Using third-party Python packages

Mach and its associated commands have a variety of 3rd-party Python dependencies. Many of these are vendored in third_party/python, while others are installed at runtime via pip.

The dependencies of Mach itself can be found at python/sites/mach.txt. Mach commands may have additional dependencies which are specified at python/sites/<site>.txt.

For example, the following Mach command would have its 3rd-party dependencies declared at python/sites/foo.txt.

# ...
def foo_it_command():
    import specific_dependency

The format of <site>_virtualenv_requirements.txt files are documented further in the MachEnvRequirements class.

Adding a Python package

There’s two ways of using 3rd-party Python dependencies:


For dependencies that meet both restrictions (dependency of Mach/build, and has native code), see the Mach/Build Native 3rd-party Dependencies section below.

pip install the package

To add a pip install-d package dependency, add it to your site’s python/sites/<site>.txt manifest file:



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.

Vendoring Python packages

To vendor a Python package, add it to third_party/python/ and then run mach vendor python. This will update the tree of pinned dependencies in third_party/python/requirements.txt and download them all into the third_party/python directory.

Next, add that package and any new transitive dependencies (you’ll see them added in third_party/python/requirements.txt) to the associated site’s dependency manifest in python/sites/<site>.txt:



The following policy applies to ALL vendored packages:

  • Vendored PyPI libraries MUST NOT be modified

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

    • When considering manually vendoring a package, discuss the situation with the #build team to ensure that other, more maintainable options are exhausted.


We require that it is possible to build Firefox using only a checkout of the source, without depending 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. Therefore, packages required by Mach core logic or for building Firefox itself must be vendored.

Mach/Build Native 3rd-party Dependencies

There are cases where Firefox is built without being able to pip install, but where native 3rd party Python dependencies enable optional functionality. This can’t be solved by vendoring the platform-specific libraries, as then each one would have to be stored multiple times in-tree according to how many platforms we wish to support.

Instead, this is solved by pre-installing such native packages onto the host system in advance, then having Mach attempt to use such packages directly from the system. This feature is only viable in very specific environments, as the system Python packages have to be compatible with Mach’s vendored packages.

To control this behaviour, the MACH_BUILD_PYTHON_NATIVE_PACKAGE_SOURCE environment variable can be used:





Mach will pip install all needed dependencies from PyPI at runtime into a Python virtual environment that’s reused in future Mach invocations.


Mach will perform the build using only vendored packages. No Python virtual environment will be created for Mach.


Mach will use the host system’s Python packages as part of doing the build. This option allows the usage of native Python packages without leaning on a pip install at build-time. This is generally slower because the system Python packages have to be asserted to be compatible with Mach. Additionally, dependency lockfiles are ignored, so there’s higher risk of breakage. Finally, as with "none", no Python virtualenv environment is created for Mach.


Same behaviour as "pip" if MOZ_AUTOMATION isn’t set. Otherwise, uses the same behaviour as "none".

There’s a couple restrictions here:

  • MACH_BUILD_PYTHON_NATIVE_PACKAGE_SOURCE only applies to the top-level "mach" site,

    the "common" site and the "build" site. All other sites will use pip install at run-time as needed.

  • MACH_BUILD_PYTHON_NATIVE_PACKAGE_SOURCE="system" is not allowed when using any site other than "mach", "common" or "build", because:

    • As described in Package compatibility below, packages used by Mach are still in scope when commands are run, and

    • The host system is practically guaranteed to be incompatible with commands’ dependency lockfiles.

The MACH_BUILD_PYTHON_NATIVE_PACKAGE_SOURCE environment variable fits into the following use cases:

Mozilla CI Builds

We need access to the native packages of zstandard and psutil to extract archives and get OS information respectively. Use MACH_BUILD_PYTHON_NATIVE_PACKAGE_SOURCE="system".

Mozilla CI non-Build Tasks

We generally don’t want to create a Mach virtual environment to avoid redundant processing, but it’s ok to pip install for specific command sites as needed, so leave MACH_BUILD_PYTHON_NATIVE_PACKAGE_SOURCE unset (MOZ_AUTOMATION implies the default behaviour of MACH_BUILD_PYTHON_NATIVE_PACKAGE_SOURCE="none").

In cases where native packages are needed by Mach, use MACH_BUILD_PYTHON_NATIVE_PACKAGE_SOURCE="pip".

Downstream CI Builds

Sometimes these builds happen in sandboxed, network-less environments, and usually these builds don’t need any of the behaviour enabled by installing native Python dependencies. Use MACH_BUILD_PYTHON_NATIVE_PACKAGE_SOURCE="none".

Gentoo Builds

When installing Firefox via the package manager, Gentoo generally builds it from source rather than distributing a compiled binary artifact. Accordingly, users doing a build of Firefox in this context don’t want stray files created in ~/.mozbuild or unnecessary pip install calls. Use MACH_BUILD_PYTHON_NATIVE_PACKAGE_SOURCE="none".

Firefox Developers

Leave MACH_BUILD_PYTHON_NATIVE_PACKAGE_SOURCE unset so that all Mach commands can be run, Python dependency lockfiles are respected, and optional behaviour is enabled by installing native packages.

Package compatibility

Mach requires that all commands’ package requirements be compatible with those of Mach itself. (This is because functions and state created by Mach are still usable from within the commands, and they may still need access to their associated 3rd-party modules).

However, it is OK for Mach commands to have package requirements which are incompatible with each other. This allows the flexibility for some Mach commands to depend on modern dependencies while other, more mature commands may still only be compatible with a much older version.


Only one version of a package may be vendored at any given time. If two Mach commands need to have conflicting packages, then at least one of them must pip install the package instead of vendoring.

If a Mach command’s dependency conflicts with a vendored package, and that vendored package isn’t needed by Mach itself, then that vendored dependency should be moved from python/sites/mach.txt to its associated environment.