Single sourcing a python package version using importlib.metadata.version()

my_package.py

try:
    # importlib.metadata is present in Python 3.8 and later
    import importlib.metadata as importlib_metadata
except ImportError:
    # use the shim package importlib-metadata pre-3.8
    import importlib_metadata as importlib_metadata

try:
    __version__ = importlib_metadata.version(__package__ or __name__)
except importlib_metadata.PackageNotFoundError:
    __version__ = "0.0.0"

pyproject.toml

### poetry ###
[tool.poetry]
name = "my_package"
version = "1.2.3"

# add this in addition to whatever else you've got in here
[tool.poetry.dependencies]
importlib-metadata = { version = "~=1.0", python = "<3.8" }

### setuptools ###
[metadata]
name = "my_package"
version = "1.2.3"

# add this in addition to whatever else you've got in here
[options]
install_requires = "importlib-metadata ~= 1.0 ; python_version < '3.8'"

### PEP621 ###
[project]
name = "my_package"
version = "1.2.3"

# add this in addition to whatever else you've got in here
dependencies = [
  "importlib-metadata ~= 1.0 ; python_version < '3.8'"
]