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 14 th, 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 callinglen ()
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=0.0if(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 blockswhile(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()fornameinnamesif(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 desc ription.
(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, parameters (a) and (b) are positional-only, whilecor (D) can be positional or keyword, andeor (f) are 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 argumentf(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 signature(len) obj,/)
. This precludes awkward calls such as:
len(OBJ='hello')# The "obj" keyword argument impairs readability
A further benefit of marking a parameter as positional-only is that it allows the parameter name to be changed in the future without risk of breaking client code. For example, in thestatistics
module, 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 ways10 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 thecollections
module:
(class) (Counter)(dict): def__ init __(self,iterable=None,/,**kwds): # Note "iterable" is a possible keyword argument
See(PEP)for a full description.
(Contributed by Pablo Galindo inbpo – 36540.)
Parallel filesystem cache for compiled bytecode files¶
The newPYTHONPYCACHEPREFIX
setting (also available as- X
pycache_prefix
) configures the implicit bytecode cache to use a separate parallel filesystem tree, rather than the default__ pycache __
subdirectories within each source directory.
The location of the cache is reported insys.pycache_prefix
(None
indicates the default location in__ Pycache __
(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 thePy_DEBUG
macro no longer implies thePy_TRACE_REFS
macro, which introduces the only ABI incompatibility. ThePy_TRACE_REFS
macro, 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- embed
option must be passed toPython3 -config- libs- embed
to 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-embed
module to embed Python into an application:(pkg-configpython-3.8-embed- libs
includes- lpython3.8
. To support both 3.8 and older, trypkg-configpython-XY-embed- libs
first and fallback topkg-configpython-XY- libs
(without- embed
) if the previous command fails (replace)XY
with the Python version).
On the other hand,pkg-configpython3.8- libs
NO longer contains- lpython3.8
. C extensions must not be linked to libpython (except on Android and Cygwin, whose cases are handled by the script); this change is backward incompatible on purpose. (Contributed by Victor Stinner inbpo – 36721.)
f-strings support=
for self-documenting expressions and debugging¶
Added an=
specifier tof-strings. An f-string such asf '{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(),7,31)>>>f'{u ser=} {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 in (bpo -) .)
PEP 578: Python Runtime Audit Hooks¶
The PEP adds an Audit Hook and Verified Open Hook. Both are available from Python and native code, allowing applications and frameworks written in pure Python code to take advantage of extra notifications, while also allowing embedders or system administrators to deploy builds of Python where auditing is always enabled.
See(PEP)for 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.
See(PEP) for a full description.
(Contributed by Jeroen Demeyer and Mark Shannon inbpo – 36974.)
Pickle protocol 5 with out-of-band data buffers¶
Whenpickle
is used to transfer large data between Python processes in order to take advantage of multi-core or multi-machine processing, it is important to optimize the transfer by reducing memory copies, and possibly by applying custom techniques such as data-dependent compression.
Thepickle
protocol 5 introduces support for out-of-band buffers where(PEP)– 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¶
-
A
continue
statement was illegal in thefinally
clause due to a problem with the implementation. In Python 3.8 this restriction was lifted. (Contributed by Serhiy Storchaka inbpo – 32489.) -
The
bool
,int
, andfractions.Fraction
types now have anas_integer_ratio ()
method like that found infloat
anddecimal.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 inbpo – 37819.) -
Constructors of
int
,float
andcomplex
will 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 in
yield
andreturn
statements 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 in (bpo – 32117.)
-
When a comma is missed in code such as
[(10,20)(30,40)]
, the compiler displays aSyntaxWarning
with a helpful suggestion . This improves on just having aTypeError
indicating that the first tuple was not callable. (Contributed by Serhiy Storchaka inbpo – 15248.) -
Arithmetic operations between subclasses of
datetime.date
ordatetime.datetime
anddatetime.timedelta
objects now return an instance of the subclass, rather than the base class. This also affects the return type of operations whose implementation (directly or indirectly) usesdatetime.timedelta
arithmetic, such asdatetime.datetime.astimezone ()
. (Contributed by Paul Ganssle inBPO – 32417.) -
When the Python interpreter is interrupted by Ctrl-C (SIGINT) and the resulting
KeyboardInterrupt
exception is not caught, the Python process now exits via a SIGINT signal or with the correct exit code such that the calling process can detect that it died due to a Ctrl-C. Shells on POSIX and Windows use this to properly terminate scripts in interactive sessions. (Contributed by Google via Gregory P. Smith inBPO – 1054041.) -
Some advanced styles of programming require updating the
types.CodeType
object for an existing function. Since code objects are immutable, a new code object needs to be created, one that is modeled on the existing code object. With 19 parameters, this was somewhat tedious. Now, the newreplace ()
method makes it possible to create a clone with a few altered parameters.Here’s an example that alters the
statistics.mean ()
function to prevent 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 the
pow ()
function now permits the exponent to be negative in the case where the base is relatively prime to the modulus. It then computes a modular inverse to the base when the exponent is- 1
, and a suitable power of that inverse for other negative exponents. For example, to compute themodular multiplicative inverseof 38 modulo 137, write:>>>pow(38,-1,137)119>>>119*%1371
Modular inverses arise in the solution ofLinear Diophantine equations. For example, to find integer solutions for
(𝑥)147 ***=369
, first rewrite as4258 𝑥≡369(mod147)
then solve:>>>x=369*pow4258,-1,147)%147>>>y=(*x-369)//-147>>>4258*x147*y369
(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 Arthuractor? Chapmanrole? Black Knightactor? Cleese>>># Dict literal>>>cast={input('role?'):input('actor?')}role? Sir Robinactor? 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 new
importlib.metadata
module provides (provisional) support for reading metadata from third-party packages. For example, it can extract an installed package’s version number, list of entry points, and more:>>># Note following example requires that the popular "requests">>># package has been installed.>>>>>>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_lineno
andend_col_offset
attributes, which give the precise location of the end of the node. (This only applies to nodes that havelineno
andcol_offset
attributes.)
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=True
causes it to return the text of(PEP)and(PEP)type comments associated with certain AST nodes; -
mode='func_type'
can be used to parse(PEP)“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 treatAsync
and(await)
as non-reserved words.)
(Contributed by Guido van Rossum inbpo – 35766.)
Asyncio¶
RunningPython(-masyncio
launches 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.)
ProactorEventLoop
now also supports UDP . (Contributed by Adam Meily and Andrew Svetlov inbpo – 29883.)
ProactorEventLoop
can now be interrupted byKeyboardInterrupt
(“CTRL C”). (Contributed by Vladimir Matveev inbpo – 23057.)
builtins¶
Thecompile ()
built-in has been improved to accept theast .PYCF_ALLOW_TOP_LEVEL_AWAIT
flag. With this new flag passed,compile ()
will allow top-levelawait
,Asyncfor
andAsyncwith
constructs that are usually considered invalid syntax. Asynchronous code object marked with theCO_COROUTINE
flag may then be returned. (Contributed by Matthias Bussonnier inbpo – 34616)
collections***
The_ asdict ()(method for)
collections.namedtuple ()
now returns Adict
instead of acollections.OrderedDict
. This works because regular dicts have guaranteed ordering since Python 3.7. If the extra features of(OrderedDict)
are required, the suggested remediation is to cast the result to the desired type:OrderedDict (nt._asdict ())
. (Contributed by Raymond Hettinger 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,(CDLL)and subclasses now accept a (Winmode) parameter to specify flags for the underlyingLoadLibraryEx
call. The default flags are set to only load DLL dependencies from trusted locations, including the path where the DLL is stored (if a full or partial path is used to load the initial DLL) and paths added byadd_dll_directory ()
. (Contributed by Steve Dower inbpo – 36085.)
GC¶
get_objects ()
can now receive an optional (generation) parameter 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.)
ABadGzipFile
exception is now raised instead ofOSError
for certain types of invalid or corrupt gzip files. (Contributed by Filip Gruszczyński, Michele Orrù, and Zackery Spytz inbpo- 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 adict
where the values are docstrings. This provides documentation options similar to what we already have forproperty ()
,classmethod ()
, andstaticmethod ()
:
(class)AudioClip: __ slots __={'bit_rate':' expressed in kilohertz to one decimal place ', 'duration':'in seconds, rounded up to an integer '} def__ init __(self,bit_rate,Duration): self.bit_rate=round(bit_rate/1000 0,(1)) self.duration=ceil(Duration)
(Contributed by Raymond Hettinger inbpo – 36326.)
io ¶
In development mode (- X
env)
) and in debug build, theio.IOBase
finalizer 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 thememfd_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_mode
will only haveS_IFLNK
set for symbolic links and not other kinds of reparse points. To identify other kinds of reparse point, check the newstat_result.st_reparse_tag
attribute.
On Windows,os.readlink ()
is now able to read directory junctions. Note thatislink ()
will returnFalse
for directory junctions, and so code that checksislink
first 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)
(pickle) ****************
extensions subclassing the C-optimizedPicklercan 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.UID
and enabled support for reading and writing NSKeyedArchiver-encoded binary plists. (Contributed by Jon Janzen inbpo – 26707.)
Shutil¶
shutil.copytree ()
now accepts a newdirs_exist_ok
keyword 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 thetarfile
module. (Contributed by CAM 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 ofstatistics.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.mean6.0>>>Temperature_feb.STDEV6. 356099432828281>>>Temperature_feb.cdf(3)# Chance of being under 3 degrees0. 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_febNormalDist (mu=10 .0, sigma=6. 830080526611674)>>>temperature_feb*((9)/5)32# Convert to FahrenheitNormalDist (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***
TheTarfile
module now defaults to the modern pax (POSIX.1- 2001) format for new archives, instead of the previous GNU-specific one. This improves cross-platform portability with a consistent encoding (UTF-8) in a standardized and extensible format, and offers several other benefits. (Contributed by CAM Gerlach inbpo – 36268.)
tokenize¶
Thetokenize
module now implicitly emits aNEWLINE
token when provided with input that does not have a trailing new line. This behavior now matches what the C tokenizer does internally. (Contributed by Ammar Askar inBPO – 33899.)
tkinter¶
Added methodsselection_from ()
,selection_present ()
,selection_range ()
andselection_to ()
in thetkinter.Spinbox
class. (Contributed by Juliette Monsel inbpo – 34829.)
Added methodmoveto ()
in thetkinter.Canvas
class. (Contributed by Juliette Monsel inBPO – 23831.)
Thetkinter.PhotoImage
class now hastransparency_get ()
andtransparency_set ()
methods. (Contributed by Zackery Spytz inBPO – 25451.)
Typing¶
The(typing)
module incorporates several new features:
-
A dictionary type with per-key types. See(PEP)and
typing.TypedDict
. TypedDict uses only string keys. By default, every key is required to be present. Specify “total=False” to allow keys to be optional:class(Location)(TypedDict,total=False) : lat_long:tuple grid_square:str xy_coordinate:tuple
-
Literal types. SeePEP 586and
typing.Literal
. Literal types indicate that a parameter or return value is constrained to one or more specific literal values:defget_status(port:int)->Literal['connected','disconnected']: ...
-
“Final” variables, functions, methods and classes. SeePEP 591,
typing.Final
andtyping.final ()
. The final qualifier instructs a static type checker to restrict subclassing, overriding, or reassignment:(pi:Final[float]=3. 1415926536
-
Protocol definitions. See(PEP),
typing.Protocol
andTyping .runtime_checkable ()
. Simple ABCs liketyping.SupportsInt
are nowProtocol
subclasses. -
New protocol class
typing.SupportsIndex
. - (New functions)
typing.get_origin ()
andtyping.get_args ()
.
unicodedata***
Theunicodedata
module has been upgraded to use theUnicode (****************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************. 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 inbpo – 32285andbpo – 37966).
unittest ¶
AddedAsyncMock
to support an asynchronous version ofMock
. Appropriate new assert functions for testing have been added as well. (Contributed by Lisa Roach inbpo – 26467).
AddedaddModuleCleanup ()
andaddClassCleanup ()
to unittest to support cleanups for(setUpModule)
andsetUpClass ()
. (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.)
unittest
module gained support for coroutines to be used as test cases withunittest.IsolatedAsyncioTestCase
. (Contributed by Andrew Svetlov inbpo – 32972.)
Example:
importunittestclassTestRequest(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 __": unittestmain()
VENV¶
VENV
now includes anActivate.ps1
script 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, thexml.dom.minidom
andxml.sax
modules no longer process external entities by default. (Contributed by Christian Heimes inbpo – 17239.)
The. find * ()
methods in thexml.etree.ElementTree
module support wildcard searches like{*} tag
which ignores the namespace and{namespace} *
which returns all tags in the given namespace. (Contributed by Stefan Behnel inbpo – 28238.)
Thexml.etree.ElementTree
module provides a new function- xml.etree.ElementTree.canonicalize ()
that implements C 14 N 2.0. (Contributed by Stefan Behnel inBPO – 13611.)
The target object ofxml.etree.ElementTree.XMLParser
can receive namespace declaration events through the new callback methodsstart_ns ()
andend_ns ()
. Additionally, thexml.etree.ElementTree.TreeBuilder
target can be configured to process events about comments and processing instructions to include them in the generated tree. (Contributed by Stefan Behnel in (bpo-) andbpo – 36673.)
Optimizations¶
-
The
(subprocess)
module 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 ()
,shuti l.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 (KiB) and amemoryview ()
– based variant ofshutil.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 a nd 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 (% making)shutil.copytree ()
especially faster on network filesystems. (Contributed by Giampaolo Rodolà inbpo – 33695.) -
The default protocol in the
pickle
module is now Protocol 4, first introduced in Python 3.4. It offers better performance and smaller size compared to Protocol 3 available since Python 3.0. - (Removed one) ******************************Py_ssize_tmember from
PyGC_Head
. All GC tracked objects (e.g. tuple, list, dict) size is reduced 4 or 8 bytes. (Contributed by Inada Naoki inbpo – 33597) -
uuid.UUID
now uses__ slots __
to reduce its memory footprint. (Contributed by Wouter Bolsterlee and Tal Einat inbpo – 30977) -
Improved performance of
operator.itemgetter ()
by 33%. Optimized argument handling and added a fast path for the common case of a single non-negative integer index into a tuple (which is the typical use case in the standard library). (Contributed by Raymond Hettinger inbpo – 35664.) -
Sped-up field lookups in
collections.namedtuple ()
. They are now more than two times faster, making them the fastest form of instance variable lookup in Python. (Contributed by Raymond Hettinger, Pablo Galindo, and Joe Jevnik, Serhiy Storchaka inbpo – 32492.) -
The
list
constructor does not overallocate the internal item buffer if the input iterable has a known length (the input implements__ len __
)). This makes the created list 12% smaller on average. (Contributed by Raymond Hettinger and Pablo Galindo 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_GLOBAL
instruction now uses new “per opcode cache” mechanism. It is about 40% faster now. (Contributed by Yury Selivanov and Inada Naoki inbpo – 26219.)
Build and C API Changes¶
-
Default
sys.abiflags
became an empty string: theM
flag 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:
-
Only
python3.8
program is installed,python3.8m
program is gone. -
Only
python3.8-config
script is installed,python3.8m-config
script is gone. -
The
M
flag has been removed from the suffix of dynamic library filenames: extension modules in the standard library as well as those produced and installed by third-party packages, like those downloaded from PyPI. On Linux, for example, the Python 3.7 suffix. cpython - (MX) _ 64 - linux-gnu.so
became. cpython - 38 - x 86 _ 64 - linux-gnu.so
in Python 3.8.
-
-
The header files have been reorganized to better separate the different kinds of APIs:
-
Include / *. H
should be the portable public stable C API. -
Include / cpython / *. H
should be the unstable C API specific to CPython; public API, with some private API prefixed by_ Py
or_ PY
. -
Include / internal / *. H
is the private internal C API very specific to CPython. This API comes with no backward compatibility warranty and should not be used outside CPython. It is only exposed for very specific needs like debuggers and profiles which has to access to CPython internals without calling functions. This API is now installed 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:
-
PyObject_INIT ()
,PyObject_INIT_VAR ()
-
Private functions:
_ PyObject_GC_TRACK ()
,_ PyObject_GC_UNTRACK ()
,_ Py_Dealloc ()
(Contributed by Victor Stinner inbpo – 35059.)
-
The
PyByteArray_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 of
PyExceptionClass_Name ()
is now of typeconstchar*
rather thanchar*
. (Contributed by Serhiy Storchaka inbpo – 33818.) -
The duality of
Modules / Setup.dist
andModules / Setup
has been removed. Previously, when updating the CPython source tree, one had to manually copyModules / Setup.dist
(inside the source tree) toModules / Setup
(inside the build tree) in order to reflect any changes upstream. This was of a small benefit to packagers at the expense of a frequent annoyance to developers following CPython development, as forgetting to copy the file could produce build failures.Now the build system always reads from
Modules / Setup
inside the source tree. People who want to customize that file are encouraged to maintain their changes in a git fork of CPython or as patch files, as they would do for any other change to the source tree.(Contributed by Antoine Pitrou inbpo – 32430.)
-
Functions that convert Python number to C integer like
PyLong_AsLong ()
and argument parsing functions likePyArg_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 (likeDecimal
andFraction
).PyNumber_Check ()
will now return1
for objects implementing__ index __ ()
.PyNumber_Long ()
,PyNumber_Float
andPyFloat_AsDouble ()
also now use the__ index __ ()
method if available. (Contributed by Serhiy Storchaka in (bpo-) andbpo – 20092. -
Heap-allocated type objects will now increase their reference count in
PyObject_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 function
PyCode_NewWithPosOnlyArgs ()
allows to create code objects likePyCode_New (
, but with an extraposonlyargcountparameter for indicating the number of positional-only arguments. (Contributed by Pablo Galindo inbpo – 37221.) -
Py_SetPath ()
now setssys. executable
to the program full path ((Py_GetProgramFullPath))
) rather than to the program name ((Py_GetProgramName)
). (Contributed by Victor Stinner inbpo – 38234.)
Deprecated¶
-
The distutils
bdist_wininst
command is now deprecated, usebdist_wheel
(wheel packages) instead. (Contributed by Victor Stinner inbpo – 37481.) -
Deprecated methods
getchildren ()
andgetiterator ()
in TheElementTree
module now emit aDeprecationWarning
instead ofPendingDeprecationWarning
. They will be removed in Python 3.9. (Contributed by Serhiy Storchaka inbpo – 29209.) -
Passing an object that is not an instance of
concurrent.futures.ThreadPoolExecutor
toasyncio.loop.set_default_executor ()
is deprecated and will be prohibited in Python 3.9. (Contributed by Elvis Pranskevichus inbpo – 34075.) -
The
__ getitem __ ()
methods ofxml.dom.pulldom.DOMEventStream
,wsgiref.util.FileWrapper
andfileinput.FileInput
have been deprecated.Implementations of these methods have been ignoring theirindexparameter, and returning the next item instead. (Contributed by Berker Peksag inbpo – 9372.)
-
The
typing.NamedTuple
class has deprecated the_ field_types
attribute in favor of the__ annotations __
attribute which has the same information. (Contributed by Raymond Hettinger inbpo – 36320 (*****************.) -
ast
classesNum
,Str
,Bytes
,NameConstant
andEllipsis
are considered deprecated and will be removed in future Python versions.Constant
should be used instead. (Contributed by Serhiy Storchaka inbpo –.) -
ast.NodeVisitor
methodsvisit_Num ()
,visit_Str ()
,visit_Bytes ()
,visit_NameConstant ()
andvisit_Ellipsis ()
are deprecated now and will not be called in future Python versions. Add thevisit_Constant ()
method to handle all constant nodes. (Contributed by Serhiy Storchaka inbpo – 36917.) -
The following functions and methods are deprecated in the
gettext
module:lgettext ()
,ldgettext ()
,lngettext ()
andldngettext ()
. They return encoded bytes, and it’s possible that you will get unexpected Unicode-related exceptions if there are encoding problems with the translated strings. It’s much better to use alternatives which return Unicode strings in Python 3. These functions have been broken for a long time.Function
bind_textdomain_codeset ()
, methodsoutput_charset ()
andset_output_charset ()
, and thecodesetparameter of functionstranslation ()
andinstall ()
are also deprecated, since they are only used for for the(l * gettext))
functions. (Contributed by Serhiy Storchaka inbpo – 33710.) -
The
isAlive ()
method ofthreading.Thread
has been deprecated. (Contributed by Dong-hee Na inbpo – 35283.) -
Many builtin and extension functions that take integer arguments will now emit a deprecation warning for
Decimal
s,Fraction
s and any other objects that can be converted to integers only with a loss (eg that have the__ int __ ()
method but do not have the__ index __ ()
method). In future version they will be errors. (Contributed by Serhiy Storchaka in (bpo-) .) -
Deprecated passing the following arguments as keyword arguments:
-
FUNCin
functools.partialmethod ()
,weakref.finalize ()
,profile.Profile.runcall ()
,cProfile.Profile.runcall ()
,bdb.Bdb.runcall ()
,trace.Trace.runfunc ()
curses.wrapper ()
. -
functionin
unittest .TestCase.addCleanup ()
. -
FNin the
(submit))
method ofconcurrent.futures.ThreadPoolExecutor
andconcurrent.futures.ProcessPoolExecutor
. -
callbackin
contextlib.ExitStack.callback ()
,contextlib .YncExitStack.callback ()
andcontextlib.AsyncExitStack.push_async_callback ()(************************************************.
-
Candtypeidin the
create ()
method ofmultiprocessing.managers.Server
andmultiprocessing .managers.SharedMemoryServer
. -
OBJin
weakref.finalize ()
.
-
In future releases of Python they will bepositional-only. (Contributed by Serhiy Storchaka inbpo – 36492.)
API and Feature Removals¶
The following features and APIs have been removed from Python 3.8:
-
The
macpath
module, deprecated in Python 3.7, has been removed. (Contributed by Victor Stinner inbpo – 35471.) -
The function
platform.popen ()
has been removed, after having been deprecated since Python 3.3: useos.popen ()
instead. (Contributed by Victor Stinner inbpo – 35345.) -
The function
time.clock ()
has been removed, after having been deprecated since Python 3.3: usetime.perf_counter ()
ortime.process_time ()
instead, depending on your requirements, to have well-defined behavior. (Contributed by Matthias Bussonnier inbpo – 36895.) -
The
Pyvenv
script 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
, andescape
are removed from thecgi
module. They are deprecated in Python 3.2 or older. They should be imported from theurllib.parse
andhtml
modules instead. -
filemode
function is removed from thetarfile
module. It is not documented and deprecated since Python 3.3. -
The
XMLParser
constructor 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 the
doctype ()
method ofXMLParser
. (Contributed by Serhiy Storchaka inbpo – 29209.) -
“unicode_internal” codec is removed. (Contributed by Inada Naoki inbpo – 36297.)
-
The
Cache
andStatement
objects of thesqlite3
module are not exposed to the user. (Contributed by Aviv Palivoda inbpo – 30262.) -
The
bufsize
keyword argument offileinput.input ()
andfileinput.FileInput ()
which was ignored and deprecated since Python 3.6 has been removed.bpo – 36952(Contributed by Matthias Bussonnier.) -
The functions
sys.set_coroutine_wrapper ()
andsys .get_coroutine_wrapper ()
deprecated in Python 3.7 have been removed;bpo – 36933(Contributed by Matthias Bussonnier .)
Porting to Python 3.8 ¶
This section lists previously described changes and other bugfixes that may require changes to your code.
Changes in Python behavior¶
-
Yield expressions (both
yield
andyieldfrom
clauses ) are now disallowed in comprehensions and generator expressions (aside from the iterable expression in the leftmostfor
clause). (Contributed by Serhiy Storchaka inbpo – 10544.) -
The compiler now produces a
SyntaxWarning
when identity checksis
andisnot
) 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
,complex
and 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.platform
doesn’t contain the major version anymore. It is always'aix'
, instead of'aix3'
..'aix7'
. Since older Python versions include the version number, so it is recommended to always 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¶
-
The
os.getcwdb ()
function now uses the UTF-8 encoding on Windows, rather than the ANSI code page: see(PEP)for the rationale. The function is no longer deprecated on Windows. (Contributed by Victor Stinner inbpo – 37412.) -
subprocess .Popen
can now useos.posix_spawn ()
in some cases for better performance. On Windows Subsystem for Linux and QEMU User Emulation, thePopen
constructor using(os.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 .Popen
is no longer compatible with subinterpreters. The use of the parameter in a subinterpreter now raisesRuntimeError
. (Contributed by Eric Snow in (bpo-) , modified by Christian Heimes inbpo – 37951.) -
The
imap.IMAP4.logout ()
method no longer ignores silently arbitrary exceptions. (Contributed by Victor Stinner inBPO – 36348.) -
The function
platform.popen ()
has been removed, after having been deprecated since Python 3.3: useos.popen ()
instead. (Contributed by Victor Stinner inbpo – 35345.) -
The
statistics.mode ()
function no longer raises an exception when given multimodal data. Instead, it returns the first mode encountered in the input data. (Contributed by Raymond Hettinger inbpo – 35892.) -
The
selection ()
method of thetkinter.ttk.Treeview
class 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. -
The
writexml ()
,toxml ()
andtoprettyxml ()
methods ofxml.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.) - (A)
dbm.dumb
database opened with flags'R'
is now read-only.dbm.dumb.open ()
with flags'r'
and'w'
no longer creates a database if it does not exist. (Contributed by Serhiy Storchaka inbpo – 32749. -
The
doctype ()
method defined in a subclass ofXMLParser
will no longer be called and will emit aRuntimeWarning
instead of aDeprecationWarning
. Define thedoctype ()
method on a target for handling an XML doctype declaration. (Contributed by Serhiy Storchaka inbpo – 29209.) - (A)
(RuntimeError)
is now raised when the custom metaclass doesn’t Provide the__ classcell __
entry in the namespace passed totype .__ new __
. ADeprecationWarning
was emitted in Python 3.6–3.7. (Contributed by Serhiy Storchaka inbpo – 23722.) -
The
cProfile.Profile
class can now be used as a context manager. (Contributed by Scott Sanderson inbpo – 29235.) -
shutil.copyfile ()
,shuti l.copy ()
,shutil.copy2 ()
,shutil. copytree ()
andshutil.move ()
use platform-specific “Fast-copy” syscalls (seePlatform-dependent efficient copy operationssection). -
shutil.copyfile ()
default buffer size on Windows was changed from 16 KiB to 1 MiB. -
The
PyGC_Head
struct has changed completely. All code that touched the struct member should be rewritten. (Seebpo – 33597.) -
The
PyInterpreterState
struct has been moved into the “internal” header files (specifically Include / internal / pycore_pystate.h). An opaquePyInterpreterState
is still available as part of the public API (and stable ABI). The docs indicate that none of the struct’s fields are public, so we hope no one has been using them. However, if you do rely on one or more of those private fields and have no alternative then please open a BPO issue. We’ll work on helping you adjust (possibly including adding accessor functions to the public API). (Seebpo – 35886.) -
Asyncio tasks can now be named, either by passing the
name
keyword 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.Task
and can also be retrieved using theget_name ()
method. -
The
mmap.flush ()
method now returns(None)
on success and raises an exception on error under all platforms. Previously, its behavior was platform-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.minidom
andxml.sax
modules no longer process external entities by default. (Contributed by Christian Heimes inbpo – 17239.) -
Deleting a key from a read-only
dbm
(database)dbm.dumb
,dbm.gnu
ordbm.ndbm
) raiseserror
(dbm.dumb.error
,dbm.gnu.error
ordbm.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 exception
asyncio.CancelledError
now inherits fromBaseException
rather than AException
. (Contributed by Yury Selivanov inBPO – 13528.)
-
DLL dependencies for extension modules and DLLs loaded with
ctypes
on Windows are now resolved more securely. Only the system paths, the directory containing the DLL or PYD file, and directories added withadd_dll_directory ()
are searched for load-time dependencies. Specifically,(PATH
and the current working directory are no longer used, and modifications to these will no longer have any effect on normal DLL resolution. If your application relies on these mechanisms, you should check 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 KB 2533625 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.CodeType
has a new parameter in the second position of the constructor (posonlyargcount) support positional-only arguments defined inPEP 570. The first argument ( (argcount) now represents the total number of positional arguments (including positional-only arguments). The newreplace ()
method oftypes.CodeType
can be used to make the code future-proof.
Changes in the C API¶
-
The
PyCompilerFlags
structure got a newcf_feature_versionfield. It should be initialized toPY_MINOR_VERSION
. The field is ignored by default, and is used if and only ifPyCF_ONLY_AST
flag is set incf_flags. (Contributed by Guido van Rossum inbpo – 35766.) -
The
PyEval_ReInitThreads ()
function has been removed from the C API. It should not be called explicitly: usePyOS_AfterFork_Child ()
instead. (Contributed by Victor Stinner in (bpo -) .) -
On Unix, C extensions are no longer linked to libpython except on Android and Cygwin. When Python is embedded,
libpython
must not be loaded withRTLD_LOCAL
, butRTLD_GLOBAL
instead. Previously, usingRTLD_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 (egPyArg_ParseTuple ()
,Py_BuildValue ()
,PyObject_CallFunction ()
, etc.) withoutPY_SSIZE_T_CLEAN
defined raisesDeprecationWarning
now. It will be removed in 3. 10 or 4.0. ReadParsing arguments and building values for 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 fromPyType_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:
-
Remove
Py_INCREF
on 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 usesPyObject_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 custom
tp_dealloc
functions of heap-allocated types decrease the type’s reference count.Example:
staticvoidfoo_dealloc(foo_struct*instance){ PyObject*type=Py_TYPE(instance); PyObject_GC_Del(instance);# if PY_VERSION_HEX>=0x 03080000 // This was not needed before Python 3.8 (Python issue 35810) Py_DECREF(type);# endif}
(Contributed by Eddie Elizondo inbpo – 35810.)
-
-
The
Py_DEPRECATED ()
macro has been implemented for MSVC. The macro now must be placed before the symbol name.Example:
Py_DEPRECATED(3.8)PyAPI_FUNC(int)Py_OldFunction(void);
(Contributed by Zackery Spytz inbpo – 33407.)
-
The interpreter does not pretend to support binary compatibility of extension types across feature releases, anymore. A
PyTypeObject
exported by a third-party extension module is supposed to have all the slots expected in the current Python version, includingtp_finalize
(Py_TPFLAGS_HAVE_FINALIZE
is not checked anymore before readingtp_finalize
).(Contributed by Antoine Pitrou inbpo – 32388.)
-
The
PyCode_New ()
has a new parameter in the second position (posonlyargcount) to support(PEP), indicating the number of positional-only arguments. -
The functions
PyNode_AddChild ()
andPyParser_AddToken ()
now accept two additional(int)
arguments (end_linenoandend_col_offset. -
The
libpython 38 .a
file to allow MinGW tools to link directly againstPython 38 .dll
is no longer included in the regular Windows distribution. If you require this file, it may be generated with thegendef
anddlltool
tools, which are part of the MinGW binutils package:gendef python 38 .dll>tmp.def dlltool --dllname python 38 .dll - def tmp.def --output-lib libpython 38
The location of an installed
pythonXY.dll
will depend on the installation options and the version and language of Windows. SeeUsing Python on Windowsfor more information. The resulting library should be placed in the same directory aspythonXY.lib
, which is generally thelibs
directory 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 for
break
,continue
andreturn
.Removed opcodes
BREAK_LOOP
,CONTINUE_LOOP
,SETUP_LOOP
and(SETUP_EXCEPT)
. Added new opcodesROT_FOUR
,BEGIN_FINALLY
,CALL_ FINALLY
andPOP_FINALLY
. Changed the behavior ofEND_FINALLY
andWITH_CLEANUP_START
.(Contributed by Mark Shannon, Antoine Pitrou and Serhiy Storchaka inbpo – 17611.)
-
Added new opcode
END_ASYNC_FOR
for handling exceptions raised when awaiting a next item in anAsyncfor
loop. (Contributed by Serhiy Storchaka inbpo – 33041.) -
The
MAP_ADD
now expects the value as the first element in the stack and the key as the second element. This change was made so the key is always evaluated before the value in dictionary comprehensions, as proposed by(PEP). (Contributed by Jörn Heissler inbpo – 35224.)
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