Hatch VCS Footgun Example

A somewhat hacky usage of the Hatch VCS build hook which ensures that the __version__ variable stays up-to-date, even when the project is installed in editable mode.

Quick summary

1. Ensure that Hatch VCS is configured in pyproject.toml.
2. Copy the contents of __init__.py and adjust to your project.
3. Add _version.py to your .gitignore file.
4. Install setuptools-scm as a development dependency.
5. Enjoy up-to-date version numbers, even in editable mode.

Background

For consistency's sake, it's good to have a single source of truth for the version number of your project. However, there are four common places where the version number appears in most modern Python projects:

1. The version field of the [project] section of pyproject.toml.
2. The dist-info METADATA file from when the project was installed.
3. The __version__ variable in the __init__.py file of the project's main package.
4. The Git tag of the release commit.

With Hatch VCS, the definitive source of truth is the Git tag. One often still needs a technique to access this version number programmatically. The quasi-standard is to store it in the __version__ variable, so that the following works:

import myproject

print(myproject.__version__)

Standard solutions

1. Dynamically read the version number from the package metadata with importlib.metadata

   For Python 3.8 and higher, one can do:

   # __init__.py
   from importlib.metadata import version
   
   __version__ = version("myproject")

   This works well in most cases, and does not require the Hatch VCS build hook.

   There are two important caveats to this approach.

   1. The version number comes from the last time the project was installed. In case you are developing your project in editable mode, the reported version may be outdated unless you remember to reinstall each time the version number changes.

   2. Parsing the METADATA file can be relatively slow. If performance is crucial and every millisecond counts (e.g. if one is writing a CLI tool), then this is not an ideal solution.

   (For compatibility with Python 3.7 and lower, see the examples here regarding importlib_metadata.)

2. Use a static _version.py file

   Using the Hatch VCS build hook, a _version.py file is generated when either building a distribution or installing the project from source.

   # __init__.py
   from myproject._version import __version__

   Since _version.py is generated dynamically, it should be added to .gitignore.

   As with the importlib.metadata approach, if the project is installed in editable mode then the _version.py file will not be updated unless the package is reinstalled (or locally rebuilt).

3. Compute the version number at runtime with setuptools_scm

   Using setuptools_scm as follows only succeeds in particular cases:

   from setuptools_scm import get_version
   
   __version__ = get_version(root="..", relative_to=__file__)

   It requires that setuptools_scm is installed in the runtime environment alongside the VCS tool (git or hg), and in order to read the tags, the project must be installed from a source repository.

   This is very fragile, but has the advantage that when it works, the version number is always up-to-date, even for an editable installation.

Note that parsing the version from pyproject.toml is usually not a viable option because this file is typically absent when a project is installed from a wheel!

Conclusion

In most cases, using importlib.metadata.version or a _version.py are the best solutions. In the second case, the Hatch VCS build hook is a good way to generate the _version.py file. However, this data can become outdated during development with an editable install. If reporting the correct version during development is important, then the hybrid approach as follows may be desirable.

Why "Footgun"?

In case you are developing in editable mode, and it is important that the version number be kept up-to-date automatically, then it is possible to use a solution similar to that illustrated in this example. Namely:

• Default to using setuptools_scm to set __version__.
• When that fails, fall back to _version.py.

However, it is somewhat of a footgun: it involves distinct version detection mechanisms between development and deployment. Furthermore, this technique is unsupported, so it must be used at your own risk.

Usage

After cloning this repository,

python -m hatch_vcs_footgun_example.main  # Prints an error because it's not installed
pip install --editable .  # Installs and creates the "_version.py" file
python -m hatch_vcs_footgun_example.main  # Prints "My version is '1.0.2'."

Without setuptools-scm installed, the version number is reported incorrectly after a new tag.

git commit --allow-empty -m "For v1.2.3"
git tag v1.2.3
python -m hatch_vcs_footgun_example.main  # My version is '1.0.2'.

With setuptools-scm installed the version is correctly reported:

pip install setuptools-scm
python -m hatch_vcs_footgun_example.main  # My version is '1.2.3'.