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

Python 3.8 was released on October 14th, 2019.

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 tothe 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)

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()fornameinnames
if(clean_name:=normalize('NFC',name))inallowed_names]

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

SeePEP 572for a full description.

(Contributed by Emily Morehouse inbpo-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 byhelp()for C
functions annotated with Larry Hastings’Argument Clinictool.

In the following example, parametersaandbare positional-only,
whilecordcan be positional or keyword, andeorfare
required to be keywords:

deff(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-inpow()function does not accept keyword arguments:

defpow(x,y,z=None,/):
   "Emulate the built in pow() function"
   r=x**y
   returnrifzisNoneelser%z

Another use case is to preclude keyword arguments when the parameter
name is not helpful. For example, the builtinlen()function has
the signaturelen(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 thestatisticsmodule, the
parameter namedistmay be changed in the future. This was made
possible with the following function specification:

defquantiles(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:

>>>deff(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 thecollectionsmodule:

classCounter(dict):

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

SeePEP 570for a full description.

(Contributed by Pablo Galindo inbpo-36540.)

Parallel filesystem cache for compiled bytecode files

The newPYTHONPYCACHEPREFIXsetting (also available as
-Xpycache_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 insys.pycache_prefix
(Noneindicates the default location in__pycache__
subdirectories).

(Contributed by Carl Meyer inbpo-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_DEBUGmacro no longer implies thePy_TRACE_REFSmacro, which
introduces the only ABI incompatibility. ThePy_TRACE_REFSmacro, which
adds thesys.getobjects()function and thePYTHONDUMPREFS
environment variable, can be set using the new./configure--with-trace-refs
build option.
(Contributed by Victor Stinner inbpo-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 inbpo-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 inbpo-36722.)

To embed Python into an application, a new--embedoption must be passed to
python3-config--libs--embedto get-lpython3.8(link the application
to libpython). To support both 3.8 and older, trypython3-config--libs
--embed
first and fallback topython3-config--libs(without--embed)
if the previous command fails.

Add a pkg-configpython-3.8-embedmodule to embed Python into an
application:pkg-configpython-3.8-embed--libsincludes-lpython3.8.
To support both 3.8 and older, trypkg-configpython-X.Y-embed--libsfirst
and fallback topkg-configpython-X.Y--libs(without--embed) if the
previous command fails (replaceX.Ywith the Python version).

On the other hand,pkg-configpython3.8--libsno 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 inbpo-36721.)

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

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

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

The usualf-string format specifiersallow 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 inbpo-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.

SeePEP 578for full details.

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.

SeePEP 590for a full description.

(Contributed by Jeroen Demeyer and Mark Shannon inbpo-36974.)

Pickle protocol 5 with out-of-band data buffers

Whenpickleis 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.

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

SeePEP 574for a full description.

(Contributed by Antoine Pitrou inbpo-36785.)

Other Language Changes

  • Acontinuestatement was illegal in thefinallyclause
    due to a problem with the implementation. In Python 3.8 this restriction
    was lifted.
    (Contributed by Serhiy Storchaka inbpo-32489.)

  • Thebool,int, andfractions.Fractiontypes
    now have anas_integer_ratio()method like that found in
    floatanddecimal.Decimal. This minor API extension
    makes it possible to writenumerator,denominator=
    x.as_integer_ratio()
    and have it work across multiple numeric types.
    (Contributed by Lisa Roach inbpo-33073and Raymond Hettinger in
    bpo-37819.)

  • Constructors ofint,floatandcomplexwill now
    use the__index__()special method, if available and the
    corresponding method__int__(),__float__()
    or__complex__()is not available.
    (Contributed by Serhiy Storchaka inbpo-20092.)

  • Added support ofN{name}escapes inregularexpressions:

    >>>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 inbpo-30688.)

  • Dict and dictviews are now iterable in reversed insertion order using
    reversed(). (Contributed by Rémi Lapeyre inbpo-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 inbpo-34641.)

  • Generalized iterable unpacking inyieldand
    returnstatements no longer requires enclosing parentheses.
    This brings theyieldandreturnsyntax into better agreement with
    normal assignment syntax:

    >>>defparse(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 inbpo-32117.)

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

  • Arithmetic operations between subclasses ofdatetime.dateor
    datetime.datetimeanddatetime.timedeltaobjects 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)
    usesdatetime.timedeltaarithmetic, such as
    datetime.datetime.astimezone().
    (Contributed by Paul Ganssle inbpo-32417.)

  • When the Python interpreter is interrupted by Ctrl-C (SIGINT) and the
    resultingKeyboardInterruptexception 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 inbpo-1054041.)

  • Some advanced styles of programming require updating the
    types.CodeTypeobject 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 newreplace()method makes
    it possible to create a clone with a few altered parameters.

    Here’s an example that alters thestatistics.mean()function to
    prevent thedataparameter from being used as a keyword argument:

    >>>fromstatisticsimportmean
    >>>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 inbpo-37032.)

  • For integers, the three-argument form of thepow()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 inverseof 38
    modulo 137, write:

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

    Modular inverses arise in the solution oflinear Diophantine
    equations
    .
    For example, to find integer solutions for4258𝑥+147𝑦=369,
    first rewrite as4258𝑥369(mod147)then solve:

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

    (Contributed by Mark Dickinson inbpo-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? ')foriinrange(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():nfornameinnames}
    {'martin von löwis': 'Martin von Löwis',
    'łukasz langa': 'Łukasz Langa',
    'walter dörwald': 'Walter Dörwald'}
    

    (Contributed by Jörn Heissler inbpo-35224.)

New Modules

  • The newimportlib.metadatamodule 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.
    >>>
    >>>fromimportlib.metadataimportversion,requires,files
    >>>version('requests')
    '2.22.0'
    >>>list(requires('requests'))
    ['chardet (=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 inbpo-34632.)

Improved Modules

ast

AST nodes now haveend_linenoandend_col_offsetattributes,
which give the precise location of the end of the node. (This only
applies to nodes that havelinenoandcol_offsetattributes.)

New functionast.get_source_segment()returns the source code
for a specific AST node.

(Contributed by Ivan Levkivskyi inbpo-33416.)

Theast.parse()function has some new flags:

  • type_comments=Truecauses it to return the text ofPEP 484and
    PEP 526type comments associated with certain AST nodes;

  • mode='func_type'can be used to parsePEP 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
    asyncandawaitas non-reserved words.)

(Contributed by Guido van Rossum inbpo-35766.)

asyncio

Runningpython-masynciolaunches a natively async REPL. This allows rapid
experimentation with code that has a top-levelawait. There is no
longer a need to directly callasyncio.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 inbpo-37028.)

On Windows, the default event loop is nowProactorEventLoop.
(Contributed by Victor Stinner inbpo-34687.)

ProactorEventLoopnow also supports UDP.
(Contributed by Adam Meily and Andrew Svetlov inbpo-29883.)

ProactorEventLoopcan now be interrupted by
KeyboardInterrupt(“CTRL+C”).
(Contributed by Vladimir Matveev inbpo-23057.)

builtins

Thecompile()built-in has been improved to accept the
ast.PyCF_ALLOW_TOP_LEVEL_AWAITflag. With this new flag passed,
compile()will allow top-levelawait,asyncforandasyncwith
constructs that are usually considered invalid syntax. Asynchronous code object
marked with theCO_COROUTINEflag may then be returned.
(Contributed by Matthias Bussonnier inbpo-34616)

collections

The_asdict()method forcollections.namedtuple()now returns
adictinstead of acollections.OrderedDict. This works because
regular dicts have guaranteed ordering since Python 3.7. If the extra
features ofOrderedDictare required, the suggested remediation is
to cast the result to the desired type:OrderedDict(nt._asdict()).
(Contributed by Raymond Hettinger inbpo-35864.)

curses

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

ctypes

On Windows,CDLLand subclasses now accept awinmodeparameter
to specify flags for the underlyingLoadLibraryExcall. 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 byadd_dll_directory().
(Contributed by Steve Dower inbpo-36085.)

gc

get_objects()can now receive an optionalgenerationparameter
indicating a generation to get objects from.
(Contributed by Pablo Galindo inbpo-36016.)

gettext

Addedpgettext()and its variants.
(Contributed by Franz Glasner, Éric Araujo, and Cheryl Sabella inbpo-2504.)

gzip

Added themtimeparameter togzip.compress()for reproducible output.
(Contributed by Guo Ci Teo inbpo-34898.)

ABadGzipFileexception is now raised instead ofOSError
for certain types of invalid or corrupt gzip files.
(Contributed by Filip Gruszczyński, Michele Orrù, and Zackery Spytz in
bpo-6584.)

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 inbpo-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 inbpo-5680andbpo-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 inbpo-17535.)

The changes above have been backported to 3.7 maintenance releases.

inspect

Theinspect.getdoc()function can now find docstrings for__slots__
if that attribute is adictwhere the values are docstrings.
This provides documentation options similar to what we already have
forproperty(),classmethod(), andstaticmethod():

classAudioClip:
   __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 inbpo-36326.)

io

In development mode (-Xenv) and in debug build, the
io.IOBasefinalizer now logs the exception if theclose()method
fails. The exception is ignored silently by default in release build.
(Contributed by Victor Stinner inbpo-18748.)

math

Added new functionmath.dist()for computing Euclidean distance
between two points. (Contributed by Raymond Hettinger inbpo-33089.)

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

Added new function,math.prod(), as analogous function tosum()
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 inbpo-35606)

Added new functionmath.isqrt()for computing integer square roots.
(Contributed by Mark Dickinson inbpo-36887.)

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

os

Added new functionadd_dll_directory()on Windows for providing
additional search paths for native dependencies when importing extension
modules or loading DLLs usingctypes.
(Contributed by Steve Dower inbpo-36085.)

A newos.memfd_create()function was added to wrap the
memfd_create()syscall.
(Contributed by Zackery Spytz and Christian Heimes inbpo-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, whileos.lstat()will only open reparse points that
identify as “name surrogates” while others are opened as foros.stat().
In all cases,stat_result.st_modewill only haveS_IFLNKset for
symbolic links and not other kinds of reparse points. To identify other kinds
of reparse point, check the newstat_result.st_reparse_tagattribute.

On Windows,os.readlink()is now able to read directory junctions. Note
thatislink()will returnFalsefor directory junctions,
and so code that checksislinkfirst will continue to treat junctions as
directories, while code that handles errors fromos.readlink()may now
treat junctions as links.

(Contributed by Steve Dower inbpo-37834.)

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 inbpo-35900)

pickleextensions subclassing the C-optimizedPickler
can now override the pickling logic of functions and classes by defining the
specialreducer_override()method.
(Contributed by Pierre Glaser and Olivier Grisel inbpo-35900)

plistlib

Added newplistlib.UIDand enabled support for reading and writing
NSKeyedArchiver-encoded binary plists.
(Contributed by Jon Janzen inbpo-26707.)

shutil

shutil.copytree()now accepts a newdirs_exist_okkeyword argument.
(Contributed by Josh Bronson inbpo-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 thetarfilemodule.
(Contributed by C.A.M. Gerlach inbpo-30661.)

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

statistics

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

Addedstatistics.geometric_mean()
(Contributed by Raymond Hettinger inbpo-27181.)

Addedstatistics.multimode()that returns a list of the most
common values. (Contributed by Raymond Hettinger inbpo-35892.)

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

Addedstatistics.NormalDist, a tool for creating
and manipulating normal distributions of a random variable.
(Contributed by Raymond Hettinger inbpo-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 newsys.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 inbpo-36829.)

tarfile

Thetarfilemodule 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 inbpo-36268.)

tokenize

Thetokenizemodule now implicitly emits aNEWLINEtoken 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 inbpo-33899.)

tkinter

Added methodsselection_from(),
selection_present(),
selection_range()and
selection_to()
in thetkinter.Spinboxclass.
(Contributed by Juliette Monsel inbpo-34829.)

Added methodmoveto()
in thetkinter.Canvasclass.
(Contributed by Juliette Monsel inbpo-23831.)

Thetkinter.PhotoImageclass now has
transparency_get()and
transparency_set()methods. (Contributed by
Zackery Spytz inbpo-25451.)

typing

Thetypingmodule incorporates several new features:

unicodedata

Theunicodedatamodule has been upgraded to use theUnicode 12.1.0release.

New functionis_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-32285andbpo-37966).

unittest

AddedAsyncMockto support an asynchronous version ofMock.
Appropriate new assert functions for testing have been added as well.
(Contributed by Lisa Roach inbpo-26467).

AddedaddModuleCleanup()and
addClassCleanup()to unittest to support
cleanups forsetUpModule()and
setUpClass().
(Contributed by Lisa Roach inbpo-24412.)

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

unittestmodule gained support for coroutines to be used as test cases
withunittest.IsolatedAsyncioTestCase.
(Contributed by Andrew Svetlov inbpo-32972.)

Example:

importunittest


classTestRequest(unittest.IsolatedAsyncioTestCase):

   asyncdefasyncSetUp(self):
       self.connection=awaitAsyncConnection()

   asyncdeftest_get(self):
       response=awaitself.connection.get("https://example.com")
       self.assertEqual(response.status_code,200)

   asyncdefasyncTearDown(self):
       awaitself.connection.close()


if__name__=="__main__":
   unittest.main()

venv

venvnow includes anActivate.ps1script on all platforms for
activating virtual environments under PowerShell Core 6.1.
(Contributed by Brett Cannon inbpo-32718.)

weakref

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

xml

As mitigation against DTD and external entity retrieval, the
xml.dom.minidomandxml.saxmodules no longer process
external entities by default.
(Contributed by Christian Heimes inbpo-17239.)

The.find*()methods in thexml.etree.ElementTreemodule
support wildcard searches like{*}tagwhich ignores the namespace
and{namespace}*which returns all tags in the given namespace.
(Contributed by Stefan Behnel inbpo-28238.)

Thexml.etree.ElementTreemodule provides a new function
–xml.etree.ElementTree.canonicalize()that implements C14N 2.0.
(Contributed by Stefan Behnel inbpo-13611.)

The target object ofxml.etree.ElementTree.XMLParsercan
receive namespace declaration events through the new callback methods
start_ns()andend_ns(). Additionally, the
xml.etree.ElementTree.TreeBuildertarget can be configured
to process events about comments and processing instructions to include
them in the generated tree.
(Contributed by Stefan Behnel inbpo-36676andbpo-36673.)

Optimizations

  • Thesubprocessmodule can now use theos.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_fdsis false;

    • preexec_fn,pass_fds,cwdandstart_new_sessionparameters
      are not set;

    • theexecutablepath contains a directory.

    (Contributed by Joannah Nanjekye and Victor Stinner inbpo-35537.)

  • shutil.copyfile(),shutil.copy(),shutil.copy2(),
    shutil.copytree()andshutil.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 Windowsshutil.copyfile()uses a bigger default buffer size (1 MiB
    instead of 16 KiB) and amemoryview()-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.
    SeePlatform-dependent efficient copy operationssection.
    (Contributed by Giampaolo Rodolà inbpo-33671.)

  • shutil.copytree()usesos.scandir()function and all copy
    functions depending from it use cachedos.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 ofos.stat()
    syscalls is reduced by 38% makingshutil.copytree()especially faster
    on network filesystems. (Contributed by Giampaolo Rodolà inbpo-33695.)

  • The default protocol in thepicklemodule 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 onePy_ssize_tmember fromPyGC_Head. All GC tracked
    objects (e.g. tuple, list, dict) size is reduced 4 or 8 bytes.
    (Contributed by Inada Naoki inbpo-33597)

  • uuid.UUIDnow uses__slots__to reduce its memory footprint.
    (Contributed by Wouter Bolsterlee and Tal Einat inbpo-30977)

  • Improved performance ofoperator.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 incollections.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 inbpo-32492.)

  • Thelistconstructor 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 inbpo-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 inbpo-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 inbpo-23867,
    bpo-35582andbpo-36127.)

  • LOAD_GLOBALinstruction 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

  • Defaultsys.abiflagsbecame an empty string: themflag for
    pymalloc became useless (builds with and without pymalloc are ABI compatible)
    and so has been removed. (Contributed by Victor Stinner inbpo-36707.)

    Example of changes:

    • Onlypython3.8program is installed,python3.8mprogram is gone.

    • Onlypython3.8-configscript is installed,python3.8m-configscript
      is gone.

    • Themflag 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.sobecame
      .cpython-38-x86_64-linux-gnu.soin Python 3.8.

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

    • Include/*.hshould be the portable public stable C API.

    • Include/cpython/*.hshould be the unstable C API specific to CPython;
      public API, with some private API prefixed by_Pyor_PY.

    • Include/internal/*.his 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 bymakeinstall.

    (Contributed by Victor Stinner inbpo-35134andbpo-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:

    (Contributed by Victor Stinner inbpo-35059.)

  • ThePyByteArray_Init()andPyByteArray_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 inbpo-35713.)

  • The result ofPyExceptionClass_Name()is now of type
    constchar*rather ofchar*.
    (Contributed by Serhiy Storchaka inbpo-33818.)

  • The duality ofModules/Setup.distandModules/Setuphas been
    removed. Previously, when updating the CPython source tree, one had
    to manually copyModules/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 fromModules/Setupinside 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 inbpo-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 (likeDecimaland
    Fraction). PyNumber_Check()will now return
    1for objects implementing__index__().
    PyNumber_Long(),PyNumber_Float()and
    PyFloat_AsDouble()also now use the__index__()method if
    available.
    (Contributed by Serhiy Storchaka inbpo-36048andbpo-20092.)

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

  • The new functionPyCode_NewWithPosOnlyArgs()allows to create
    code objects likePyCode_New(), but with an extraposonlyargcount
    parameter for indicating the number of positional-only arguments.
    (Contributed by Pablo Galindo inbpo-37221.)

  • Py_SetPath()now setssys.executableto the program full
    path (Py_GetProgramFullPath()) rather than to the program name
    (Py_GetProgramName()).
    (Contributed by Victor Stinner inbpo-38234.)

Deprecated

API and Feature Removals

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

  • Themacpathmodule, deprecated in Python 3.7, has been removed.
    (Contributed by Victor Stinner inbpo-35471.)

  • The functionplatform.popen()has been removed, after having been
    deprecated since Python 3.3: useos.popen()instead.
    (Contributed by Victor Stinner inbpo-35345.)

  • The functiontime.clock()has been removed, after having been
    deprecated since Python 3.3: usetime.perf_counter()or
    time.process_time()instead, depending
    on your requirements, to have well-defined behavior.
    (Contributed by Matthias Bussonnier inbpo-36895.)

  • Thepyvenvscript has been removed in favor ofpython3.8-mvenv
    to help eliminate confusion as to what Python interpreter thepyvenv
    script is tied to. (Contributed by Brett Cannon inbpo-25427.)

  • parse_qs,parse_qsl, andescapeare removed from thecgi
    module. They are deprecated in Python 3.2 or older. They should be imported
    from theurllib.parseandhtmlmodules instead.

  • filemodefunction is removed from thetarfilemodule.
    It is not documented and deprecated since Python 3.3.

  • TheXMLParserconstructor no longer accepts
    thehtmlargument. It never had an effect and was deprecated in Python 3.4.
    All other parameters are nowkeyword-only.
    (Contributed by Serhiy Storchaka inbpo-29209.)

  • Removed thedoctype()method ofXMLParser.
    (Contributed by Serhiy Storchaka inbpo-29209.)

  • “unicode_internal” codec is removed.
    (Contributed by Inada Naoki inbpo-36297.)

  • TheCacheandStatementobjects of thesqlite3module are not
    exposed to the user.
    (Contributed by Aviv Palivoda inbpo-30262.)

  • Thebufsizekeyword argument offileinput.input()and
    fileinput.FileInput()which was ignored and deprecated since Python 3.6
    has been removed.bpo-36952(Contributed by Matthias Bussonnier.)

  • The functionssys.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 (bothyieldandyieldfromclauses) are now disallowed
    in comprehensions and generator expressions (aside from the iterable expression
    in the leftmostforclause).
    (Contributed by Serhiy Storchaka inbpo-10544.)

  • The compiler now produces aSyntaxWarningwhen identity checks
    (isandisnot) 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 inbpo-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 inbpo-35459.)

  • Removed__str__implementations from builtin typesbool,
    int,float,complexand few classes from
    the standard library. They now inherit__str__()fromobject.
    As result, defining the__repr__()method in the subclass of these
    classes will affect their string representation.
    (Contributed by Serhiy Storchaka inbpo-36793.)

  • On AIX,sys.platformdoesn’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 usesys.platform.startswith('aix').
    (Contributed by M. Felt inbpo-36588.)

  • PyEval_AcquireLock()andPyEval_AcquireThread()now
    terminate the current thread if called while the interpreter is
    finalizing, making them consistent withPyEval_RestoreThread(),
    Py_END_ALLOW_THREADS(), andPyGILState_Ensure(). If this
    behavior is not desired, guard the call by checking_Py_IsFinalizing()
    orsys.is_finalizing().
    (Contributed by Joannah Nanjekye inbpo-36475.)

Changes in the Python API

  • Theos.getcwdb()function now uses the UTF-8 encoding on Windows,
    rather than the ANSI code page: seePEP 529for the rationale. The
    function is no longer deprecated on Windows.
    (Contributed by Victor Stinner inbpo-37412.)

  • subprocess.Popencan now useos.posix_spawn()in some cases
    for better performance. On Windows Subsystem for Linux and QEMU User
    Emulation, thePopenconstructor usingos.posix_spawn()no longer raises an
    exception on errors like “missing program”. Instead the child process fails with a
    non-zeroreturncode.
    (Contributed by Joannah Nanjekye and Victor Stinner inbpo-35537.)

  • Thepreexec_fnargument of *subprocess.Popenis no longer
    compatible with subinterpreters. The use of the parameter in a
    subinterpreter now raisesRuntimeError.
    (Contributed by Eric Snow inbpo-34651, modified by Christian Heimes
    inbpo-37951.)

  • Theimap.IMAP4.logout()method no longer ignores silently arbitrary
    exceptions.
    (Contributed by Victor Stinner inbpo-36348.)

  • The functionplatform.popen()has been removed, after having been deprecated since
    Python 3.3: useos.popen()instead.
    (Contributed by Victor Stinner inbpo-35345.)

  • Thestatistics.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
    inbpo-35892.)

  • Theselection()method of the
    tkinter.ttk.Treeviewclass no longer takes arguments. Using it with
    arguments for changing the selection was deprecated in Python 3.6. Use
    specialized methods likeselection_set()for
    changing the selection. (Contributed by Serhiy Storchaka inbpo-31508.)

  • Thewritexml(),toxml()andtoprettyxml()methods of
    xml.dom.minidom, and thewrite()method ofxml.etree,
    now preserve the attribute order specified by the user.
    (Contributed by Diego Rojas and Raymond Hettinger inbpo-34160.)

  • Adbm.dumbdatabase 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 inbpo-32749.)

  • Thedoctype()method defined in a subclass of
    XMLParserwill no longer be called and will
    emit aRuntimeWarninginstead of aDeprecationWarning.
    Define thedoctype()
    method on a target for handling an XML doctype declaration.
    (Contributed by Serhiy Storchaka inbpo-29209.)

  • ARuntimeErroris now raised when the custom metaclass doesn’t
    provide the__classcell__entry in the namespace passed to
    type.__new__. ADeprecationWarningwas emitted in Python
    3.6–3.7. (Contributed by Serhiy Storchaka inbpo-23722.)

  • ThecProfile.Profileclass can now be used as a context
    manager. (Contributed by Scott Sanderson inbpo-29235.)

  • shutil.copyfile(),shutil.copy(),shutil.copy2(),
    shutil.copytree()andshutil.move()use platform-specific
    “fast-copy” syscalls (see
    Platform-dependent efficient copy operationssection).

  • shutil.copyfile()default buffer size on Windows was changed from
    16 KiB to 1 MiB.

  • ThePyGC_Headstruct has changed completely. All code that touched the
    struct member should be rewritten. (Seebpo-33597.)

  • ThePyInterpreterStatestruct has been moved into the “internal”
    header files (specifically Include/internal/pycore_pystate.h). An
    opaquePyInterpreterStateis 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). (Seebpo-35886.)

  • Asyncio tasks can now be named, either by passing thenamekeyword
    argument toasyncio.create_task()or
    thecreate_task()event loop method, or by
    calling theset_name()method on the task object. The
    task name is visible in therepr()output ofasyncio.Taskand
    can also be retrieved using theget_name()method.

  • Themmap.flush()method now returnsNoneon
    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 inbpo-2122.)

  • xml.dom.minidomandxml.saxmodules no longer process
    external entities by default.
    (Contributed by Christian Heimes inbpo-17239.)

  • Deleting a key from a read-onlydbmdatabase (dbm.dumb,
    dbm.gnuordbm.ndbm) raiseserror(dbm.dumb.error,
    dbm.gnu.errorordbm.ndbm.error) instead ofKeyError.
    (Contributed by Xiang Zhang inbpo-33106.)

  • expanduser()on Windows now prefers theUSERPROFILE
    environment variable and does not useHOME, which is not normally
    set for regular user accounts.
    (Contributed by Anthony Sottile inbpo-36264.)

  • The exceptionasyncio.CancelledErrornow inherits from
    BaseExceptionrather than aException.
    (Contributed by Yury Selivanov inbpo-13528.)

  • DLL dependencies for extension modules and DLLs loaded withctypeson
    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,PATHand 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
    foradd_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).
    (Contributed by Steve Dower inbpo-36085.)

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

  • types.CodeTypehas a new parameter in the second position of the
    constructor (posonlyargcount) to support positional-only arguments defined
    inPEP 570. The first argument (argcount) now represents the total
    number of positional arguments (including positional-only arguments). The new
    replace()method oftypes.CodeTypecan be used to make the code
    future-proof.

Changes in the C API

  • ThePyCompilerFlagsstructure got a newcf_feature_version
    field. It should be initialized toPY_MINOR_VERSION. The field is ignored
    by default, and is used if and only ifPyCF_ONLY_ASTflag is set in
    cf_flags.
    (Contributed by Guido van Rossum inbpo-35766.)

  • ThePyEval_ReInitThreads()function has been removed from the C API.
    It should not be called explicitly: usePyOS_AfterFork_Child()
    instead.
    (Contributed by Victor Stinner inbpo-36728.)

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

  • Use of#variants of formats in parsing or building value (e.g.
    PyArg_ParseTuple(),Py_BuildValue(),PyObject_CallFunction(),
    etc.) withoutPY_SSIZE_T_CLEANdefined raisesDeprecationWarningnow.
    It will be removed in 3.10 or 4.0. ReadParsing arguments and building valuesfor detail.
    (Contributed by Inada Naoki inbpo-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()andPyObject_INIT().
    This makes types created throughPyType_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:

    • RemovePy_INCREFon the type object after allocating an
      instance – if any.
      This may happen after callingPyObject_New(),
      PyObject_NewVar(),PyObject_GC_New(),
      PyObject_GC_NewVar(), or any other custom allocator that uses
      PyObject_Init()orPyObject_INIT().

      Example:

      staticfoo_struct*
      foo_new(PyObject*type){
         foo_struct*foo=PyObject_GC_New(foo_struct,(PyTypeObject*)type);
         if(foo==NULL)
             returnNULL;
      #if PY_VERSION_HEX
         // Workaround for Python issue 35810; no longer necessary in Python 3.8
         PY_INCREF(type)
      #endif
         returnfoo;
      }
      
    • Ensure that all customtp_deallocfunctions of heap-allocated types
      decrease the type’s reference count.

      Example:

      staticvoid
      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 inbpo-35810.)

  • ThePy_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 inbpo-33407.)

  • The interpreter does not pretend to support binary compatibility of
    extension types across feature releases, anymore. APyTypeObject
    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 readingtp_finalize).

    (Contributed by Antoine Pitrou inbpo-32388.)

  • ThePyCode_New()has a new parameter in the second position (posonlyargcount)
    to supportPEP 570, indicating the number of positional-only arguments.

  • The functionsPyNode_AddChild()andPyParser_AddToken()now accept
    two additionalintargumentsend_linenoandend_col_offset.

  • Thelibpython38.afile to allow MinGW tools to link directly against
    python38.dllis no longer included in the regular Windows distribution.
    If you require this file, it may be generated with thegendefand
    dlltooltools, 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 installedpythonXY.dllwill depend on the
    installation options and the version and language of Windows. See
    Using Python on Windowsfor more information. The resulting library should be
    placed in the same directory aspythonXY.lib, which is generally the
    libsdirectory under your Python installation.

    (Contributed by Steve Dower inbpo-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 forbreak,continueand
    return.

    Removed opcodesBREAK_LOOP,CONTINUE_LOOP,
    SETUP_LOOPandSETUP_EXCEPT. Added new opcodes
    ROT_FOUR,BEGIN_FINALLY,CALL_FINALLYand
    POP_FINALLY. Changed the behavior ofEND_FINALLY
    andWITH_CLEANUP_START.

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

  • Added new opcodeEND_ASYNC_FORfor handling exceptions raised
    when awaiting a next item in anasyncforloop.
    (Contributed by Serhiy Storchaka inbpo-33041.)

  • TheMAP_ADDnow 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 byPEP 572. (Contributed by Jörn Heissler inbpo-35224.)

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