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This is Python version 3.9.18

See the end of this file for further copyright and license information.

Contents

General Information
Contributing to CPython
Using Python
Build Instructions
- Profile Guided Optimization
- Link Time Optimization
What's New
Documentation
Converting From Python 2.x to 3.x
Testing
Installing multiple versions
Issue Tracker and Mailing List
Proposals for enhancement
Release Schedule
Copyright and License Information

General Information

Website: https://www.python.org
Source code: https://github.com/python/cpython
Issue tracker: https://bugs.python.org
Documentation: https://docs.python.org
Developer's Guide: https://devguide.python.org/

Contributing to CPython

For more complete instructions on contributing to CPython development, see the Developer Guide.

Using Python

Installable Python kits, and information about using Python, are available at python.org.

You can pass many options to the configure script; run ./configure --help to find out more. On macOS case-insensitive file systems and on Cygwin, the executable is called python.exe; elsewhere it's just python.

Building a complete Python installation requires the use of various additional third-party libraries, depending on your build platform and configure options. Not all standard library modules are buildable or useable on all platforms. Refer to the Install dependencies section of the Developer Guide for current detailed information on dependencies for various Linux distributions and macOS.

On macOS, there are additional configure and build options related to macOS framework and universal builds. Refer to Mac/README.rst.

On Windows, see PCbuild/readme.txt.

If you wish, you can create a subdirectory and invoke configure from there. For example:

mkdir debug
cd debug
../configure --with-pydebug
make
make test

(This will fail if you also built at the top-level directory. You should do a make clean at the top-level first.)

To get an optimized build of Python, configure --enable-optimizations before you run make. This sets the default make targets up to enable Profile Guided Optimization (PGO) and may be used to auto-enable Link Time Optimization (LTO) on some platforms. For more details, see the sections below.

Profile Guided Optimization

PGO takes advantage of recent versions of the GCC or Clang compilers. If used, either via configure --enable-optimizations or by manually running make profile-opt regardless of configure flags, the optimized build process will perform the following steps:

The entire Python directory is cleaned of temporary files that may have resulted from a previous compilation.

An instrumented version of the interpreter is built, using suitable compiler flags for each flavor. Note that this is just an intermediary step. The binary resulting from this step is not good for real-life workloads as it has profiling instructions embedded inside.

After the instrumented interpreter is built, the Makefile will run a training workload. This is necessary in order to profile the interpreter's execution. Note also that any output, both stdout and stderr, that may appear at this step is suppressed.

The final step is to build the actual interpreter, using the information collected from the instrumented one. The end result will be a Python binary that is optimized; suitable for distribution or production installation.

Link Time Optimization

Enabled via configure's --with-lto flag. LTO takes advantage of the ability of recent compiler toolchains to optimize across the otherwise arbitrary .o file boundary when building final executables or shared libraries for additional performance gains.

What's New

We have a comprehensive overview of the changes in the What's New in Python 3.9 document. For a more detailed change log, read Misc/NEWS, but a full accounting of changes can only be gleaned from the commit history.

If you want to install multiple versions of Python, see the section below entitled "Installing multiple versions".

Documentation

Documentation for Python 3.9 is online, updated daily.

It can also be downloaded in many formats for faster access. The documentation is downloadable in HTML, PDF, and reStructuredText formats; the latter version is primarily for documentation authors, translators, and people with special formatting requirements.

For information about building Python's documentation, refer to Doc/README.rst.

Converting From Python 2.x to 3.x

Significant backward incompatible changes were made for the release of Python 3.0, which may cause programs written for Python 2 to fail when run with Python 3. For more information about porting your code from Python 2 to Python 3, see the Porting HOWTO.

Testing

To test the interpreter, type make test in the top-level directory. The test set produces some output. You can generally ignore the messages about skipped tests due to optional features which can't be imported. If a message is printed about a failed test or a traceback or core dump is produced, something is wrong.

By default, tests are prevented from overusing resources like disk space and memory. To enable these tests, run make testall.

If any tests fail, you can re-run the failing test(s) in verbose mode. For example, if test_os and test_gdb failed, you can run:

make test TESTOPTS="-v test_os test_gdb"

If the failure persists and appears to be a problem with Python rather than your environment, you can file a bug report and include relevant output from that command to show the issue.

See Running & Writing Tests for more on running tests.

Installing multiple versions

On Unix and Mac systems if you intend to install multiple versions of Python using the same installation prefix (--prefix argument to the configure script) you must take care that your primary python executable is not overwritten by the installation of a different version. All files and directories installed using make altinstall contain the major and minor version and can thus live side-by-side. make install also creates ${prefix}/bin/python3 which refers to ${prefix}/bin/pythonX.Y. If you intend to install multiple versions using the same prefix you must decide which version (if any) is your "primary" version. Install that version using make install. Install all other versions using make altinstall.

For example, if you want to install Python 2.7, 3.6, and 3.9 with 3.9 being the primary version, you would execute make install in your 3.9 build directory and make altinstall in the others.

Issue Tracker and Mailing List

Bug reports are welcome! You can use the issue tracker to report bugs, and/or submit pull requests on GitHub.

You can also follow development discussion on the python-dev mailing list.

Proposals for enhancement

If you have a proposal to change Python, you may want to send an email to the comp.lang.python or python-ideas mailing lists for initial feedback. A Python Enhancement Proposal (PEP) may be submitted if your idea gains ground. All current PEPs, as well as guidelines for submitting a new PEP, are listed at python.org/dev/peps/.

Release Schedule

See PEP 596 for Python 3.9 release details.

Copyright and License Information

See the file "LICENSE" for information on the history of this software, terms & conditions for usage, and a DISCLAIMER OF ALL WARRANTIES.

This Python distribution contains no GNU General Public License (GPL) code, so it may be used in proprietary projects. There are interfaces to some GNU code but these are entirely optional.

All trademarks referenced herein are property of their respective holders.

License

python/cpython

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