dynvertest: test dynamic versioning in Python#

Does Python packaging tooling support calling a function to get the version name at package build time? It depends on the tooling in question. This is an experiment to answer that question for packages built with setuptools and pyproject.toml.

At the time of this experiment, 2023-06-17, with setuptools 65.5.0 running on Python 3.11.4, a version can be set to a function.

See this with:

python3 -m venv venv
. venv/bin/activate
pip install -e .

It will show a specific version being installed, like:

(venv) user@host> pip install -e .
Obtaining file:///Users/mrled/Documents/Repositories/pptt2
  Installing build dependencies ... done
  Checking if build backend supports build_editable ... done
  Getting requirements to build editable ... done
  Preparing editable metadata (pyproject.toml) ... done
Building wheels for collected packages: dynvertest
  Building editable for dynvertest (pyproject.toml) ... done
  Created wheel for dynvertest: filename=dynvertest-0.0.1689623616-0.editable-py3-none-any.whl size=2748 sha256=9aa55b4a860a2fde124d628b4b50e0f2865b4ce4b0acbb083d884853363ae136
  Stored in directory: /private/var/folders/pn/r59j98xn3z30mhhv5_hhh8m80000gn/T/pip-ephem-wheel-cache-ztgj5en7/wheels/c4/4b/16/05aa356d92007c1e0ad97f36a6b05a5361226ff6a0f41edd01
Successfully built dynvertest
Installing collected packages: dynvertest
  Attempting uninstall: dynvertest
    Found existing installation: dynvertest 0.0.1689623603
    Uninstalling dynvertest-0.0.1689623603:
      Successfully uninstalled dynvertest-0.0.1689623603
Successfully installed dynvertest-0.0.1689623616

That version is based on the epoch at installation time, based on version = {attr = "dynvertest.dynamicversion"} in pyproject.toml. Reinstalling the pacakge will result in a new package version, based on the epoch at reinstallation time.

Not officially supported?#

It’s worth noting that the setuptools has an official answer for this:

attr is meant to be used when the module attribute is statically specified (e.g. as a string, list or tuple). As a rule of thumb, the attribute should be able to be parsed with `ast.literal_eval() <https://docs.python.org/3/library/ast.html#ast.literal_eval>`__, and should not be modified or re-assigned.

This seems to say that only literals like numbers or strings are supported. ast.literal_eval() will not run a function. In practice, though, this doesn’t seem to be what setuptools actually uses to handle dynamic versions in pyproject.toml.

What’s it doing in the source code#

In config/expand.py, we can see:

try:
    return getattr(StaticModule(module_name, spec), attr_name)
except Exception:
    # fallback to evaluate module
    module = _load_spec(spec, module_name)
    return getattr(module, attr_name)

The docstring for that function says

This function will try to read the attributed statically first (via :func:ast.literal_eval), and only evaluate the module if it fails.

So it’s specifically built to do this, but it’s not the preferred way.

Used in previous versions with setup.cfg#

When progfiguration was first written, it relied on setup.cfg’s support for dynamic versions. The build script added a version file, and the dynamic version would import that file and return the version it defined, and return a fallback version on import error.

When I wanted to update it to use pyproject.toml only, I found the footnote quoted above from the setuptools documentation which claimed it isn’t possible.

I plan to keep using it, at least for now, even if it isn’t supported, but only in my progfigsite package. Using it isn’t part of the core progfiguration package, and your own progfigsite package(s) can use whatever method you like.

Why is this useful?#

It is useful in progfigsite repositories because it allows date-based versioning at build time. When writing system configuration, it’s useful to be able to make a change and deploy it iteratively very quickly. We want to avoid toil from updating a version file between each cycle.

Alternatives to setuptools?#

Note that this is not pyproject.toml support, it’s setuptools support. Other build backends might support it officially, or might not support it.

According to this report, Flit allows dynamic version functions as well, but Flit’s documentation doesn’t specifically claim support.

See also#