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

Release:
   3.9.0a1

Date:
   December 19, 2019

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, "sys.argv[0]" 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 development mode and in debug build, *encoding* and *errors*
  arguments are now checked on 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)


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.)


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.


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.)


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.)


pathlib
-------

Added "pathlib.Path.readlink()" which acts similarly to
"os.readlink()". (Contributed by Girts Folkmanis in bpo-30618)


pprint
------

"pprint" can now pretty-print "types.SimpleNamespace". (Contributed by
Carl Bordum Hansen in bpo-37376.)


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.)


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)


Optimizations
=============


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

* Provide "Py_EnterRecursiveCall()" and "Py_LeaveRecursiveCall()" as
  regular functions for the limited API. Previously, there were
  defined as macros, but these macros didn't work with the limited API
  which cannot access "PyThreadState.recursion_depth" field. Remove
  "_Py_CheckRecursionLimit" from the stable ABI. (Contributed by
  Victor Stinner in bpo-38644.)

* 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.)


Deprecated
==========

* 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.)


Removed
=======

* "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 abstract base classes in "collections.abc" no longer are
  exposed in the regular "collections" module.  This will help create
  a clearer distinction between the concrete classes and the abstract
  base classes.

* 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)


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 """".


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.)
