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Using fromager

Modes

Fromager has different modes for bootstrapping and production builds. The bootstrap mode recursively processes all dependencies starting from the requirements specifications given to determine what needs to be built and what order to build it. The production build commands separate these steps and avoid recursive processing so each step can be performed in isolation.

Bootstrapping

The bootstrap command

  • Creates an empty package repository for wheels
  • Downloads the source distributions for the input packages and places them under sdists-repo/downloads/.
  • Recurses through the dependencies
    • Firstly, any build system dependency specified in the pyproject.toml build-system.requires section as per PEP517
    • Secondly, any build backend dependency returned from the get_requires_for_build_wheel() build backend hook (PEP517 again)
    • Lastly, any install-time dependencies of the project as per the wheel’s core metadata Requires-Dist list.
  • As each wheel is built, it is placed in a PEP503 "simple" package repository under wheels-repo/simple generated by pypi-mirror.
  • The order the dependencies need to be built bottom-up is written to build-order.json.

Wheels are built by running pip wheel configured so it will only download dependencies from the local wheel repository. This ensures that all dependencies are being built in the correct order.

Production Builds

Production builds use separate commands for the steps described as part of bootstrapping, and accept arguments to control the servers that are used for downloading source or built wheels.

All-in-one commands

Two commands support building wheels from source.

The build command takes as input the distribution name and version to build, the variant, and the URL where it is acceptable to download source distributions. The server URL is usually a simple index URL for an internal package index. The outputs are one patched source distribution and one built wheel.

The build-sequence command takes a build-order file and the variant. The outputs are patched source distributions and built wheels for each item in the build-order file.

Step-by-step commands

Occasionally it is necessary to perform additional tasks between build steps, or to run the different steps in different configurations (with or without network access, for example). Using the step subcommands, it is possible to script the same operations performed by the build and build-sequence commands.

The step download-source-archive command finds the source distribution for a specific version of a dependency on the specified package index and downloads it. It will be common to run this step with pypi.org, but for truly isolated and reproducible builds a private index server is more robust.

The step prepare-source command unpacks the source archive downloaded from the previous step and applies any patches (refer to customization for details about patching).

The step prepare-build command creates a virtualenv with the build dependencies for building the wheel. It expects a --wheel-server-url as argument to control where built wheels can be downloaded.

The step build-sdist command turns the prepared source tree into a new source distribution ("sdist"), including any patches or vendored code.

The step build-wheel command creates a wheel using the build environment and prepared source, compiling any extensions using the appropriate override environment settings (refer to customization for details about overrides).