What's New In Python 3.9¶
- Release
3.9.0a4
- Date
3月 23, 2020
This article explains the new features in Python 3.9, compared to 3.8.
For full details, see the changelog.
備註
Prerelease users should be aware that this document is currently in draft form. It will be updated substantially as Python 3.9 moves towards release, so it's worth checking back even after reading earlier versions.
Summary -- Release highlights¶
You should check for DeprecationWarning in your code¶
When Python 2.7 was still supported, many functions were kept for backward
compatibility with Python 2.7. With the end of Python 2.7 support, these
backward compatibility layers have been removed, or will be removed soon.
Most of them emitted a DeprecationWarning
warning for several years. For
example, using collections.Mapping
instead of collections.abc.Mapping
emits a DeprecationWarning
since Python 3.3, released in 2012.
Test your application with the -W
default
command-line option to see
DeprecationWarning
and PendingDeprecationWarning
, or even with
-W
error
to treat them as errors. Warnings Filter can be used to ignore warnings from third-party code.
It has been decided to keep a few backward compatibility layers for one last release, to give more time to Python projects maintainers to organize the removal of the Python 2 support and add support for Python 3.9.
Aliases to Abstract Base Classes in
the collections
module, like collections.Mapping
alias to
collections.abc.Mapping
, are kept for one last release for backward
compatibility. They will be removed from Python 3.10.
More generally, try to run your tests in the Python Development Mode which helps to prepare your code to make it compatible with the next Python version.
New Features¶
Other Language Changes¶
__import__()
now raisesImportError
instead ofValueError
, which used to occur when a relative import went past its top-level package. (Contributed by Ngalim Siregar in bpo-37444.)Python now gets the absolute path of the script filename specified on the command line (ex:
python3 script.py
): the__file__
attribute of the__main__
module became an absolute path, rather than a relative path. These paths now remain valid after the current directory is changed byos.chdir()
. As a side effect, the traceback also displays the absolute path for__main__
module frames in this case. (Contributed by Victor Stinner in bpo-20443.)In the Python Development Mode and in debug build, the encoding and errors arguments are now checked for string encoding and decoding operations. Examples:
open()
,str.encode()
andbytes.decode()
.By default, for best performance, the errors argument is only checked at the first encoding/decoding error and the encoding argument is sometimes ignored for empty strings. (Contributed by Victor Stinner in bpo-37388.)
"".replace("", s, n)
now returnss
instead of an empty string for all non-zeron
. It is now consistent with"".replace("", s)
. There are similar changes forbytes
andbytearray
objects. (Contributed by Serhiy Storchaka in bpo-28029.)Any valid expression can now be used as a decorator. Previously, the grammar was much more restrictive. See PEP 614 for details. (Contributed by Brandt Bucher in bpo-39702.)
New Modules¶
None yet.
Improved Modules¶
ast¶
Added the indent option to dump()
which allows it to produce a
multiline indented output.
(Contributed by Serhiy Storchaka in bpo-37995.)
Added ast.unparse()
as a function in the ast
module that can
be used to unparse an ast.AST
object and produce a string with code
that would produce an equivalent ast.AST
object when parsed.
(Contributed by Pablo Galindo and Batuhan Taskaya in bpo-38870.)
Added docstrings to AST nodes that contains the ASDL signature used to construct that node. (Contributed by Batuhan Taskaya in bpo-39638.)
asyncio¶
Due to significant security concerns, the reuse_address parameter of
asyncio.loop.create_datagram_endpoint()
is no longer supported. This is
because of the behavior of the socket option SO_REUSEADDR
in UDP. For more
details, see the documentation for loop.create_datagram_endpoint()
.
(Contributed by Kyle Stanley, Antoine Pitrou, and Yury Selivanov in
bpo-37228.)
Added a new coroutine shutdown_default_executor()
that schedules a shutdown for the default executor that waits on the
ThreadPoolExecutor
to finish closing. Also,
asyncio.run()
has been updated to use the new coroutine.
(Contributed by Kyle Stanley in bpo-34037.)
Added asyncio.PidfdChildWatcher
, a Linux-specific child watcher
implementation that polls process file descriptors. (bpo-38692)
concurrent.futures¶
Added a new cancel_futures parameter to
concurrent.futures.Executor.shutdown()
that cancels all pending futures
which have not started running, instead of waiting for them to complete before
shutting down the executor.
(Contributed by Kyle Stanley in bpo-39349.)
curses¶
Add curses.get_escdelay()
, curses.set_escdelay()
,
curses.get_tabsize()
, and curses.set_tabsize()
functions.
(Contributed by Anthony Sottile in bpo-38312.)
fcntl¶
Added constants F_OFD_GETLK
, F_OFD_SETLK
and F_OFD_SETLKW
.
(Contributed by Dong-hee Na in bpo-38602.)
ftplib¶
FTP
and FTP_TLS
now raise a ValueError
if the given timeout for their constructor is zero to prevent the creation of
a non-blocking socket. (Contributed by Dong-hee Na in bpo-39259.)
functools¶
Add the functools.TopologicalSorter
class to offer functionality to perform
topological sorting of graphs. (Contributed by Pablo Galindo, Tim Peters and Larry
Hastings in bpo-17005.)
gc¶
When the garbage collector makes a collection in which some objects resurrect (they are reachable from outside the isolated cycles after the finalizers have been executed), do not block the collection of all objects that are still unreachable. (Contributed by Pablo Galindo and Tim Peters in bpo-38379.)
Added a new function gc.is_finalized()
to check if an object has been
finalized by the garbage collector. (Contributed by Pablo Galindo in
bpo-39322.)
http¶
HTTP status codes 103 EARLY_HINTS
, 418 IM_A_TEAPOT
and 425 TOO_EARLY
are added to
http.HTTPStatus
. (Contributed by Dong-hee Na in bpo-39509 and Ross Rhodes in bpo-39507.)
imaplib¶
IMAP4
and IMAP4_SSL
now have
an optional timeout parameter for their constructors.
Also, the open()
method now has an optional timeout parameter
with this change. The overridden methods of IMAP4_SSL
and
IMAP4_stream
were applied to this change.
(Contributed by Dong-hee Na in bpo-38615.)
importlib¶
To improve consistency with import statements, importlib.util.resolve_name()
now raises ImportError
instead of ValueError
for invalid relative
import attempts.
(Contributed by Ngalim Siregar in bpo-37444.)
inspect¶
inspect.BoundArguments.arguments
is changed from OrderedDict
to regular
dict. (Contributed by Inada Naoki in bpo-36350 and bpo-39775.)
ipaddress¶
ipaddress
now supports IPv6 Scoped Addresses (IPv6 address with suffix %<scope_id>
).
Scoped IPv6 addresses can be parsed using ipaddress.IPv6Address
.
If present, scope zone ID is available through the scope_id
attribute.
(Contributed by Oleksandr Pavliuk in bpo-34788.)
math¶
Expanded the math.gcd()
function to handle multiple arguments.
Formerly, it only supported two arguments.
(Contributed by Serhiy Storchaka in bpo-39648.)
Add math.lcm()
: return the least common multiple of specified arguments.
(Contributed by Mark Dickinson, Ananthakrishnan and Serhiy Storchaka in
bpo-39479 and bpo-39648.)
Add math.nextafter()
: return the next floating-point value after x
towards y.
(Contributed by Victor Stinner in bpo-39288.)
Add math.ulp()
: return the value of the least significant bit
of a float.
(Contributed by Victor Stinner in bpo-39310.)
nntplib¶
NNTP
and NNTP_SSL
now raise a ValueError
if the given timeout for their constructor is zero to prevent the creation of
a non-blocking socket. (Contributed by Dong-hee Na in bpo-39259.)
os¶
Added CLD_KILLED
and CLD_STOPPED
for si_code
.
(Contributed by Dong-hee Na in bpo-38493.)
Exposed the Linux-specific os.pidfd_open()
(bpo-38692) and
os.P_PIDFD
(bpo-38713) for process management with file
descriptors.
The os.unsetenv()
function is now also available on Windows.
(Contributed by Victor Stinner in bpo-39413.)
The os.putenv()
and os.unsetenv()
functions are now always
available.
(Contributed by Victor Stinner in bpo-39395.)
pathlib¶
Added pathlib.Path.readlink()
which acts similarly to
os.readlink()
.
(Contributed by Girts Folkmanis in bpo-30618)
poplib¶
POP3
and POP3_SSL
now raise a ValueError
if the given timeout for their constructor is zero to prevent the creation of
a non-blocking socket. (Contributed by Dong-hee Na in bpo-39259.)
pprint¶
pprint
can now pretty-print types.SimpleNamespace
.
(Contributed by Carl Bordum Hansen in bpo-37376.)
signal¶
Exposed the Linux-specific signal.pidfd_send_signal()
for sending to
signals to a process using a file descriptor instead of a pid. (bpo-38712)
smtplib¶
SMTP
and SMTP_SSL
now raise a ValueError
if the given timeout for their constructor is zero to prevent the creation of
a non-blocking socket. (Contributed by Dong-hee Na in bpo-39259.)
LMTP
constructor now has an optional timeout parameter.
(Contributed by Dong-hee Na in bpo-39329.)
threading¶
In a subinterpreter, spawning a daemon thread now raises a RuntimeError
. Daemon
threads were never supported in subinterpreters. Previously, the subinterpreter
finalization crashed with a Python fatal error if a daemon thread was still
running.
(Contributed by Victor Stinner in bpo-37266.)
sys¶
Add a new sys.platlibdir
attribute: name of the platform-specific
library directory. It is used to build the path of platform-specific dynamic
libraries and the path of the standard library. It is equal to "lib"
on
most platforms. On Fedora and SuSE, it is equal to "lib64"
on 64-bit
platforms.
(Contributed by Jan Matějek, Matěj Cepl, Charalampos Stratakis and Victor Stinner in bpo-1294959.)
typing¶
PEP 593 introduced an typing.Annotated
type to decorate existing
types with context-specific metadata and new include_extras
parameter to
typing.get_type_hints()
to access the metadata at runtime. (Contributed
by Till Varoquaux and Konstantin Kashin.)
venv¶
The activation scripts provided by venv
now all specify their prompt
customization consistently by always using the value specified by
__VENV_PROMPT__
. Previously some scripts unconditionally used
__VENV_PROMPT__
, others only if it happened to be set (which was the default
case), and one used __VENV_NAME__
instead.
(Contributed by Brett Cannon in bpo-37663.)
Optimizations¶
Optimized the idiom for assignment a temporary variable in comprehensions. Now
for y in [expr]
in comprehensions is as fast as a simple assignmenty = expr
. For example:sums = [s for s in [0] for x in data for s in [s + x]]
Unlike to the
:=
operator this idiom does not leak a variable to the outer scope.(Contributed by Serhiy Storchaka in bpo-32856.)
Optimize signal handling in multithreaded applications. If a thread different than the main thread gets a signal, the bytecode evaluation loop is no longer interrupted at each bytecode instruction to check for pending signals which cannot be handled. Only the main thread of the main interpreter can handle signals.
Previously, the bytecode evaluation loop was interrupted at each instruction until the main thread handles signals. (Contributed by Victor Stinner in bpo-40010.)
Build and C API Changes¶
New
PyThreadState_GetInterpreter()
andPyInterpreterState_Get()
functions to get the interpreter. NewPyThreadState_GetFrame()
function to get the current frame of a Python thread state. (Contributed by Victor Stinner in bpo-39947.)Add
--with-platlibdir
option to theconfigure
script: name of the platform-specific library directory, stored in the newsys.platlibdir
attribute. Seesys.platlibdir
attribute for more information. (Contributed by Jan Matějek, Matěj Cepl, Charalampos Stratakis and Victor Stinner in bpo-1294959.)Add a new public
PyObject_CallNoArgs()
function to the C API, which calls a callable Python object without any arguments. It is the most efficient way to call a callable Python object without any argument. (Contributed by Victor Stinner in bpo-37194.)The global variable
PyStructSequence_UnnamedField
is now a constant and refers to a constant string. (Contributed by Serhiy Storchaka in bpo-38650.)Exclude
PyFPE_START_PROTECT()
andPyFPE_END_PROTECT()
macros ofpyfpe.h
fromPy_LIMITED_API
(stable API). (Contributed by Victor Stinner in bpo-38835.)Remove
PyMethod_ClearFreeList()
andPyCFunction_ClearFreeList()
functions: the free lists of bound method objects have been removed. (Contributed by Inada Naoki and Victor Stinner in bpo-37340.)Remove
PyUnicode_ClearFreeList()
function: the Unicode free list has been removed in Python 3.3. (Contributed by Victor Stinner in bpo-38896.)The
tp_print
slot of PyTypeObject has been removed. It was used for printing objects to files in Python 2.7 and before. Since Python 3.0, it has been ignored and unused. (Contributed by Jeroen Demeyer in bpo-36974.)On non-Windows platforms, the
setenv()
andunsetenv()
functions are now required to build Python. (Contributed by Victor Stinner in bpo-39395.)The
COUNT_ALLOCS
special build macro has been removed. (Contributed by Victor Stinner in bpo-39489.)Changes in the limited C API (if
Py_LIMITED_API
macro is defined):Provide
Py_EnterRecursiveCall()
andPy_LeaveRecursiveCall()
as regular functions for the limited API. Previously, there were defined as macros, but these macros didn't compile with the limited C API which cannot accessPyThreadState.recursion_depth
field (the structure is opaque in the limited C API).Exclude the following functions from the limited C API:
_Py_CheckRecursionLimit
_Py_NewReference()
_Py_ForgetReference()
_PyTraceMalloc_NewReference()
_Py_GetRefTotal()
The trashcan mechanism which never worked in the limited C API.
PyTrash_UNWIND_LEVEL
Py_TRASHCAN_BEGIN_CONDITION
Py_TRASHCAN_BEGIN
Py_TRASHCAN_END
Py_TRASHCAN_SAFE_BEGIN
Py_TRASHCAN_SAFE_END
The following static inline functions or macros become regular "opaque" function to hide implementation details:
_Py_NewReference()
PyObject_INIT()
andPyObject_INIT_VAR()
become aliases toPyObject_Init()
andPyObject_InitVar()
in the limited C API, but are overriden with static inline function otherwise. Thanks to that, it was possible to exclude_Py_NewReference()
from the limited C API.
Move following functions and definitions to the internal C API:
_PyDebug_PrintTotalRefs()
_Py_PrintReferences()
_Py_PrintReferenceAddresses()
_Py_tracemalloc_config
_Py_AddToAllObjects()
(specific toPy_TRACE_REFS
build)
PyInterpreterState.eval_frame
(PEP 523) now requires a new mandatory tstate parameter (PyThreadState*
). (Contributed by Victor Stinner in bpo-38500.)Extension modules:
m_traverse
,m_clear
andm_free
functions ofPyModuleDef
are no longer called if the module state was requested but is not allocated yet. This is the case immediately after the module is created and before the module is executed (Py_mod_exec
function). More precisely, these functions are not called ifm_size
is greater than 0 and the module state (as returned byPyModule_GetState()
) isNULL
.Extension modules without module state (
m_size <= 0
) are not affected.If
Py_AddPendingCall()
is called in a subinterpreter, the function is now scheduled to be called from the subinterpreter, rather than being called from the main interpreter. Each subinterpreter now has its own list of scheduled calls. (Contributed by Victor Stinner in bpo-39984.)Remove
_PyRuntime.getframe
hook and remove_PyThreadState_GetFrame
macro which was an alias to_PyRuntime.getframe
. They were only exposed by the internal C API. Remove alsoPyThreadFrameGetter
type. (Contributed by Victor Stinner in bpo-39946.)The
PyModule_AddType()
function is added to help adding a type to a module. (Contributed by Dong-hee Na in bpo-40024.)
Deprecated¶
The distutils
bdist_msi
command is now deprecated, usebdist_wheel
(wheel packages) instead. (Contributed by Hugo van Kemenade in bpo-39586.)Currently
math.factorial()
acceptsfloat
instances with non-negative integer values (like5.0
). It raises aValueError
for non-integral and negative floats. It is now deprecated. In future Python versions it will raise aTypeError
for all floats. (Contributed by Serhiy Storchaka in bpo-37315.)The
parser
module is deprecated and will be removed in future versions of Python. For the majority of use cases, users can leverage the Abstract Syntax Tree (AST) generation and compilation stage, using theast
module.Using
NotImplemented
in a boolean context has been deprecated, as it is almost exclusively the result of incorrect rich comparator implementations. It will be made aTypeError
in a future version of Python. (Contributed by Josh Rosenberg in bpo-35712.)The
random
module currently accepts any hashable type as a possible seed value. Unfortunately, some of those types are not guaranteed to have a deterministic hash value. After Python 3.9, the module will restrict its seeds toNone
,int
,float
,str
,bytes
, andbytearray
.Opening the
GzipFile
file for writing without specifying the mode argument is deprecated. In future Python versions it will always be opened for reading by default. Specify the mode argument for opening it for writing and silencing a warning. (Contributed by Serhiy Storchaka in bpo-28286.)Deprecated the
split()
method of_tkinter.TkappType
in favour of thesplitlist()
method which has more consistent and predicable behavior. (Contributed by Serhiy Storchaka in bpo-38371.)The explicit passing of coroutine objects to
asyncio.wait()
has been deprecated and will be removed in version 3.11. (Contributed by Yury Selivanov and Kyle Stanley in bpo-34790.)binhex4 and hexbin4 standards are now deprecated. The :binhex module and the following
binascii
functions are now deprecated:(Contributed by Victor Stinner in bpo-39353.)
ast
classesslice
,Index
andExtSlice
are considered deprecated and will be removed in future Python versions.value
itself should be used instead ofIndex(value)
.Tuple(slices, Load())
should be used instead ofExtSlice(slices)
. (Contributed by Serhiy Storchaka in bpo-32892.)ast
classesSuite
,Param
,AugLoad
andAugStore
are considered deprecated and will be removed in future Python versions. They were not generated by the parser and not accepted by the code generator in Python 3. (Contributed by Batuhan Taskaya in bpo-39639 and bpo-39969 and Serhiy Storchaka in bpo-39988.)The
PyEval_InitThreads()
andPyEval_ThreadsInitialized()
functions are now deprecated and will be removed in Python 3.11. CallingPyEval_InitThreads()
now does nothing. The GIL is initialized byPy_Initialize()
since Python 3.7. (Contributed by Victor Stinner in bpo-39877.)
Removed¶
The erroneous version at
unittest.mock.__version__
has been removed.nntplib.NNTP
:xpath()
andxgtitle()
methods have been removed. These methods are deprecated since Python 3.3. Generally, these extensions are not supported or not enabled by NNTP server administrators. Forxgtitle()
, please usenntplib.NNTP.descriptions()
ornntplib.NNTP.description()
instead. (Contributed by Dong-hee Na in bpo-39366.)array.array
:tostring()
andfromstring()
methods have been removed. They were aliases totobytes()
andfrombytes()
, deprecated since Python 3.2. (Contributed by Victor Stinner in bpo-38916.)The undocumented
sys.callstats()
function has been removed. Since Python 3.7, it was deprecated and always returnedNone
. It required a special build optionCALL_PROFILE
which was already removed in Python 3.7. (Contributed by Victor Stinner in bpo-37414.)The
sys.getcheckinterval()
andsys.setcheckinterval()
functions have been removed. They were deprecated since Python 3.2. Usesys.getswitchinterval()
andsys.setswitchinterval()
instead. (Contributed by Victor Stinner in bpo-37392.)The C function
PyImport_Cleanup()
has been removed. It was documented as: "Empty the module table. For internal use only." (Contributed by Victor Stinner in bpo-36710.)_dummy_thread
anddummy_threading
modules have been removed. These modules were deprecated since Python 3.7 which requires threading support. (Contributed by Victor Stinner in bpo-37312.)aifc.openfp()
alias toaifc.open()
,sunau.openfp()
alias tosunau.open()
, andwave.openfp()
alias towave.open()
have been removed. They were deprecated since Python 3.7. (Contributed by Victor Stinner in bpo-37320.)The
isAlive()
method ofthreading.Thread
has been removed. It was deprecated since Python 3.8. Useis_alive()
instead. (Contributed by Dong-hee Na in bpo-37804.)Methods
getchildren()
andgetiterator()
of classesElementTree
andElement
in theElementTree
module have been removed. They were deprecated in Python 3.2. Useiter(x)
orlist(x)
instead ofx.getchildren()
andx.iter()
orlist(x.iter())
instead ofx.getiterator()
. Thexml.etree.cElementTree
module has been removed. (Contributed by Serhiy Storchaka in bpo-36543.)The old
plistlib
API has been removed, it was deprecated since Python 3.4. Use theload()
,loads()
,dump()
, anddumps()
functions. Additionally, the use_builtin_types parameter was removed, standardbytes
objects are always used instead. (Contributed by Jon Janzen in bpo-36409.)The C function
PyThreadState_DeleteCurrent()
has been removed. It was not documented. (Contributed by Joannah Nanjekye in bpo-37878.)The C function
PyGen_NeedsFinalizing
has been removed. It was not documented, tested, or used anywhere within CPython after the implementation of PEP 442. Patch by Joannah Nanjekye. (Contributed by Joannah Nanjekye in bpo-15088)base64.encodestring()
andbase64.decodestring()
, aliases deprecated since Python 3.1, have been removed: usebase64.encodebytes()
andbase64.decodebytes()
instead. (Contributed by Victor Stinner in bpo-39351.)fractions.gcd()
function has been removed, it was deprecated since Python 3.5 (bpo-22486): usemath.gcd()
instead. (Contributed by Victor Stinner in bpo-39350.)The buffering parameter of
bz2.BZ2File
has been removed. Since Python 3.0, it was ignored and using it emitted aDeprecationWarning
. Pass an open file object to control how the file is opened. (Contributed by Victor Stinner in bpo-39357.)The encoding parameter of
json.loads()
has been removed. As of Python 3.1, it was deprecated and ignored; using it has emitted aDeprecationWarning
since Python 3.8. (Contributed by Inada Naoki in bpo-39377)with (await asyncio.lock):
andwith (yield from asyncio.lock):
statements are not longer supported, useasync with lock
instead. The same is correct forasyncio.Condition
andasyncio.Semaphore
. (Contributed by Andrew Svetlov in bpo-34793.)The
sys.getcounts()
function, the-X showalloccount
command line option and theshow_alloc_count
field of the C structurePyConfig
have been removed. They required a special Python build by definingCOUNT_ALLOCS
macro. (Contributed by Victor Stinner in bpo-39489.)
Porting to Python 3.9¶
This section lists previously described changes and other bugfixes that may require changes to your code.
Changes in the Python API¶
__import__()
andimportlib.util.resolve_name()
now raiseImportError
where it previously raisedValueError
. Callers catching the specific exception type and supporting both Python 3.9 and earlier versions will need to catch both usingexcept (ImportError, ValueError):
.The
venv
activation scripts no longer special-case when__VENV_PROMPT__
is set to""
.The
select.epoll.unregister()
method no longer ignores theEBADF
error. (Contributed by Victor Stinner in bpo-39239.)The compresslevel parameter of
bz2.BZ2File
became keyword-only, since the buffering parameter has been removed. (Contributed by Victor Stinner in bpo-39357.)Simplified AST for subscription. Simple indices will be represented by their value, extended slices will be represented as tuples.
Index(value)
will return avalue
itself,ExtSlice(slices)
will returnTuple(slices, Load())
. (Contributed by Serhiy Storchaka in bpo-34822.)The
importlib
module now ignores thePYTHONCASEOK
environment variable when the-E
or-I
command line options are being used.
CPython bytecode changes¶
The
LOAD_ASSERTION_ERROR
opcode was added for handling theassert
statement. Previously, the assert statement would not work correctly if theAssertionError
exception was being shadowed. (Contributed by Zackery Spytz in bpo-34880.)