1. Getting Started¶
These instructions cover how to get a working copy of the source code and a compiled version of the CPython interpreter (CPython is the version of Python available from https://www.python.org/). It also gives an overview of the directory structure of the CPython source code.
Alternatively, if you have Docker installed you might want to use our official images. These contain the latest releases of several Python versions, along with git head, and are provided for development and testing purposes only.
The Quick Reference gives brief summary of the process from installing git to submitting a pull request.
CPython is developed using git for version control. The git
command line program is named
git; this is also used to refer to git
itself. git is easily available for all common operating systems.
As the CPython repo is hosted on GitHub, please refer to either the GitHub setup instructions or the git project instructions for step-by-step installation directions. You may also want to consider a graphical client such as TortoiseGit or GitHub Desktop.
Configure your name and email and create an SSH key as this will allow you to interact with GitHub without typing a username and password each time you execute a command, such as
git push, or
git fetch. On Windows, you should also enable autocrlf.
1.2. Get the source code¶
The CPython repo is hosted on GitHub. To get a copy of the source code you should fork the Python repository on GitHub, create a local clone of your personal fork, and configure the remotes.
You will only need to execute these steps once:
Press Fork on the top right.
When asked where to fork the repository, choose to fork it to your username.
Your fork will be created at
Clone your GitHub fork (replace
<username>with your username):
$ git clone firstname.lastname@example.org:<username>/cpython.git
(You can use both SSH-based or HTTPS-based URLs.)
$ cd cpython $ git remote add upstream email@example.com:python/cpython.git
Verify that your setup is correct:
$ git remote -v origin firstname.lastname@example.org:<your-username>/cpython.git (fetch) origin email@example.com:<your-username>/cpython.git (push) upstream firstname.lastname@example.org:python/cpython.git (fetch) upstream email@example.com:python/cpython.git (push)
If you did everything correctly, you should now have a copy of the code
cpython directory and two remotes that refer to your own GitHub fork
origin) and the official CPython repository (
If you want a working copy of an already-released version of Python,
i.e., a version in maintenance mode, you can checkout
a release branch. For instance, to checkout a working copy of Python 3.8,
git checkout 3.8.
You will need to re-compile CPython when you do such an update.
Do note that CPython will notice that it is being run from a working copy. This means that if you edit CPython’s source code in your working copy, changes to Python code will be picked up by the interpreter for immediate use and testing. (If you change C code, you will need to recompile the affected files as described below.)
Patches for the documentation can be made from the same repository; see Documenting Python.
1.3. Compile and build¶
CPython provides several compilation flags which help with debugging various
things. While all of the known flags can be found in the
Misc/SpecialBuilds.txt file, the most critical one is the
which creates what is known as a “pydebug” build. This flag turns on various
extra sanity checks which help catch common issues. The use of the flag is so
common that turning on the flag is a basic compile option.
You should always develop under a pydebug build of CPython (the only instance of when you shouldn’t is if you are taking performance measurements). Even when working only on pure Python code the pydebug build provides several useful checks that one should not skip.
The core CPython interpreter only needs a C compiler to be built,
however, some of the extension modules will need development headers
for additional libraries (such as the
zlib library for compression).
Depending on what you intend to work on, you might need to install these
additional requirements so that the compiled interpreter supports the
If you want to install these optional dependencies, consult the Install dependencies section below.
If you don’t need to install them, the basic steps for building Python for development is to configure it and then compile it.
Configuration is typically:
More flags are available to
configure, but this is the minimum you should
do to get a pydebug build of CPython.
You might need to run
make clean before or after re-running
in a particular build directory.
configure is done, you can then compile CPython with:
make -s -j2
This will build CPython with only warnings and errors being printed to
stderr and utilize up to 2 CPU cores. If you are using a multi-core machine
with more than 2 cores (or a single-core machine), you can adjust the number
passed into the
-j flag to match the number of cores you have (or if your
version of Make supports it, you can use
-j without a number and Make
will not limit the number of steps that can run simultaneously.).
At the end of the build you should see a success message, possibly followed by a list of extension modules that haven’t been built because their dependencies were missing:
Python build finished successfully! The necessary bits to build these optional modules were not found: _bz2 _dbm _gdbm _lzma _sqlite3 _ssl _tkinter _uuid readline zlib To find the necessary bits, look in setup.py in detect_modules() for the module's name.
If the build failed and you are using a C89 or C99-compliant compiler, please open a bug report.
If you decide to Install dependencies, you will need to re-run both
Once CPython is done building you will then have a working build
that can be run in-place;
./python on most machines (and what is used in
./python.exe wherever a case-insensitive filesystem is used
(e.g. on OS X by default), in order to avoid conflicts with the
directory. There is normally no need to install your built copy
of Python! The interpreter will realize where it is being run from
and thus use the files found in the working copy. If you are worried
you might accidentally install your working copy build, you can add
--prefix=/tmp/python to the configuration step. When running from your
working directory, it is best to avoid using the
configure; unless you are very careful, you may accidentally run
with code from an older, installed shared Python library rather than from
the interpreter you just built.
If you are using clang to build CPython, some flags you might want to set to
quiet some standard warnings which are specifically superfluous to CPython are
-Wno-unused-value -Wno-empty-body -Qunused-arguments. You can set your
CFLAGS environment variable to these flags when running
If you are using clang with ccache, turn off the noisy
parentheses-equality warnings with the
These warnings are caused by clang not having enough information to detect
that extraneous parentheses in expanded macros are valid, because the
preprocessing is done separately by ccache.
If you are using LLVM 2.8, also use the
-no-integrated-as flag in order to
ctypes module (without the flag the rest of CPython will
still build properly).
For a quick guide to building you can read this documentation from Victor Stinner.
Python 3.6 and later can use Microsoft Visual Studio 2017. You can download and use any of the free or paid versions of Visual Studio 2017.
When installing Visual Studio 2017, select the Python development workload and the optional Python native development tools component to obtain all of the necessary build tools. If you do not already have git installed, you can find git for Windows on the Individual components tab of the installer.
If you want to build MSI installers, be aware that the build toolchain for them has a dependency on the Microsoft .NET Framework Version 3.5 (which may not be configured on recent versions of Windows, such as Windows 10). If you are building on a recent Windows version, use the Control Panel (Programs | Programs and Features | Turn Windows Features on or off) and ensure that the entry “.NET Framework 3.5 (includes .NET 2.0 and 3.0)” is enabled.
Your first build should use the command line to ensure any external dependencies are downloaded:
After this build succeeds, you can open the
PCBuild\pcbuild.sln solution in
Visual Studio to continue development.
See the readme for more details on what other software is necessary and how to build.
If you are using the Windows Subsystem for Linux (WSL), clone the repository from a native Windows terminal program like cmd.exe command prompt or PowerShell as well as use a build of git targeted for Windows, e.g., the official one from https://git-scm.com. Otherwise, Visual Studio will not be able to find all the project’s files and will fail the build.
1.4. Install dependencies¶
For UNIX based systems, we try to use system libraries whenever available. This means optional components will only build if the relevant system headers are available. The best way to obtain the appropriate headers will vary by distribution, but the appropriate commands for some popular distributions are below.
On Fedora, Red Hat Enterprise Linux and other
yum based systems:
$ sudo yum install yum-utils $ sudo yum-builddep python3
On Fedora and other
DNF based systems:
$ sudo dnf install dnf-plugins-core # install this to use 'dnf builddep' $ sudo dnf builddep python3
On Debian, Ubuntu, and other
apt based systems, try to get the
dependencies for the Python you’re working on by using the
First, make sure you have enabled the source packages in the sources list.
You can do this by adding the location of the source packages, including
URL, distribution name and component name, to
Take Ubuntu Bionic for example:
deb-src http://archive.ubuntu.com/ubuntu/ bionic main
For other distributions, like Debian, change the URL and names to correspond with the specific distribution.
Then you should update the packages index:
$ sudo apt-get update
Now you can install the build dependencies via
$ sudo apt-get build-dep python3.6
If that package is not available for your system, try reducing the minor version until you find a package that is available.
1.4.2. macOS and OS X¶
For macOS systems (versions 10.12+) and OS X 10.9 and later, the Developer Tools can be downloaded and installed automatically; you do not need to download the complete Xcode application.
If necessary, run the following:
$ xcode-select --install
This will also ensure that the system header files are installed into
On Mac OS X systems (versions 10.0 - 10.7) and OS X 10.8, use the C compiler and other development utilities provided by Apple’s Xcode Developer Tools. The Developer Tools are not shipped with Mac OS X.
For these older releases (versions 10.0 - 10.8), you will need to download either the correct version of the Command Line Tools, if available, or install them from the full Xcode app or package for that OS X release. Older versions may be available either as a no-cost download through Apple’s App Store or from the Apple Developer web site.
Also note that OS X does not include several libraries used by the Python
standard library, including
libzma, so expect to see some extension module
build failures unless you install local copies of them. As of OS X 10.11,
Apple no longer provides header files for the deprecated system version of
OpenSSL which means that you will not be able to build the
One solution is to install these libraries from a third-party package
manager, like Homebrew or MacPorts, and then add the appropriate paths
for the header and library files to your
configure command. For example,
$ brew install openssl xz gdbm
configure Python versions >= 3.7:
./configure --with-pydebug --with-openssl=$(brew --prefix openssl)
configure Python versions < 3.7:
$ CPPFLAGS="-I$(brew --prefix openssl)/include" \ LDFLAGS="-L$(brew --prefix openssl)/lib" \ ./configure --with-pydebug
$ make -s -j2
$ sudo port install pkgconfig openssl xz gdbm
$ CPPFLAGS="-I/opt/local/include" \ LDFLAGS="-L/opt/local/lib" \ ./configure --with-pydebug
$ make -s -j2
There will sometimes be optional modules added for a new release which won’t yet be identified in the OS level build dependencies. In those cases, just ask for assistance on the core-mentorship list.
Explaining how to build optional dependencies on a UNIX based system without root access is beyond the scope of this guide.
While you need a C compiler to build CPython, you don’t need any knowledge of the C language to contribute! Vast areas of CPython are written completely in Python: as of this writing, CPython contains slightly more Python code than C.
If a change is made to Python which relies on some POSIX system-specific
functionality (such as using a new system call), it is necessary to update the
configure script to test for availability of the functionality.
configure script is generated from
configure.ac using Autoconf.
Instead of editing
configure.ac and then run
autoreconf to regenerate
configure and a number of other files (such as
When submitting a patch with changes made to
configure.ac, you should also
include the generated files.
Note that running
autoreconf is not the same as running
autoconf by itself will not regenerate
autoconf and a number of other tools repeatedly as is
configure.ac script typically requires a specific version of
Autoconf. At the moment, this reads:
If the system copy of Autoconf does not match this version, you will need to install your own copy of Autoconf.
1.6. Troubleshoot the build¶
This section lists some of the common problems that may arise during the compilation of Python, with proposed solutions.
1.6.1. Avoid recreating auto-generated files¶
Under some circumstances you may encounter Python errors in scripts like
Python/makeopcodetargets.py while running
Python auto-generates some of its own code, and a full build from scratch needs
to run the auto-generation scripts. However, this makes the Python build require
an already installed Python interpreter; this can also cause version mismatches
when trying to build an old (2.x) Python with a new (3.x) Python installed, or
To overcome this problem, auto-generated files are also checked into the Git repository. So if you don’t touch the auto-generation scripts, there’s no real need to auto-generate anything.
1.7. Editors and Tools¶
Python is used widely enough that practically all code editors have some form of support for writing Python code. Various coding tools also include Python support.
For editors and tools which the core developers have felt some special comment is needed for coding in Python, see Additional Resources.
1.8. Directory structure¶
There are several top-level directories in the CPython source tree. Knowing what each one is meant to hold will help you find where a certain piece of functionality is implemented. Do realize, though, there are always exceptions to every rule.
Contains the EBNF grammar file for Python.
Contains all interpreter-wide header files.
The part of the standard library implemented in pure Python.
Mac-specific code (e.g., using IDLE as an OS X application).
Things that do not belong elsewhere. Typically this is varying kinds of developer-specific documentation.
The part of the standard library (plus some other code) that is implemented in C.
Code for all built-in types.
Build files for the version of MSVC currently used for the Windows installers provided on python.org.
Code related to the parser. The definition of the AST nodes is also kept here.
Source code for C executables, including the main function for the CPython interpreter (in versions prior to Python 3.5, these files are in the Modules directory).
The code that makes up the core CPython runtime. This includes the compiler, eval loop and various built-in modules.
Various tools that are (or have been) used to maintain Python.