Use fallback_version for source = "call"

[nokados]

I am migrating from setuptools to pdm-backend. I want to use a dynamic version extracted from a VERSION file, which only contains the version without any Python syntax. For example:

1.0.0

So, I choose this setting for dynamic versioning:

[tool.pdm.version]
source = "call"
getter = "get_version:get_version"

And my get_version.py is:

"""Read version from setuptools compatible VERSION file."""

def get_version():
    try:
        with open("VERSION") as f:
            return f.read().strip()
    except FileNotFoundError:
        return "0.0.0"

I would like to have a fallback_version in the .toml file for two reasons:

1. To handle exceptions from the script, allowing me to avoid the except FileNotFoundError: return "0.0.0" block.
2. To handle an invalid version, for example, if the VERSION file contains a branch+commit string.

[nokados]

Thank you!