python - What is the correct way to share package version with setup.py and the package?

Question

With distutils, setuptools, etc. a package version is specified in setup.py:

# file: setup.py
...
setup(
name='foobar',
version='1.0.0',
# other attributes
)

I would like to be able to access the same version number from within the package:

>>> import foobar
>>> foobar.__version__
'1.0.0'

---

I could add __version__ = '1.0.0' to my package's __init__.py, but I would also like to include additional imports in my package to create a simplified interface to the package:

# file: __init__.py

from foobar import foo
from foobar.bar import Bar

__version__ = '1.0.0'

and

# file: setup.py

from foobar import __version__
...
setup(
name='foobar',
version=__version__,
# other attributes
)

However, these additional imports can cause the installation of foobar to fail if they import other packages that are not yet installed. What is the correct way to share package version with setup.py and the package?

Answer 1

Set the version in setup.py only, and read your own version with pkg_resources, effectively querying the setuptools metadata:

file: setup.py

setup(
    name='foobar',
    version='1.0.0',
    # other attributes
)

file: __init__.py

from pkg_resources import get_distribution

__version__ = get_distribution('foobar').version

To make this work in all cases, where you could end up running this without having installed it, test for DistributionNotFound and the distribution location:

from pkg_resources import get_distribution, DistributionNotFound
import os.path

try:
    _dist = get_distribution('foobar')
    # Normalize case for Windows systems
    dist_loc = os.path.normcase(_dist.location)
    here = os.path.normcase(__file__)
    if not here.startswith(os.path.join(dist_loc, 'foobar')):
        # not installed, but there is another version that *is*
        raise DistributionNotFound
except DistributionNotFound:
    __version__ = 'Please install this project with setup.py'
else:
    __version__ = _dist.version