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eamanu and cjw296 bpo-24928: Add test case for patch.dict using OrderedDict (GH -11437)
* add test for path.dict using OrderedDict

Co-authored-by: Yu Tomita
Latest commit 1d0c5e1 Jan 24, 2020
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.azure-pipelines Fix Windows release builds (GH-17550) Dec 10, 2019
.github Run doctests in GitHub actions Docs targer (GH-18041) Jan 17, 2020
Doc bpo-17005: Minor improvements to the documentation of TopologicalSort… Jan 23, 2020
Grammar bpo-35814: Allow unpacking in r.h.s of annotated assignment expressio… Jun 3, 2019
Include bpo-39320: Handle unpacking of *values in compiler (GH-17984) Jan 23, 2020
Lib bpo-24928: Add test case for patch.dict using OrderedDict (GH -11437) Jan 24, 2020
Mac Update copyright year in macOS installer license copy (GH-17806) Jan 3, 2020
Misc bpo-17005: Add a class to perform topological sorting to the standard… Jan 23, 2020
Modules bpo-39421: Fix posible crash in heapq with custom comparison operators ( Jan 23, 2020
Parser Allow pgen to produce a DOT format dump of the grammar (GH-18005) Jan 14, 2020
Programs bpo-38304: PyConfig_InitPythonConfig() cannot fail anymore (GH-16509) Oct 1, 2019
Python bpo-39320: Handle unpacking of *values in compiler (GH-17984) Jan 23, 2020
Tools bpo-39156: Break up COMPARE_OP into four logically distinct opcodes. (G… Jan 14, 2020
m4 bpo-39160 Align the verbs, grammar and defaults for `./configure --he… Jan 14, 2020
.gitattributes bpo-37760: Mark all generated Unicode data headers as generated. (GH-… Aug 13, 2019
.gitignore Be more specific about the `.so` gitignore patterns (GH-17328) Nov 27, 2019
.travis.yml bpo-39035: travis: Don't use beta group (GH-17602) Dec 14, 2019 Fix markup and minor grammar improvements in (GH-1… May 28, 2019
LICENSE Bring Python into the next decade. (GH-17801) Jan 3, 2020 bpo-39144 Align ctags and etags behaviours in the makefile and includ… Dec 28, 2019
README.rst Bring Python into the next decade. (GH-17801) Jan 3, 2020
aclocal.m4 bpo-34652 again: Remove lchmod from the default AC_CHECK_FUNCS list. (G… Sep 9, 2019
config.guess bpo-33393: Update config.guess and config.sub files (#6658) Apr 30, 2018
config.sub bpo-33393: Update config.guess and config.sub files (#6658) Apr 30, 2018
configure bpo-39406: Implement os.putenv() with setenv() if available (GH-18128) Jan 22, 2020 bpo-39406: Implement os.putenv() with setenv() if available (GH-18128) Jan 22, 2020
install-sh bpo-34765: install-sh is executable (GH-10225) Oct 29, 2018
netlify.toml bpo-37860: Add netlify deploy preview for docs (GH-15288) Aug 21, 2019 bpo-39406: Implement os.putenv() with setenv() if available (GH-18128) Jan 22, 2020 bpo-37798: Add C fastpath for statistics.NormalDist.inv_cdf() (GH-15266) Aug 23, 2019


This is Python version 3.9.0 alpha 2

CPython build status on Travis CI CPython build status on GitHub Actions CPython build status on Azure DevOps CPython code coverage on Codecov Python Zulip chat

Copyright (c) 2001-2020 Python Software Foundation. All rights reserved.

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

General Information

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

Build Instructions

On Unix, Linux, BSD, macOS, and Cygwin:

make test
sudo make install

This will install Python as python3.

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 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 flavour. 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 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 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.


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

Release Schedule

See PEP 596 for Python 3.9 release details.

Copyright and License Information

Copyright (c) 2001-2020 Python Software Foundation. All rights reserved.

Copyright (c) 2000 All rights reserved.

Copyright (c) 1995-2001 Corporation for National Research Initiatives. All rights reserved.

Copyright (c) 1991-1995 Stichting Mathematisch Centrum. All rights reserved.

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.

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