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Updating Pants BUILD files programmatically

· 5 min read
Alexey Tereshenkov


Working with a monorepository implies dealing with build metadata files that provide information about the source code and how it should be built. Pants build system uses BUILD files which are valid Python files and are evaluated using a Python interpreter as a list of statements. When adding support for Pants in a codebase, one can use the ./pants tailor command which will generate the minimal BUILD files necessary to get started.

As the adoption of Pants goes on, you may want to update your BUILD files across some parts or throughout the whole monorepository. This can be the case when you'd like to update your python_sources targets to enable/disable some code formatters or linters (for instance, by adding skip_black and skip_flake8 arguments). Likewise, updating BUILD files may be necessary if you'd need to tag multiple python_tests targets by setting the tags argument.

Although in most cases Pants does not require you to declare the dependencies between targets (thanks to dependency inference), manually updating the BUILD files for any codebase of a decent size will get tedious rather soon which is why it is necessary to have appropriate tooling for updating the BUILD files programmatically.

Modifying BUILD files by using source code refactoring tools

Since the BUILD files are valid Python files, it is possible to use an AST / CST (abstract or concrete syntax tree) based approach and refactor the target declarations. This is fairly easy given the low complexity of the files in question.

You may be already using some automated refactoring tools developed in the Python community for large refactorings of the source code. If not, you may explore some tools such as rope, redbaron, astor, or LibCST.

Modifying BUILD files by using Bazel build tools

If your use case is rather simple, and you don't have enough resources to develop Python refactoring tools in house, you may want to look at the out-of-the-box tools that will let you update the BUILD files programmatically. Because BUILD files in Pants share a lot of syntax with the BUILD files used in Bazel, it is often possible to use the Bazel build toolchain to interact with the Pants BUILD files.

When adopting Pants incrementally, potentially for only a segment of the monorepository, you would create BUILD files only in the projects (i.e. directories) of interest. However, it may be the case that after you have already enabled Pants for a part of your monorepository, you'd like to have a more fine-grained control over the target declarations. For instance, you may want to edit BUILD files within one or more projects by modifying the current arguments values, adding new or removing existing ones.

When using the Bazel build tools, keep in mind that they reformat the BUILD files following the Bazel formatting style which may be not necessarily how you expect your BUILD files to be formatted. If you are migrating from Bazel to Pants, however, you may find it convenient to preserve the same, familiar formatting style of the BUILD files used in the monorepository. Either way, if you are already formatting the BUILD files using black, then you may take advantage of the ./pants update-build-files command which will format all BUILD files in your project using black (support for yapf will come in the future releases).

To modify the Pants BUILD files in your project, you can use buildozer. This is how you would add additional arguments to all existing targets across your monorepository:

$ buildozer 'set skip_yapf True' 'set skip_mypy True' ///...:%python_tests
$ buildozer 'set skip_yapf True' 'set skip_mypy True' ///...:%python_sources

The input target declaration


would be turned into

skip_flake8 = True,
skip_mypy = True,
skip_yapf = True,

If you run a Python formatter on the BUILD files afterwards, it would be formatted to something like below:


There are ways to have a more fine-grained control over what targets you'd like to run the editing command on, but you may need to consult the Bazel docs as the semantics of addressing a particular target or targets within a directory may differ from the ones used in Pants. This command, for example, would update only python_sources targets within a single project directory:

$ buildozer 'set skip_yapf False' projectdir/subdir:%python_sources

Querying the Pants BUILD files before running the editing commands

The ./pants peek command would be of help if you would need to query your BUILD files for information necessary to update them (e.g. you want to update only those targets that have a particular argument set to a particular value). The peek command would also help you confirm that programmatic updates of the BUILD files succeeded and made the changes you wanted to make.

For instance, with a little help from jq to parse JSON, this command would report all pex_binary targets that have something else than /usr/bin/python3 as its shebang:

$ ./pants peek project-name:: | jq '.[] | select(.target_type == "pex_binary" and .shebang != "/usr/bin/python3")'

If you already have experience using buildozer, you may want to prefer using its querying functionality to learn more about BUILD files targets before running the editing commands. This command would print the name and the dependencies arguments for all pex_binary targets for a particular project:

$ buildozer 'print name dependencies' project-name:%pex_binary

Buildozer documentation has lots of code snippets that will help you learn how to update your BUILD files.

Happy coding!

Want to explore more of Pants open source project or its first-class support for Python? Check out example-python and come say hi on the Pants community Slack! It's a friendly and supportive community that is happy to respond to your questions and feedback.