Reading other people’s code is a great way to learn. Recently, I was looking at the numpy repository and they had a hidden gem within their setup.py. In large scale repositories like numpy, we can have different dependencies that we rely upon via git submodules. The following function will check to see that these dependencies are loaded and in a “clean” state.

def check_submodules():
    """ verify that the submodules are checked out and clean
        use `git submodule update --init`; on failure
    """
    if not os.path.exists('.git'):
        return
    with open('.gitmodules') as f:
        for line in f:
            if 'path' in line:
                p = line.split('=')[-1].strip()
                if not os.path.exists(p):
                    raise ValueError('Submodule {} missing'.format(p))

    proc = subprocess.Popen(['git', 'submodule', 'status'],
                            stdout=subprocess.PIPE)
    status, _ = proc.communicate()
    status = status.decode("ascii", "replace")
    for line in status.splitlines():
        if line.startswith('-') or line.startswith('+'):
            raise ValueError('Submodule not clean: {}'.format(line))

To get a better idea of what this piece of code is doing, let’s trace through its execution. The .gitmodules file defines where each of the submodules are stored in the repository. Here’s the file for numpy’s repository.

[submodule "doc/source/_static/scipy-mathjax"]
	path = doc/source/_static/scipy-mathjax
	url = https://github.com/scipy/scipy-mathjax.git
[submodule "numpy/core/src/umath/svml"]
	path = numpy/core/src/umath/svml
	url = https://github.com/numpy/SVML.git

The function then checks if each of the path locations are defined in the filesystem.

# Check #1
if not os.path.exists("doc/source/_static/scipy-mathjax"):
    raise ValueError('Submodule {} missing'.format("doc/source/_static/scipy-mathjax"))

# Check #2
if not os.path.exists("numpy/core/src/umath/svml"):
    raise ValueError('Submodule {} missing'.format("numpy/core/src/umath/svml"))

Now it is possible for these submodules to be checked out, but be in some sort of unclean state. For example let’s say that I edit numpy/core/src/umath/svml/README.md and commit that change. Then the git submodule status command will look like the following.

$ git submodule status
36f4c898f2255e0c98eb6949cd67381552d5ffea ../doc/source/_static/scipy-mathjax (heads/master)
+1f2b5b3cae704ffa6210a638dfd084946da366bf core/src/umath/svml (heads/main-1-g1f2b5b3)

Notice the + behind the second submodule. According to the man page for git submodule

Show the status of the submodules. This will print the SHA-1 of the currently checked out commit for each submodule, along with the submodule path and the output of git describe for the SHA-1.

Each SHA-1 will possibly be prefixed with

• - if the submodule is not initialized
• + if the currently checked out submodule commit does not match the SHA-1 found in the index of the containing repository
• U if the submodule has merge conflicts.

The existence of the + would make this check fail. However, no prefixes will make the check succeed.

By executing this function within the repositories setup.py, it verifies that the needed dependencies are checked out and clean before installing the python package. Checks like these make python packages feel a little more stable.