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

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?

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

I don't believe there's a canonical answer to this, but my method (either directly copied or slightly tweaked from what I've seen in various other places) is as follows:

Folder heirarchy (relevant files only):

package_root/
 |- main_package/
 |   |- __init__.py
 |   `- _version.py
 `- setup.py

main_package/_version.py:

"""Version information."""

# The following line *must* be the last in the module, exactly as formatted:
__version__ = "1.0.0"

main_package/__init__.py:

"""Something nice and descriptive."""

from main_package.some_module import some_function_or_class
# ... etc.
from main_package._version import __version__

__all__ = (
    some_function_or_class,
    # ... etc.
)

setup.py:

from setuptools import setup

setup(
    version=open("main_package/_version.py").readlines()[-1].split()[-1].strip("\"'"),
    # ... etc.
)

... which is ugly as sin ... but it works, and I've seen it or something like it in packages distributed by people who I'd expect to know a better way if there were one.

I agree with @stefano-m 's philosophy about:

Having version = "x.y.z" in the source and parsing it within setup.py is definitely the correct solution, IMHO. Much better than (the other way around) relying on run time magic.

And this answer is derived from @zero-piraeus 's answer. The whole point is "don't use imports in setup.py, instead, read the version from a file".

I use regex to parse the __version__ so that it does not need to be the last line of a dedicated file at all. In fact, I still put the single-source-of-truth __version__ inside my project's __init__.py.

Folder heirarchy (relevant files only):

package_root/
 |- main_package/
 |   `- __init__.py
 `- setup.py

main_package/__init__.py:

# You can have other dependency if you really need to
from main_package.some_module import some_function_or_class

# Define your version number in the way you mother told you,
# which is so straightforward that even your grandma will understand.
__version__ = "1.2.3"

__all__ = (
    some_function_or_class,
    # ... etc.
)

setup.py:

from setuptools import setup
import re, io

__version__ = re.search(
    r'__version__\s*=\s*[\'"]([^\'"]*)[\'"]',  # It excludes inline comment too
    io.open('main_package/__init__.py', encoding='utf_8_sig').read()
    ).group(1)
# The beautiful part is, I don't even need to check exceptions here.
# If something messes up, let the build process fail noisy, BEFORE my release!

setup(
    version=__version__,
    # ... etc.
)

... which is still not ideal ... but it works.

And by the way, at this point you can test your new toy in this way:

python setup.py --version
1.2.3

PS: This official Python packaging document (and its mirror) describes more options. Its first option is also using regex. (Depends on the exact regex you use, it may or may not handle quotation marks inside version string. Generally not a big issue though.)

PPS: The fix in ADAL Python is now backported into this answer.