What’s New In Python 3.8

Editor:

Raymond Hettinger

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

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 <= 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()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-indivmod()function does not accept keyword arguments:

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

Another use case is to preclude keyword arguments when the parameter name is not helpful. For example, the 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 anddebug buildsare 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-refsbuild option. (Contributed by Victor Stinner inbpo-36465.)

On Unix, C extensions are no longer linked to lib Python 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 Python 3-config--libs--embedto get-l Python 3.8(link the application to lib Python ). To support both 3.8 and older, tryPython 3-config--libs --embedfirst and fallback toPython 3-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-l Python 3.8. 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-configPython 3.8--libsno longer contains -l Python 3.8.C extensions must not be linked to lib Python (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.

PEP 587: Python Initialization Configuration

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

New structures:

New functions:

This PEP also adds_PyRuntimeState.preconfig(PyPreConfigtype) andPyInterpreterState.config(PyConfigtype) fields to these internal structures.PyInterpreterState.configbecomes the new reference configuration, replacing global configuration variables and other private variables.

SeePython Initialization Configurationfor the documentation.

SeePEP 587for a full description.

(Contributed by Victor Stinner inbpo-36763.)

PEP 590: Vectorcall: a fast calling protocol for CPython

The Vectorcall Protocolis added to the Python/C API. It is meant to formalize existing optimizations which were already done for various classes. Anystatic typeimplementing 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, Mark Shannon and Petr Viktorin 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 of\N{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 maggie')
('SIMPSONS', 'homer', 'marge', 'bart', 'lisa', 'maggie')

    (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 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 as4258𝑥≡369(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.)

  • Theobject.__reduce__()method can now return a tuple from two to six elements long. Formerly, five was the limit. The new, optional sixth element is a callable with a(obj,state)signature. This allows the direct control over the state-updating behavior of a specific object. If notNone,this callable will have priority over the object’s __setstate__()method. (Contributed by Pierre Glaser and Olivier Grisel inbpo-35900.)

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 (<3.1.0,>=3.0.2)']
>>>list(files('requests'))[:5]
[PackagePath('requests-2.22.0.dist-info/INSTALLER'),
PackagePath('requests-2.22.0.dist-info/LICENSE'),
PackagePath('requests-2.22.0.dist-info/METADATA'),
PackagePath('requests-2.22.0.dist-info/RECORD'),
PackagePath('requests-2.22.0.dist-info/WHEEL')]

    (Contributed by Barry Warsaw and Jason R. Coombs 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

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

importasyncio

asyncdefmain():
awaitasyncio.sleep(0)
return42

asyncio.run(main())

This isroughlyequivalent to:

importasyncio

asyncdefmain():
awaitasyncio.sleep(0)
return42

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

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

(Contributed by Yury Selivanov inbpo-32314.)

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.)

The exceptionasyncio.CancelledErrornow inherits from BaseExceptionrather thanExceptionand no longer inherits fromconcurrent.futures.CancelledError. (Contributed by Yury Selivanov inbpo-32528.)

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.)

Addedasyncio.Task.get_coro()for getting the wrapped coroutine within anasyncio.Task. (Contributed by Alex Grönholm inbpo-36999.)

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. (Contributed by Alex Grönholm inbpo-34270.)

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

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

cProfile

ThecProfile.Profileclass can now be used as a context manager. Profile a block of code by running:

importcProfile

withcProfile.Profile()asprofiler:
# code to be profiled
...

(Contributed by Scott Sanderson inbpo-29235.)

csv

Thecsv.DictReadernow returns instances ofdictinstead of acollections.OrderedDict.The tool is now faster and uses less memory while still preserving the field order. (Contributed by Michael Selik inbpo-34003.)

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.)

datetime

Added new alternate constructorsdatetime.date.fromisocalendar()and datetime.datetime.fromisocalendar(),which constructdateand datetimeobjects respectively from ISO year, week number, and weekday; these are the inverse of each class’sisocalendarmethod. (Contributed by Paul Ganssle inbpo-36004.)

functools

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

@lru_cache
deff(x):
...

@lru_cache(maxsize=256)
deff(x):
...

(Contributed by Raymond Hettinger inbpo-36772.)

Added a newfunctools.cached_property()decorator, for computed properties cached for the life of the instance.

importfunctools
importstatistics

classDataset:
def__init__(self,sequence_of_numbers):
self.data=sequence_of_numbers

@functools.cached_property
defvariance(self):
returnstatistics.variance(self.data)

(Contributed by Carl Meyer inbpo-21145)

Added a newfunctools.singledispatchmethod()decorator that converts methods intogeneric functionsusing single dispatch:

fromfunctoolsimportsingledispatchmethod
fromcontextlibimportsuppress

classTaskManager:

def__init__(self,tasks):
self.tasks=list(tasks)

@singledispatchmethod
defdiscard(self,value):
withsuppress(ValueError):
self.tasks.remove(value)

@discard.register(list)
def_(self,tasks):
targets=set(tasks)
self.tasks=[xforxinself.tasksifxnotintargets]

(Contributed by Ethan Smith inbpo-32380)

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 press()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.)

IDLE and idlelib

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

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

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

New in 3.8.1:

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

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

The changes above have been backported to 3.7 maintenance releases.

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

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 indebug 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.)

itertools

Theitertools.accumulate()function added an optioninitialkeyword argument to specify an initial value:

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

(Contributed by Lisa Roach inbpo-34659.)

json.tool

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

logging

Added aforcekeyword argument tologging.basicConfig() When set to true, any existing handlers attached to the root logger are removed and closed before carrying out the configuration specified by the other arguments.

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

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

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 two new combinatoric functionsmath.perm()andmath b():

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

(Contributed by Yash Aggarwal, Keller Fuchs, Serhiy Storchaka, and Raymond Hettinger inbpo-37128,bpo-37178,andbpo-35431.)

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

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

(Contributed by Mark Dickinson inbpo-36887.)

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

mmap

Themmap.mmapclass now has anmadvise()method to access themadvise()system call. (Contributed by Zackery Spytz inbpo-32941.)

multiprocessing

Added newmultiprocessing.shared_memorymodule. (Contributed by Davin Potts inbpo-35813.)

On macOS, thespawnstart method is now used by default. (Contributed by Victor Stinner inbpo-33725.)

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.)

os.path

os.pathfunctions that return a boolean result like exists(),lexists(),isdir(), isfile(),islink(),andismount() now returnFalseinstead of raisingValueErroror its subclasses UnicodeEncodeErrorandUnicodeDecodeErrorfor paths that contain characters or bytes unrepresentable at the OS level. (Contributed by Serhiy Storchaka inbpo-33721.)

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.)

isdir()on Windows no longer returnsTruefor a link to a non-existent directory.

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

(Contributed by Steve Dower inbpo-37834.)

pathlib

pathlib.Pathmethods 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 returnFalseinstead of raising ValueErroror its subclassUnicodeEncodeErrorfor paths that contain characters unrepresentable at the OS level. (Contributed by Serhiy Storchaka inbpo-33721.)

Addedpathlib.Path.link_to()which creates a hard link pointing to a path. (Contributed by Joannah Nanjekye inbpo-26978) Note thatlink_towas deprecated in 3.10 and removed in 3.12 in favor of ahardlink_tomethod added in 3.10 which matches the semantics of the existingsymlink_tomethod.

pickle

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.)

pprint

Thepprintmodule added asort_dictsparameter to several functions. By default, those functions continue to sort dictionaries before rendering or printing. However, ifsort_dictsis set to false, the dictionaries retain the order that keys were inserted. This can be useful for comparison to JSON inputs during debugging.

In addition, there is a convenience new function,pprint.pp()that is likepprint.pprint()but withsort_dictsdefaulting toFalse:

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

(Contributed by Rémi Lapeyre inbpo-30670.)

py_compile

py_compile pile()now supports silent mode. (Contributed by Joannah Nanjekye inbpo-22640.)

shlex

The newshlex.join()function acts as the inverse ofshlex.split(). (Contributed by Bo Bayles inbpo-32102.)

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.)

socket

Addedcreate_server()andhas_dualstack_ipv6() convenience functions to automate the necessary tasks usually involved when creating a server socket, including accepting both IPv4 and IPv6 connections on the same socket. (Contributed by Giampaolo Rodolà inbpo-17561.)

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

ssl

Addedpost_handshake_authto enable and verify_client_post_handshake()to initiate TLS 1.3 post-handshake authentication. (Contributed by Christian Heimes inbpo-34670.)

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.)

threading

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

Add a newthreading.get_native_id()function and anative_id attribute to thethreading.Threadclass. 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_idfor more information. (Contributed by Jake Tesler inbpo-36084.)

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.)

time

Added new clockCLOCK_UPTIME_RAWfor macOS 10.12. (Contributed by Joannah Nanjekye inbpo-35702.)

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 of Mock.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")
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.)

xmlrpc

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

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:

    • OnlyPython 3.8program is installed,Python 3.8mprogram is gone.

    • OnlyPython 3.8-configscript is installed,Python 3.8m-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 .c Python -37m-x86_64-linux-gnu.sobecame .c Python -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/c Python /*.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:

  • Starting with Python 3.3, importing ABCs fromcollectionswas deprecated, and importing should be done fromcollections.abc.Being able to import from collections was marked for removal in 3.8, but has been delayed to 3.9. (Seegh-81134.)

  • 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 ofPython 3.8-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 silently ignores 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.)

  • 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.)

  • Simplified AST for literals. All constants will be represented as ast.Constantinstances. Instantiating old classesNum, Str,Bytes,NameConstantandEllipsiswill return an instance ofConstant. (Contributed by Serhiy Storchaka inbpo-32892.)

  • 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 thanExceptionand no longer inherits fromconcurrent.futures.CancelledError. (Contributed by Yury Selivanov inbpo-32528.)

  • The functionasyncio.wait_for()now correctly waits for cancellation when using an instance ofasyncio.Task.Previously, upon reaching timeout,it was cancelled and immediately returned. (Contributed by Elvis Pranskevichus inbpo-32751.)

  • The functionasyncio.BaseTransport.get_extra_info()now returns a safe to use socket object when ‘socket’ is passed to thenameparameter. (Contributed by Yury Selivanov inbpo-37027.)

  • asyncio.BufferedProtocolhas graduated to the stable API.

  • 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 KB2533623 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.

  • The parameterdigestmodforhmac.new()no longer uses the MD5 digest by default.

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 lib Python except on Android and Cygwin. When Python is embedded,lib Pythonmust 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 tolib Python,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 typesare 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 < 0x03080000
// 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.)

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

  • Thelib Python 38.afile to allow MinGW tools to link directly against Python 38.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-Python 38.dll>tmp.def
dlltool--dllnamePython 38.dll--deftmp.def--output-liblib Python 38.a

    The location of an installedPython XY.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 asPython XY.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.)

Demos and Tools

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

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

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

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

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

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

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

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

Timing loop:
loop_overhead 0.3 0.5 0.6 0.4 0.3 0.3

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

Notable changes in Python 3.8.1

Due to significant security concerns, thereuse_addressparameter of asyncio.loop.create_datagram_endpoint()is no longer supported. This is because of the behavior of the socket optionSO_REUSEADDRin UDP. For more details, see the documentation forloop.create_datagram_endpoint(). (Contributed by Kyle Stanley, Antoine Pitrou, and Yury Selivanov in bpo-37228.)

Notable changes in Python 3.8.2

Fixed a regression with theignorecallback ofshutil.copytree(). The argument types are now str and List[str] again. (Contributed by Manuel Barkhau and Giampaolo Rodola ingh-83571.)

Notable changes in Python 3.8.3

The constant values of future flags in the__future__module are updated in order to prevent collision with compiler flags. Previously PyCF_ALLOW_TOP_LEVEL_AWAITwas clashing withCO_FUTURE_DIVISION. (Contributed by Batuhan Taskaya ingh-83743)

Notable changes in Python 3.8.8

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

Notable changes in Python 3.8.9

A security fix alters theftplib.FTPbehavior to not trust the IPv4 address sent from the remote server when setting up a passive data channel. We reuse the ftp server IP address instead. For unusual code requiring the old behavior, set atrust_server_pasv_ipv4_address attribute on your FTP instance toTrue.(Seegh-87451)

Notable changes in Python 3.8.10

macOS 11.0 (Big Sur) and Apple Silicon Mac support

As of 3.8.10, Python now supports building and running on macOS 11 (Big Sur) and on Apple Silicon Macs (based on theARM64architecture). A new universal build variant,universal2,is now available to natively support bothARM64andIntel64in one set of executables. Note that support for “weaklinking”, building binaries targeted for newer versions of macOS that will also run correctly on older versions by testing at runtime for missing features, is not included in this backport from Python 3.9; to support a range of macOS versions, continue to target for and build on the oldest version in the range.

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

Notable changes in Python 3.8.10

urllib.parse

The presence of newline or tab characters in parts of a URL allows for some forms of attacks. Following the WHATWG specification that updatesRFC 3986, ASCII newline\n,\rand tab\tcharacters are stripped from the URL by the parser inurllib.parsepreventing such attacks. The removal characters are controlled by a new module level variable urllib.parse._UNSAFE_URL_BYTES_TO_REMOVE.(Seebpo-43882)

Notable changes in Python 3.8.12

Changes in the Python API

Starting with Python 3.8.12 theipaddressmodule no longer accepts any leading zeros in IPv4 address strings. Leading zeros are ambiguous and interpreted as octal notation by some libraries. For example the legacy functionsocket.inet_aton()treats leading zeros as octal notation. glibc implementation of moderninet_pton()does not accept any leading zeros.

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

Notable security feature in 3.8.14

Converting betweenintandstrin bases other than 2 (binary), 4, 8 (octal), 16 (hexadecimal), or 32 such as base 10 (decimal) now raises aValueErrorif the number of digits in string form is above a limit to avoid potential denial of service attacks due to the algorithmic complexity. This is a mitigation forCVE 2020-10735. This limit can be configured or disabled by environment variable, command line flag, orsysAPIs. See theinteger string conversion length limitationdocumentation. The default limit is 4300 digits in string form.

Notable changes in 3.8.17

tarfile

  • The extraction methods intarfile,andshutil.unpack_archive(), have a new afilterargument that allows limiting tar features than may be surprising or dangerous, such as creating files outside the destination directory. SeeExtraction filtersfor details. In Python 3.12, use without thefilterargument will show a DeprecationWarning. In Python 3.14, the default will switch to'data'. (Contributed by Petr Viktorin inPEP 706.)