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

Editor:
   Raymond Hettinger

This article explains the new features in Python 3.8, compared to 3.7.
For full details, see the changelog.

Prerelease users should be aware that this document is currently in
draft form. It will be updated as Python 3.8 moves towards release, so
it's worth checking back even after reading earlier versions. Some
notable items not yet covered are:

* **PEP 578** - Runtime audit hooks for potentially sensitive
  operations

* "python -m asyncio" runs a natively async REPL


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 "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 "pow()" function does not accept keyword
arguments:

   def pow(x, y, z=None, /):
       "Emulate the built in pow() function"
       r = x ** y
       return r if z is None else r%z

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 except 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 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 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.)


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 extension 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 and Mark Shannon 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. See 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 sally')
     ('SIMPSONS', 'homer', 'marge', 'bart', 'lisa', 'sally')

  (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
  "datetime.datetime.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'}


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 in bpo-34632 by Barry Warsaw and Jason R. Coombs.)


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.)

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.)

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


asyncio
-------

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.)


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.)


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()".


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.)


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.)


gettext
-------

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


idlelib and IDLE
----------------

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.)

Add 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.)

The changes above have been backported to 3.7 maintenance releases.


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)


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.)


gc
--

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


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.)


idlelib and IDLE
----------------

Add optional line numbers for IDLE editor windows. Windows open
without line numbers unless set otherwise in the General tab of the
configuration dialog. (Contributed by Tal Einat and Saimadhav Heblikar
in bpo-17535.)

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.)

The changes above have been backported to 3.7 maintenance releases.


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

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


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]
   >>> (link: http://math.prod) math.prod(likelihoods, start=prior)
   0.126

(Contributed by Pablo Galindo in bpo-35606)

Added new function "math.isqrt()" for computing integer square roots.
(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 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".

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.


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.

"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.


ncurses
-------

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


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)


pickle
------

Reduction methods can now include a 6th item in the tuple they return.
This item should specify a custom state-setting method that's called
instead of the regular "__setstate__" method. (Contributed by Pierre
Glaser and Olivier Grisel in bpo-35900)

"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.)


py_compile
----------

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


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 Rodola 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.)


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.


ssl
---

Added "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
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:

* Protocol definitions.  See **PEP 544**, "typing.Protocol" and
  "typing.runtime_checkable()".  Simple ABCs like "typing.SupportsInt"
  are now "Protocol" subclasses.

* A dictionary type with per-key types.  See **PEP 589** and
  "typing.TypedDict".

* Literal types.  See **PEP 586** and "typing.Literal".

* "Final" variables, functions, methods and classes.  See **PEP
  591**, "typing.Final" and "typing.final()".

* 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. (Contributed by Max Belanger and David
  Euresti in bpo-32285).


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.)


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.)


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 Rodola' 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 Rodola' 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.

* 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.)


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 emit now 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
  "asyncio.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 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 for the "l*gettext()" functions.

  (Contributed by Serhiy Storchaka in bpo-33710.)

* The "isAlive()" method of "threading.Thread" has been deprecated.
  (Contributed by Dong-hee 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:

* 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, it was
  deprecated since Python 3.3: use "os.popen()" instead. (Contributed
  by Victor Stinner in bpo-35345.)

* The function "time.clock()" has been removed, it was deprecated
  since Python 3.3: use "time.perf_counter()" or "time.process_time()"
  instead, depending on your requirements, to have a 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 "cgi"
  module. They are deprecated from Python 3.2 or older.

* "filemode" function is removed from "tarfile" module. It is not
  documented and deprecated since Python 3.3.

* The "XMLParser" constructor no longer accepts the *html* argument.
  It never had 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, ints).  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 less 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
  they 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, it is recommended to
  always use the "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
  behaviour is not desired, guard the call by checking
  "_Py_IsFinalizing()" or "sys.is_finalizing()".


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, Popen constructor using "os.posix_spawn()" no longer
  raise an exception on errors like missing program, but 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 ignores silently
  arbitrary exceptions.

* The function "platform.popen()" has been removed, it was
  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 the
  "xml.dom.minidom" module, and "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 cause emitting 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.)

* 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.

* The "mmap.flush()" method now returns "None" on success and raises
  an exception on error under all platforms.  Previously, its behavior
  was platform-depended: 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.)

* "expanduser()" on Windows now prefers the "USERPROFILE"
  environment variable and does not use "HOME", which is not normally
  set for regular user accounts.

* 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 KB2533625 has been installed (this is also verified by the
  installer). (See 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). A new "replace()" method of
  "types.CodeType" can be used to make the code future-proof.


Changes in the C API
--------------------

* The "PyCompilerFlags" structure gets a new *cf_feature_version*
  field. It should be initialized to "PY_MINOR_VERSION". The field is
  ignored by default, it is used if and only if "PyCF_ONLY_AST" flag
  is set in *cf_flags*.

* 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 "PyCode_New()" has a new parameter in the second position
  (*posonlyargcount*) to support **PEP 570**, indicating the number of
  positional-only arguments.

* 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 在
  Windows 上使用 Python 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.


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.)
