Single source of truth for Python project version in presence of pyproject.toml

The pyproject.toml specification affords the ability to specify the project version, e.g.

[project]
name = "foo"
version = "0.0.1"

However, it is also a common Python idiom to put __version__ = "0.0.1" in foo/__init__.py so that users can query it.

Is there a standard way of extracting the version from the pyproject.toml and getting it into the foo/__init__.py?

Solution

There are two approaches you can take here.

Keep version in pyproject.toml and get it from the package metadata in the source code. So, in your mypkg/__init__.py or wherever:
```
from importlib.metadata import version
__version__ = version("mypkg")
```
importlib.metadata.version is available since Python 3.8. For earlier Python versions, you can do similar with the importlib_metadata backport.
Keep the version in the source code and instruct the build system to get it from there. For a setuptools build backend, it looks like this in pyproject.toml:
```
[project]
name = "mypkg"
dynamic = ["version"]

[tool.setuptools.dynamic]
version = {attr = "mypkg.__version__"}
```

My recommendation is (🥁) ... neither! Don't keep a __version__ attribute in the source code at all. It's an outdated habit which we can do without these days. Version is already a required field in the package metadata, it's redundant to keep the same string as an attribute in the package/module namespace.