What’s New In Python 3.9

Release

3.9.0a0

Date

August 25, 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

  • builtins.__import__() now raises ImportError instead of ValueError as 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 performances, 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.)

New Modules

  • None yet.

Improved Modules

threading

In a subinterpreter, spawning a daemon thread now raises an exception. 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.)

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.)

Optimizations

Build and C API Changes

  • Add a new public PyObject_CallNoArgs() function to the C API: call 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.)

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 deprecated now. 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.

Removed

  • 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.)

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

  • builtins.__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: except (ImportError, ValueError):

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