feature: write __version__.py automatically

[lsth]

Dear Matthew,

thanks a lot for poetry-dynamic-versioning. It is great!

Additionally it would be interesting if poetry could write the file

    <package>/__version__.py

automatically and fill it with the dynamic version number string.

This feature could be implemented with just three lines at the end of _get_version():

    if config["versionfile"]:
        version_file = pyproject_path.parent / pyproject["tool"]["poetry"]["name"] / "__version__.py"
        version_file.write_text(_VERSION_FILE_TEMPLATE.format(serialized))

Would you consider a pull request?

Best regards
Lars

[mtkennerly]

Hey there! I think this would definitely make sense to add to the plugin, although it's a little more complicated than just that change. If you'd like to send a PR, that would be great, or I can take a look as soon as I have a chance.

Some thoughts:

• I think _version.py would be a better default than __version__.py. If you have a foo/__version__.py and forget to add from foo.__version__ import __version__ in foo/__init__.py, then if an end user runs from foo import __version__, they'll get the module itself instead of a string, which would be quite surprising.
• The module folder won't always match pyproject["tool"]["poetry"]["name"]. For example, hyphens are converted to underscores, as in this repo's poetry_dynamic_versioning folder. Poetry also supports an src layout and arbitrary package lists.
• After poetry build finishes, the change should be reverted, like how the plugin currently reverts pyproject.toml, to make sure users won't end up accidentally committing the auto-generated change. (Once Poetry plugins are officially supported, maybe we could add it to the distribution archive directly without even making/changing the file on disk.)

I'm not sure the best way to handle the second point. We could use some info from pyproject.toml and maybe some info exposed by Poetry to make a reasonable guess on a default location, but we'd probably need a fallback option to specify the file explicitly.

[miracle2k]

I'm researching this issue right now. First of all, note the discussion here python-poetry/poetry#144 (comment) Apparently the idea of adding this to poetry existed at some point.

Also, it seems that with 3.8 (3.7 backported) it is possible to access the version from code: python-poetry/poetry#144 (comment)

For me personally, this would solve the problem of accessing the version at runtime and make it unnecessary to dynamically write a __version__.py file.

[mtkennerly]

This is now available in 0.4.0! Instead of creating a __version__.py from scratch, the plugin will update any placeholder __version__ = ... declarations it finds, so you have more control over how it's handled. You can customize the files and placeholders it looks for, but by default, it will update __version__ = "0.0.0" (for example) in any of these:

*.py              # e.g., my_package.py
*/__init__.py     # e.g., my_package/__init__.py
*/__version__.py
*/_version.py