1This is Python version 3.9.1
2============================
3
4.. image:: https://travis-ci.org/python/cpython.svg?branch=3.9
5   :alt: CPython build status on Travis CI
6   :target: https://travis-ci.org/python/cpython
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15
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17   :alt: CPython code coverage on Codecov
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23
24
25Copyright (c) 2001-2020 Python Software Foundation.  All rights reserved.
26
27See the end of this file for further copyright and license information.
28
29.. contents::
30
31General Information
32-------------------
33
34- Website: https://www.python.org
35- Source code: https://github.com/python/cpython
36- Issue tracker: https://bugs.python.org
37- Documentation: https://docs.python.org
38- Developer's Guide: https://devguide.python.org/
39
40Contributing to CPython
41-----------------------
42
43For more complete instructions on contributing to CPython development,
44see the `Developer Guide`_.
45
46.. _Developer Guide: https://devguide.python.org/
47
48Using Python
49------------
50
51Installable Python kits, and information about using Python, are available at
52`python.org`_.
53
54.. _python.org: https://www.python.org/
55
56Build Instructions
57------------------
58
59On Unix, Linux, BSD, macOS, and Cygwin::
60
61    ./configure
62    make
63    make test
64    sudo make install
65
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 various
73additional third-party libraries, depending on your build platform and
74configure options.  Not all standard library modules are buildable or
75useable on all platforms.  Refer to the
76`Install dependencies <https://devguide.python.org/setup/#install-dependencies>`_
77section of the `Developer Guide`_ for current detailed information on
78dependencies for various Linux distributions and macOS.
79
80On macOS, there are additional configure and build options related
81to macOS framework and universal builds.  Refer to `Mac/README.rst
82<https://github.com/python/cpython/blob/3.9/Mac/README.rst>`_.
83
84On Windows, see `PCbuild/readme.txt
85<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 debug
91    cd debug
92    ../configure --with-pydebug
93    make
94    make test
95
96(This will fail if you *also* built at the top-level directory.  You should do
97a ``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 enable
101Profile Guided Optimization (PGO) and may be used to auto-enable Link Time
102Optimization (LTO) on some platforms.  For more details, see the sections
103below.
104
105Profile Guided Optimization
106^^^^^^^^^^^^^^^^^^^^^^^^^^^
107
108PGO takes advantage of recent versions of the GCC or Clang compilers.  If used,
109either via ``configure --enable-optimizations`` or by manually running
110``make profile-opt`` regardless of configure flags, the optimized build
111process will perform the following steps:
112
113The entire Python directory is cleaned of temporary files that may have
114resulted from a previous compilation.
115
116An instrumented version of the interpreter is built, using suitable compiler
117flags for each flavour. Note that this is just an intermediary step.  The
118binary resulting from this step is not good for real life workloads as it has
119profiling instructions embedded inside.
120
121After the instrumented interpreter is built, the Makefile will run a training
122workload.  This is necessary in order to profile the interpreter execution.
123Note also that any output, both stdout and stderr, that may appear at this step
124is suppressed.
125
126The final step is to build the actual interpreter, using the information
127collected from the instrumented one.  The end result will be a Python binary
128that is optimized; suitable for distribution or production installation.
129
130
131Link Time Optimization
132^^^^^^^^^^^^^^^^^^^^^^
133
134Enabled via configure's ``--with-lto`` flag.  LTO takes advantage of the
135ability of recent compiler toolchains to optimize across the otherwise
136arbitrary ``.o`` file boundary when building final executables or shared
137libraries for additional performance gains.
138
139
140What's New
141----------
142
143We have a comprehensive overview of the changes in the `What's New in Python
1443.9 <https://docs.python.org/3.9/whatsnew/3.9.html>`_ document.  For a more
145detailed change log, read `Misc/NEWS
146<https://github.com/python/cpython/blob/3.9/Misc/NEWS.d>`_, but a full
147accounting of changes can only be gleaned from the `commit history
148<https://github.com/python/cpython/commits/3.9>`_.
149
150If you want to install multiple versions of Python, see the section below
151entitled "Installing multiple versions".
152
153
154Documentation
155-------------
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 documentation
161is downloadable in HTML, PDF, and reStructuredText formats; the latter version
162is primarily for documentation authors, translators, and people with special
163formatting requirements.
164
165For information about building Python's documentation, refer to `Doc/README.rst
166<https://github.com/python/cpython/blob/3.9/Doc/README.rst>`_.
167
168
169Converting From Python 2.x to 3.x
170---------------------------------
171
172Significant backward incompatible changes were made for the release of Python
1733.0, which may cause programs written for Python 2 to fail when run with Python
1743.  For more information about porting your code from Python 2 to Python 3, see
175the `Porting HOWTO <https://docs.python.org/3/howto/pyporting.html>`_.
176
177
178Testing
179-------
180
181To test the interpreter, type ``make test`` in the top-level directory.  The
182test set produces some output.  You can generally ignore the messages about
183skipped tests due to optional features which can't be imported.  If a message
184is 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 and
188memory.  To enable these tests, run ``make testall``.
189
190If any tests fail, you can re-run the failing test(s) in verbose mode.  For
191example, 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 than
196your environment, you can `file a bug report <https://bugs.python.org>`_ and
197include 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 versions
203----------------------------
204
205On Unix and Mac systems if you intend to install multiple versions of Python
206using the same installation prefix (``--prefix`` argument to the configure
207script) you must take care that your primary python executable is not
208overwritten by the installation of a different version.  All files and
209directories installed using ``make altinstall`` contain the major and minor
210version and can thus live side-by-side.  ``make install`` also creates
211``${prefix}/bin/python3`` which refers to ``${prefix}/bin/pythonX.Y``.  If you
212intend to install multiple versions using the same prefix you must decide which
213version (if any) is your "primary" version.  Install that version using ``make
214install``.  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 the
217primary version, you would execute ``make install`` in your 3.9 build directory
218and ``make altinstall`` in the others.
219
220
221Issue Tracker and Mailing List
222------------------------------
223
224Bug reports are welcome!  You can use the `issue tracker
225<https://bugs.python.org>`_ to report bugs, and/or submit pull requests `on
226GitHub <https://github.com/python/cpython>`_.
227
228You can also follow development discussion on the `python-dev mailing list
229<https://mail.python.org/mailman/listinfo/python-dev/>`_.
230
231
232Proposals for enhancement
233-------------------------
234
235If you have a proposal to change Python, you may want to send an email to the
236comp.lang.python or `python-ideas`_ mailing lists for initial feedback.  A
237Python 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 at
239`python.org/dev/peps/ <https://www.python.org/dev/peps/>`_.
240
241.. _python-ideas: https://mail.python.org/mailman/listinfo/python-ideas/
242
243
244Release Schedule
245----------------
246
247See :pep:`596` for Python 3.9 release details.
248
249
250Copyright and License Information
251---------------------------------
252
253Copyright (c) 2001-2020 Python Software Foundation.  All rights reserved.
254
255Copyright (c) 2000 BeOpen.com.  All rights reserved.
256
257Copyright (c) 1995-2001 Corporation for National Research Initiatives.  All
258rights reserved.
259
260Copyright (c) 1991-1995 Stichting Mathematisch Centrum.  All rights reserved.
261
262See the file "LICENSE" for information on the history of this software, terms &
263conditions for usage, and a DISCLAIMER OF ALL WARRANTIES.
264
265This Python distribution contains *no* GNU General Public License (GPL) code,
266so it may be used in proprietary projects.  There are interfaces to some GNU
267code but these are entirely optional.
268
269All trademarks referenced herein are property of their respective holders.
270