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1This is Python version 3.9.252=============================3 4.. image:: https://travis-ci.org/python/cpython.svg?branch=3.95   :alt: CPython build status on Travis CI6   :target: https://travis-ci.org/python/cpython7 8.. image:: https://github.com/python/cpython/workflows/Tests/badge.svg9   :alt: CPython build status on GitHub Actions10   :target: https://github.com/python/cpython/actions11 12.. image:: https://dev.azure.com/python/cpython/_apis/build/status/Azure%20Pipelines%20CI?branchName=3.913   :alt: CPython build status on Azure DevOps14   :target: https://dev.azure.com/python/cpython/_build/latest?definitionId=4&branchName=3.915 16.. image:: https://codecov.io/gh/python/cpython/branch/3.9/graph/badge.svg17   :alt: CPython code coverage on Codecov18   :target: https://codecov.io/gh/python/cpython19 20.. image:: https://img.shields.io/badge/discourse-join_chat-brightgreen.svg21   :alt: Python Discourse chat22   :target: https://discuss.python.org/23 24 25Copyright © 2001-2023 Python Software Foundation.  All rights reserved.26 27See the end of this file for further copyright and license information.28 29.. contents::30 31General Information32-------------------33 34- Website: https://www.python.org35- Source code: https://github.com/python/cpython36- Issue tracker: https://bugs.python.org37- Documentation: https://docs.python.org38- Developer's Guide: https://devguide.python.org/39 40Contributing to CPython41-----------------------42 43For more complete instructions on contributing to CPython development,44see the `Developer Guide`_.45 46.. _Developer Guide: https://devguide.python.org/47 48Using Python49------------50 51Installable Python kits, and information about using Python, are available at52`python.org`_.53 54.. _python.org: https://www.python.org/55 56Build Instructions57------------------58 59On Unix, Linux, BSD, macOS, and Cygwin::60 61    ./configure62    make63    make test64    sudo make install65 66This will install Python as ``python3``.67 68You can pass many options to the configure script; run ``./configure --help``69to find out more.  On macOS case-insensitive file systems and on Cygwin,70the executable is called ``python.exe``; elsewhere it's just ``python``.71 72Building a complete Python installation requires the use of various73additional third-party libraries, depending on your build platform and74configure options.  Not all standard library modules are buildable or75useable on all platforms.  Refer to the76`Install dependencies <https://devguide.python.org/setup/#install-dependencies>`_77section of the `Developer Guide`_ for current detailed information on78dependencies for various Linux distributions and macOS.79 80On macOS, there are additional configure and build options related81to macOS framework and universal builds.  Refer to `Mac/README.rst82<https://github.com/python/cpython/blob/3.9/Mac/README.rst>`_.83 84On Windows, see `PCbuild/readme.txt85<https://github.com/python/cpython/blob/3.9/PCbuild/readme.txt>`_.86 87If you wish, you can create a subdirectory and invoke configure from there.88For example::89 90    mkdir debug91    cd debug92    ../configure --with-pydebug93    make94    make test95 96(This will fail if you *also* built at the top-level directory.  You should do97a ``make clean`` at the top-level first.)98 99To get an optimized build of Python, ``configure --enable-optimizations``100before you run ``make``.  This sets the default make targets up to enable101Profile Guided Optimization (PGO) and may be used to auto-enable Link Time102Optimization (LTO) on some platforms.  For more details, see the sections103below.104 105Profile Guided Optimization106^^^^^^^^^^^^^^^^^^^^^^^^^^^107 108PGO takes advantage of recent versions of the GCC or Clang compilers.  If used,109either via ``configure --enable-optimizations`` or by manually running110``make profile-opt`` regardless of configure flags, the optimized build111process will perform the following steps:112 113The entire Python directory is cleaned of temporary files that may have114resulted from a previous compilation.115 116An instrumented version of the interpreter is built, using suitable compiler117flags for each flavor. Note that this is just an intermediary step.  The118binary resulting from this step is not good for real-life workloads as it has119profiling instructions embedded inside.120 121After the instrumented interpreter is built, the Makefile will run a training122workload.  This is necessary in order to profile the interpreter's execution.123Note also that any output, both stdout and stderr, that may appear at this step124is suppressed.125 126The final step is to build the actual interpreter, using the information127collected from the instrumented one.  The end result will be a Python binary128that is optimized; suitable for distribution or production installation.129 130 131Link Time Optimization132^^^^^^^^^^^^^^^^^^^^^^133 134Enabled via configure's ``--with-lto`` flag.  LTO takes advantage of the135ability of recent compiler toolchains to optimize across the otherwise136arbitrary ``.o`` file boundary when building final executables or shared137libraries for additional performance gains.138 139 140What's New141----------142 143We have a comprehensive overview of the changes in the `What's New in Python1443.9 <https://docs.python.org/3.9/whatsnew/3.9.html>`_ document.  For a more145detailed change log, read `Misc/NEWS146<https://github.com/python/cpython/blob/3.9/Misc/NEWS.d>`_, but a full147accounting of changes can only be gleaned from the `commit history148<https://github.com/python/cpython/commits/3.9>`_.149 150If you want to install multiple versions of Python, see the section below151entitled "Installing multiple versions".152 153 154Documentation155-------------156 157`Documentation for Python 3.9 <https://docs.python.org/3.9/>`_ is online,158updated daily.159 160It can also be downloaded in many formats for faster access.  The documentation161is downloadable in HTML, PDF, and reStructuredText formats; the latter version162is primarily for documentation authors, translators, and people with special163formatting requirements.164 165For information about building Python's documentation, refer to `Doc/README.rst166<https://github.com/python/cpython/blob/3.9/Doc/README.rst>`_.167 168 169Converting From Python 2.x to 3.x170---------------------------------171 172Significant backward incompatible changes were made for the release of Python1733.0, which may cause programs written for Python 2 to fail when run with Python1743.  For more information about porting your code from Python 2 to Python 3, see175the `Porting HOWTO <https://docs.python.org/3/howto/pyporting.html>`_.176 177 178Testing179-------180 181To test the interpreter, type ``make test`` in the top-level directory.  The182test set produces some output.  You can generally ignore the messages about183skipped tests due to optional features which can't be imported.  If a message184is printed about a failed test or a traceback or core dump is produced,185something is wrong.186 187By default, tests are prevented from overusing resources like disk space and188memory.  To enable these tests, run ``make testall``.189 190If any tests fail, you can re-run the failing test(s) in verbose mode.  For191example, if ``test_os`` and ``test_gdb`` failed, you can run::192 193    make test TESTOPTS="-v test_os test_gdb"194 195If the failure persists and appears to be a problem with Python rather than196your environment, you can `file a bug report <https://bugs.python.org>`_ and197include relevant output from that command to show the issue.198 199See `Running & Writing Tests <https://devguide.python.org/runtests/>`_200for more on running tests.201 202Installing multiple versions203----------------------------204 205On Unix and Mac systems if you intend to install multiple versions of Python206using the same installation prefix (``--prefix`` argument to the configure207script) you must take care that your primary python executable is not208overwritten by the installation of a different version.  All files and209directories installed using ``make altinstall`` contain the major and minor210version and can thus live side-by-side.  ``make install`` also creates211``${prefix}/bin/python3`` which refers to ``${prefix}/bin/pythonX.Y``.  If you212intend to install multiple versions using the same prefix you must decide which213version (if any) is your "primary" version.  Install that version using ``make214install``.  Install all other versions using ``make altinstall``.215 216For example, if you want to install Python 2.7, 3.6, and 3.9 with 3.9 being the217primary version, you would execute ``make install`` in your 3.9 build directory218and ``make altinstall`` in the others.219 220 221Issue Tracker and Mailing List222------------------------------223 224Bug reports are welcome!  You can use the `issue tracker225<https://bugs.python.org>`_ to report bugs, and/or submit pull requests `on226GitHub <https://github.com/python/cpython>`_.227 228You can also follow development discussion on the `python-dev mailing list229<https://mail.python.org/mailman/listinfo/python-dev/>`_.230 231 232Proposals for enhancement233-------------------------234 235If you have a proposal to change Python, you may want to send an email to the236`comp.lang.python`_ or `python-ideas`_ mailing lists for initial feedback.  A237Python Enhancement Proposal (PEP) may be submitted if your idea gains ground.238All current PEPs, as well as guidelines for submitting a new PEP, are listed at239`python.org/dev/peps/ <https://www.python.org/dev/peps/>`_.240 241.. _python-ideas: https://mail.python.org/mailman/listinfo/python-ideas/242.. _comp.lang.python: https://mail.python.org/mailman/listinfo/python-list243 244 245Release Schedule246----------------247 248See :pep:`596` for Python 3.9 release details.249 250 251Copyright and License Information252---------------------------------253 254Copyright © 2001-2023 Python Software Foundation.  All rights reserved.255 256Copyright © 2000 BeOpen.com.  All rights reserved.257 258Copyright © 1995-2001 Corporation for National Research Initiatives.  All259rights reserved.260 261Copyright © 1991-1995 Stichting Mathematisch Centrum.  All rights reserved.262 263See the file "LICENSE" for information on the history of this software, terms &264conditions for usage, and a DISCLAIMER OF ALL WARRANTIES.265 266This Python distribution contains *no* GNU General Public License (GPL) code,267so it may be used in proprietary projects.  There are interfaces to some GNU268code but these are entirely optional.269 270All trademarks referenced herein are property of their respective holders.