.  See **PEP 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 **PEP 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 **PEP 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 **PEP 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 "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 bpo-32285 and bpo-37966).


unittest
--------

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

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

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

"unittest" module gained support for coroutines to be used as test
cases with "unittest.IsolatedAsyncioTestCase". (Contributed by Andrew
Svetlov in bpo-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 bpo-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 bpo-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 bpo-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 bpo-28238.)

The "xml.etree.ElementTree" module provides a new function
"–xml.etree.ElementTree.canonicalize()" that implements C14N 2.0.
(Contributed by Stefan Behnel in bpo-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 bpo-36676
and bpo-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 bpo-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 bpo-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 Platform-dependent efficient copy
  operations section. (Contributed by Giampaolo Rodolà in bpo-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 bpo-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 bpo-33597.)

* "uuid.UUID" now uses "__slots__" to reduce its memory footprint.
  (Contributed by Wouter Bolsterlee and Tal Einat in bpo-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 bpo-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 bpo-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
  bpo-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 bpo-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 bpo-23867, bpo-35582 and bpo-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 bpo-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 bpo-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 bpo-35134 and bpo-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 bpo-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 bpo-35713.)

* The result of "PyExceptionClass_Name()" is now of type "const char
  *" rather of "char *". (Contributed by Serhiy Storchaka in
  bpo-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 bpo-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 "__index__()" special method instead of
  "__int__()", if available.  The deprecation warning will be emitted
  for objects with the "__int__()" method but without the
  "__index__()" method (like "Decimal" and "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 bpo-36048 and
  bpo-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 bpo-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 bpo-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
  bpo-38234.)


Deprecated
==========

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

* Deprecated methods "getchildren()" and "getiterator()" in the
  "ElementTree" module now emit a "DeprecationWarning" instead of
  "PendingDeprecationWarning". They will be removed in Python 3.9.
  (Contributed by Serhiy Storchaka in bpo-29209.)

* Passing an object that is not an instance of
  "concurrent.futures.ThreadPoolExecutor" to
  "loop.set_default_executor()" is deprecated and will be prohibited
  in Python 3.9. (Contributed by Elvis Pranskevichus in bpo-34075.)

* The "__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 bpo-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 bpo-36320.)

* "ast" classes "Num", "Str", "Bytes", "NameConstant" and "Ellipsis"
  are considered deprecated and will be removed in future Python
  versions. "Constant" should be used instead. (Contributed by Serhiy
  Storchaka in bpo-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 "visit_Constant()" method to handle all constant nodes.
  (Contributed by Serhiy Storchaka in bpo-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 bpo-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 bpo-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 "output_charset()" and
  "set_output_charset()", and the *codeset* parameter of functions
  "translation()" and "install()" are also deprecated, since they are
  only used for the "l*gettext()" functions. (Contributed by Serhiy
  Storchaka in bpo-33710.)

* The "isAlive()" method of "threading.Thread" has been deprecated.
  (Contributed by Donghee Na in bpo-35283.)

* Many builtin and extension functions that take integer arguments
  will now emit a deprecation warning for "Decimal"s, "Fraction"s and
  any other objects that can be converted to integers only with a loss
  (e.g. that have the "__int__()" method but do not have the
  "__index__()" method).  In future version they will be errors.
  (Contributed by Serhiy Storchaka in bpo-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 "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.
  (Contributed by Serhiy Storchaka in bpo-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 gh-81134.)

* The "macpath" module, deprecated in Python 3.7, has been removed.
  (Contributed by Victor Stinner in bpo-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 bpo-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
  bpo-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
  bpo-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 "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. (Contributed by Serhiy Storchaka in
  bpo-29209.)

* Removed the "doctype()" method of "XMLParser". (Contributed by
  Serhiy Storchaka in bpo-29209.)

* "unicode_internal" codec is removed. (Contributed by Inada Naoki in
  bpo-36297.)

* The "Cache" and "Statement" objects of the "sqlite3" module are not
  exposed to the user. (Contributed by Aviv Palivoda in bpo-30262.)

* The "bufsize" keyword argument of "fileinput.input()" and
  "fileinput.FileInput()" which was ignored and deprecated since
  Python 3.6 has been removed. bpo-36952 (Contributed by Matthias
  Bussonnier.)

* The functions "sys.set_coroutine_wrapper()" and
  "sys.get_coroutine_wrapper()" deprecated in Python 3.7 have been
  removed; bpo-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 bpo-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 bpo-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 bpo-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
  bpo-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 bpo-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 bpo-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 **PEP 529** for the rationale.
  The function is no longer deprecated on Windows. (Contributed by
  Victor Stinner in bpo-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 "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 "returncode".
  (Contributed by Joannah Nanjekye and Victor Stinner in bpo-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 bpo-34651, modified by Christian Heimes in bpo-37951.)

* The "imap.IMAP4.logout()" method no longer silently ignores
  arbitrary exceptions. (Contributed by Victor Stinner in bpo-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 bpo-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
  bpo-35892.)

* The "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 "selection_set()" for changing the selection.  (Contributed by
  Serhiy Storchaka in bpo-31508.)

* The "writexml()", "toxml()" and "toprettyxml()" methods of
  "xml.dom.minidom", and the "write()" method of "xml.etree", now
  preserve the attribute order specified by the user. (Contributed by
  Diego Rojas and Raymond Hettinger in bpo-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
  bpo-32749.)

* The "doctype()" method defined in a subclass of "XMLParser" will no
  longer be called and will emit a "RuntimeWarning" instead of a
  "DeprecationWarning". Define the "doctype()" method on a target for
  handling an XML doctype declaration. (Contributed by Serhiy
  Storchaka in bpo-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 bpo-23722.)

* The "cProfile.Profile" class can now be used as a context manager.
  (Contributed by Scott Sanderson in bpo-29235.)

* "shutil.copyfile()", "shutil.copy()", "shutil.copy2()",
  "shutil.copytree()" and "shutil.move()" use platform-specific "fast-
  copy" syscalls (see 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 bpo-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 bpo-35886.)

* The "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 bpo-2122.)

* "xml.dom.minidom" and "xml.sax" modules no longer process external
  entities by default. (Contributed by Christian Heimes in bpo-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 bpo-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 bpo-32892.)

* "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
  bpo-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 bpo-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 bpo-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 bpo-37027.)

* "asyncio.BufferedProtocol" has graduated to the stable API.

* DLL dependencies for extension modules and DLLs loaded with "ctypes"
  on Windows are now resolved more securely. Only the system paths,
  the directory containing the DLL or PYD file, and directories added
  with "add_dll_directory()" are searched for load-time dependencies.
  Specifically, "PATH" and the current working directory are no longer
  used, and modifications to these will no longer have any effect on
  normal DLL resolution. If your application relies on these
  mechanisms, you should check for "add_dll_directory()" and if it
  exists, use it to add your DLLs directory while loading your
  library. Note that Windows 7 users will need to ensure that Windows
  Update KB2533623 has been installed (this is also verified by the
  installer). (Contributed by Steve Dower in bpo-36085.)

* The header files and functions related to pgen have been removed
  after its replacement by a pure Python implementation. (Contributed
  by Pablo Galindo in bpo-36623.)

* "types.CodeType" has a new parameter in the second position of the
  constructor (*posonlyargcount*) to support positional-only arguments
  defined in **PEP 570**. The first argument (*argcount*) now
  represents the total number of positional arguments (including
  positional-only arguments). The new "replace()" method of
  "types.CodeType" can be used to make the code future-proof.

* The parameter "digestmod" for "hmac.new()" no longer uses the MD5
  digest by default.


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
  bpo-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
  bpo-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 bpo-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 Parsing arguments and
  building values for detail. (Contributed by Inada Naoki in
  bpo-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 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 bpo-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 bpo-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
  "tp_finalize" ("Py_TPFLAGS_HAVE_FINALIZE" is not checked anymore
  before reading "tp_finalize").

  (Contributed by Antoine Pitrou in bpo-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 Python 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 bpo-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
  bpo-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 bpo-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 **PEP 572**. (Contributed by Jörn
  Heissler in bpo-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 bpo-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 bpo-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 gh-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 gh-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 bpo-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 gh-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
gh-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
**RFC 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 bpo-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 "inet_pton()" does not accept
any leading zeros.

(Originally contributed by Christian Heimes in bpo-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 **CVE 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 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 Extraction filters 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 **PEP 706**.)
