website/content/blog/python-check-submodule-load.md

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---
title: "Quick Python: Check Submodule Load"
date: 2022-12-01T21:12:03-05:00
draft: false
tags: ["Python"]
math: false
---
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.
```python
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.
```ini
[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.
```python
# 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.
```bash
$ 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.