What’s New in Python 3.8, Hacker News

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 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 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 thecollectionsmodule:

(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 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 __

(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_DEBUGmacro no longer implies thePy_TRACE_REFSmacro, which introduces the only ABI incompatibility. ThePy_TRACE_REFSmacro, which adds thesys.getobjects ()function and thePYTHONDUMPREFSenvironment 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 libpython except on Android and Cygwin. It is now possible for a statically linked Python to load a C extension built using a shared library Python. (Contributed by Victor Stinner inbpo – 21536.)

On Unix, when Python is built in debug mode, import now also looks for C extensions compiled in release mode and for C extensions compiled with the stable ABI. (Contributed by Victor Stinner inbpo – 36722.)

To embed Python into an application, a new- embedoption must be passed toPython3 -config- libs- embedto get- lpython3.8(link the application to libpython). To support both 3.8 and older, trypython3-config- libs- embedfirst and fallback topython3-config- libs(without- embed) if the previous command fails.

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

On the other hand,pkg-configpython3.8- libsNO longer contains- lpython3.8. C extensions must not be linked to libpython (except on Android and Cygwin, whose cases are handled by the script); this change is backward incompatible on purpose. (Contributed by Victor Stinner inbpo – 36721.)

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

Added an=specifier tof-strings. An f-string such 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¶

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 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.)

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 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 treatAsyncand(await)as non-reserved words.)

(Contributed by Guido van Rossum inbpo – 35766.)

Asyncio¶

RunningPython(-masynciolaunches a natively async REPL. This allows rapid experimentation with code that has a top-levelawait. There is no longer a need to directly callasyncio.run ()which would spawn a new event loop on every invocation:

$ python -m asyncio asyncio REPL 3.8.0 Use "await" directly instead of "asyncio.run ()". Type "help", "copyright", "credits" or "license" for more information.>>>import asyncio>>>await asyncio.sleep (10, result='hello') hello

(Contributed by Yury Selivanov inbpo – 37028.)

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

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

ProactorEventLoopcan now be interrupted byKeyboardInterrupt(“CTRL C”). (Contributed by Vladimir Matveev inbpo – 23057.)

builtins¶

Thecompile ()built-in has been improved to accept theast .PYCF_ALLOW_TOP_LEVEL_AWAITflag. With this new flag passed,compile ()will allow top-levelawait,AsyncforandAsyncwithconstructs 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 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 underlyingLoadLibraryExcall. The default flags are set to only load DLL dependencies from trusted locations, including the path where the DLL is stored (if a full or partial path is used to load the initial DLL) and paths added byadd_dll_directory (). (Contributed by Steve Dower inbpo – 36085.)

GC¶

get_objects ()can now receive an 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.)

ABadGzipFileexception is now raised instead ofOSErrorfor 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 adictwhere 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 (- Xenv)) and in debug build, theio.IOBasefinalizer now logs the exception if theclose ()method fails. The exception is ignored silently by default in release build. (Contributed by Victor Stinner inbpo – 18748.)

math (¶)

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

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

Added new function,math.prod (), as analogous function tosum ()that returns the product of a ‘start’ value (default: 1) times an iterable of numbers:

>>>prior=0.8>>>likelihoods=[0.625,0.84,0.30]>>>math.prod(likelihoods,start=prior)0. 126

(Contributed by Pablo Galindo inbpo – 35606)

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

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

OS¶

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

A newos.memfd_create ()function was added to wrap 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_modewill only haveS_IFLNKset for symbolic links and not other kinds of reparse points. To identify other kinds of reparse point, check the newstat_result.st_reparse_tagattribute.

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

(Contributed by Steve Dower inbpo – 37834.)

Pickle¶

Reduction methods can now include a 6th item in the tuple they return. This item should specify a custom state-setting method that’s called instead of the regular__setstate __method. (Contributed by Pierre Glaser and Olivier Grisel inbpo – 35900)

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

Shutil¶

shutil.copytree ()now accepts a newdirs_exist_okkeyword argument. (Contributed by Josh Bronson inbpo – 20849.)

shutil.make_archive ()now defaults to the modern pax (POSIX.1) – 2001) format for new archives to improve portability and standards conformance, inherited from the corresponding change to thetarfilemodule. (Contributed by 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***

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 CAM Gerlach inbpo – 36268.)

tokenize¶

Thetokenizemodule now implicitly emits aNEWLINEtoken when provided with input that does not have a trailing new line. This behavior now matches what the C tokenizer does internally. (Contributed by Ammar Askar inBPO – 33899.)

tkinter¶

Added methodsselection_from (),selection_present (),selection_range ()andselection_to ()in thetkinter.Spinboxclass. (Contributed by Juliette Monsel inbpo – 34829.)

Added methodmoveto ()in thetkinter.Canvasclass. (Contributed by Juliette Monsel inBPO – 23831.)

Thetkinter.PhotoImageclass 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)andtyping.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 586andtyping.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.Finalandtyping.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.ProtocolandTyping .runtime_checkable (). Simple ABCs liketyping.SupportsIntare nowProtocolsubclasses.

• New protocol classtyping.SupportsIndex.

• (New functions)typing.get_origin ()andtyping.get_args ().

unicodedata***

Theunicodedatamodule 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 ¶

AddedAsyncMockto 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.)

unittestmodule 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¶

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, thexml.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 C 14 N 2.0. (Contributed by Stefan Behnel inBPO – 13611.)

The target object ofxml.etree.ElementTree.XMLParsercan receive namespace declaration events through the new callback methodsstart_ns ()andend_ns (). Additionally, thexml.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 in (bpo-) andbpo – 36673.)

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 checksisandisnot) 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: see(PEP)for 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 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 .Popenis 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.)

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

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

• Thestatistics.mode ()function no longer raises an exception when given multimodal data. Instead, it returns the first mode encountered in the input data. (Contributed by Raymond Hettinger inbpo – 35892.)

• Theselection ()method of thetkinter.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 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.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 ofXMLParserwill 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.)

• (A)(RuntimeError)is now raised when the custom metaclass doesn’t Provide the__ classcell __entry in the namespace passed totype .__ 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 (),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.

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

• ThePyInterpreterStatestruct has been moved into the “internal” header files (specifically Include / internal / pycore_pystate.h). An opaquePyInterpreterStateis still available as part of the public API (and stable ABI). The docs indicate that none of the struct’s fields are public, so we hope no one has been using them. However, if you do rely on one or more of those private fields and have no alternative then please open a BPO issue. We’ll work on helping you adjust (possibly including adding accessor functions to the public API). (Seebpo – 35886.)

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

• Themmap.flush ()method now 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.minidomandxml.saxmodules no longer process external entities by default. (Contributed by Christian Heimes inbpo – 17239.)

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

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

• The exceptionasyncio.CancelledErrornow inherits fromBaseExceptionrather than AException. (Contributed by Yury Selivanov inBPO – 13528.)

• DLL dependencies for extension modules and DLLs loaded withctypeson Windows are now resolved more securely. Only the system paths, the directory containing the DLL or PYD file, and directories added withadd_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 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.CodeTypehas 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.CodeTypecan be used to make the code future-proof.

Changes in the C API¶

• ThePyCompilerFlagsstructure 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_ASTflag is set incf_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 in (bpo -) .)

• On Unix, C extensions are no longer linked to libpython except on Android and Cygwin. When Python is embedded,libpythonmust not be loaded withRTLD_LOCAL, butRTLD_GLOBALinstead. 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_CLEANdefined raisesDeprecationWarningnow. 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 withPyType_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:

  • 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 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 customtp_deallocfunctions 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.)

• 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. APyTypeObjectexported by a third-party extension module is supposed to have all the slots expected in the current Python version, includingtp_finalize(Py_TPFLAGS_HAVE_FINALIZEis not checked anymore before readingtp_finalize).

  (Contributed by Antoine Pitrou inbpo – 32388.)

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

• The functionsPyNode_AddChild ()andPyParser_AddToken ()now accept two additional(int)arguments (end_linenoandend_col_offset.

• Thelibpython 38 .afile to allow MinGW tools to link directly againstPython 38 .dllis no longer included in the regular Windows distribution. If you require this file, it may be generated with thegendefanddlltooltools, 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 installedpythonXY.dllwill 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 thelibsdirectory 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,continueandreturn.

  Removed opcodesBREAK_LOOP,CONTINUE_LOOP,SETUP_LOOPand(SETUP_EXCEPT). Added new opcodesROT_FOUR,BEGIN_FINALLY,CALL_ FINALLYandPOP_FINALLY. Changed the behavior ofEND_FINALLYandWITH_CLEANUP_START.

  (Contributed by Mark Shannon, Antoine Pitrou and Serhiy Storchaka inbpo – 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 by(PEP). (Contributed by Jörn Heissler inbpo – 35224.)