What's New In Python 3.9
************************

Release:
   3.9.0a3

Date:
   February 25, 2020

This article explains the new features in Python 3.9, compared to 3.8.

For full details, see the changelog.

Note:

  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
=============================


New Features
============


Other Language Changes
======================

* "__import__()" now raises "ImportError" instead of "ValueError",
  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 and "sys.path[0]" become an
  absolute path, rather than a relative path. These paths now remain
  valid after the current directory is changed by "os.chdir()". As a
  side effect, a 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()" and
  "bytes.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 returns "s" instead of an empty string
  for all non-zero "n".  It is now consistent with """.replace("",
  s)". There are similar changes for "bytes" and "bytearray" objects.
  (Contributed by Serhiy Storchaka in bpo-28029.)


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.)


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.)


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.)


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.)


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 assignment "y = 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.)


Build and C API Changes
=======================

* 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()" and "PyFPE_END_PROTECT()" macros of
  "pyfpe.h" from "Py_LIMITED_API" (stable API). (Contributed by Victor
  Stinner in bpo-38835.)

* Remove "PyMethod_ClearFreeList()" and "PyCFunction_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()" and "unsetenv()" 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()" and "Py_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 access "PyThreadState.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()" and "PyObject_INIT_VAR()"  become aliases to
      "PyObject_Init()" and "PyObject_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 to "Py_TRACE_REFS" build)

  (Contributed by Victor Stinner in bpo-38644 and bpo-39542.)


Deprecated
==========

* The distutils "bdist_msi" command is now deprecated, use
  "bdist_wheel" (wheel packages) instead. (Contributed by Hugo van
  Kemenade in bpo-39586.)

* Currently "math.factorial()" accepts "float" instances with non-
  negative integer values (like "5.0").  It raises a "ValueError" for
  non-integral and negative floats.  It is now deprecated.  In future
  Python versions it will raise a "TypeError" 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 the "ast" module.

* 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 to "None", "int", "float", "str",
  "bytes", and "bytearray".

* 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
  the "splitlist()" 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:

  * "b2a_hqx()", "a2b_hqx()"

  * "rlecode_hqx()", "rledecode_hqx()"

  (Contributed by Victor Stinner in bpo-39353.)


Removed
=======

* The erroneous version at "unittest.mock.__version__" has been
  removed.

* "nntplib.NNTP": "xpath()" and "xgtitle()" 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. For "xgtitle()", please use
  "nntplib.NNTP.descriptions()" or "nntplib.NNTP.description()"
  instead. (Contributed by Dong-hee Na in bpo-39366.)

* "array.array": "tostring()" and "fromstring()" methods have been
  removed. They were aliases to "tobytes()" and "frombytes()",
  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 returned "None". It
  required a special build option "CALL_PROFILE" which was already
  removed in Python 3.7. (Contributed by Victor Stinner in bpo-37414.)

* The "sys.getcheckinterval()" and "sys.setcheckinterval()" functions
  have been removed. They were deprecated since Python 3.2. Use
  "sys.getswitchinterval()" and "sys.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" and "dummy_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 to "aifc.open()", "sunau.openfp()" alias to
  "sunau.open()", and "wave.openfp()" alias to "wave.open()" have been
  removed. They were deprecated since Python 3.7. (Contributed by
  Victor Stinner in bpo-37320.)

* The "isAlive()" method of "threading.Thread" has been removed. It
  was deprecated since Python 3.8. Use "is_alive()" instead.
  (Contributed by Dong-hee Na in bpo-37804.)

* Methods "getchildren()" and "getiterator()" in the "ElementTree"
  module have been removed.  They were deprecated in Python 3.2.  Use
  functions "list()" and "iter()" instead.  The
  "xml.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 the "load()", "loads()", "dump()", and "dumps()"
  functions. Additionally, the *use_builtin_types* parameter was
  removed, standard "bytes" 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()" and "base64.decodestring()", aliases
  deprecated since Python 3.1, have been removed: use
  "base64.encodebytes()" and "base64.decodebytes()" instead.
  (Contributed by Victor Stinner in bpo-39351.)

* "fractions.gcd()" function has been removed, it was deprecated since
  Python 3.5 (bpo-22486): use "math.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 a
  "DeprecationWarning". 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 a
  "DeprecationWarning" since Python 3.8. (Contributed by Inada Naoki
  in bpo-39377)

* "with (await asyncio.lock):" and "with (yield from asyncio.lock):"
  statements are not longer supported, use "async with lock" instead.
  The same is correct for "asyncio.Condition" and "asyncio.Semaphore".
  (Contributed by Andrew Svetlov in bpo-34793.)

* The "sys.getcounts()" function, the "-X showalloccount" command line
  option and the "show_alloc_count" field of the C structure
  "PyConfig" have been removed. They required a special Python build
  by defining "COUNT_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
-------------------------

* "open()", "io.open()", "codecs.open()" and "fileinput.FileInput" no
  longer accept "'U'" ("universal newline") in the file mode. This
  flag was deprecated since Python 3.3. In Python 3, the "universal
  newline" is used by default when a file is open in text mode.  The
  newline parameter of "open()" controls how universal newlines works.
  (Contributed by Victor Stinner in bpo-37330.)

* "__import__()" and "importlib.util.resolve_name()" now raise
  "ImportError" where it previously raised "ValueError". Callers
  catching the specific exception type and supporting both Python 3.9
  and earlier versions will need to catch both using "except
  (ImportError, ValueError):".

* The "venv" activation scripts no longer special-case when
  "__VENV_PROMPT__" is set to """".

* The "select.epoll.unregister()" method no longer ignores the "EBADF"
  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.)


CPython bytecode changes
------------------------

* The "LOAD_ASSERTION_ERROR" opcode was added for handling the
  "assert" statement. Previously, the assert statement would not work
  correctly if the "AssertionError" exception was being shadowed.
  (Contributed by Zackery Spytz in bpo-34880.)
