Using third-party Python packages

When using third-party Python packages, there are two options:

  1. Install/use a vendored version of the package.

  2. Install the package from a package index, such as PyPI or our internal mirror.

Vendoring Python packages

If the Python package is to be used in the building of Firefox itself, then we MUST use a vendored version. This ensures that to build Firefox we only require a checkout of the source, and do not depend on a package index. This ensures that building Firefox is deterministic and dependable, avoids packages from changing out from under us, and means we’re not affected when 3rd party services are offline. We don’t want a DoS against PyPI or a random package maintainer removing an old tarball to delay a Firefox chemspill.

Where possible, the following policy applies to ALL vendored packages:

  • Vendored libraries SHOULD NOT be modified except as required to successfully vendor them.

  • Vendored libraries SHOULD be released copies of libraries available on PyPI.

Adding a Python package

To vendor a Python package, run mach vendor python [PACKAGE], where [PACKAGE] is one or more package names along with a version number in the format pytest==3.5.1. The package will be installed, transient dependencies will be determined, and a requirements.txt file will be generated with the full list of dependencies. The requirements file is then used with pip to download and extract the source distributions of all packages into the third_party/python directory.

Alternatively, you can also modify the direct dependencies in third_party/python/ and then run mach vendor python for your changes to take effect.

What if the package isn’t on PyPI?

If the package isn’t available on any Python package index, then you can manually copy the source distribution into the third_party/python directory.

Using a Python package index

If the Python package is not used in the building of Firefox then it can be installed from a package index. Some tasks are not permitted to use external resources, and for those we can publish packages to an internal PyPI mirror. See how to upload to internal PyPI for more details. If you are not restricted, you can install packages from PyPI or another package index.

All packages installed from a package index MUST specify hashes to ensure compatibility and protect against remote tampering. Hash-checking mode can be forced on when using pip be specifying the --require-hashes command-line option. See hash-checking mode for more details.

Note that when using a Python package index there is a risk that the service could be unavailable, or packages may be updated or even pulled without notice. These issues are less likely with our internal PyPI mirror, but still possible. If this is undesirable, then consider vendoring the package.