Reading a git staged file in Python

Question

Is it possible for python to read a locally staged version of a file instead of the locally saved version of it?

For some context, I've got a pre-commit hook in my repo that calls a Python script to make sure some files include certain lines before staged changes can be committed. Currently I'm just using something like the following to read the files:

with open(path, 'r') as f: text = f.read() // do something with the text

My problem is that the code checks the latest local save of each file as opposed to the staged copy of them. So you can run into a situation where the file is staged with mistakes, the mistakes are fixed and saved (but not committed), and then the pre-commit hook passes. Is there any way to access the staged version instead?

You can access it with git show :path (see here). I believe the way to do that in Python is subprocess.check_output(['git', 'show', ':' + path]). — David Conrad
– David Conrad, Commented Sep 29, 2023 at 18:58

larsks · Accepted Answer · 2023-09-29 13:52:17Z

To read a staged file in a Git repository, you'll need to run git show :<file>; the : means "read from index". In your Python code, you could do something like:

text = subprocess.check_output(['git', 'show', f':{path}')

But this isn't strictly necessary; a better option may be to use the git checkout-index command to check out all files in the index to a temporary location and run your checks there.

E.g:

git checkout-index --prefix=$tmpdir -af cd $tmpdir ...check things here...

If you're doing everything in Python, that might look like:

import os import tempfile import subprocess with tempfile.TemporaryDirectory(prefix='precommit') as tmpdir: subprocess.check_call(['git', 'checkout-index', '--prefix', f'{tmpdir}/', '-af']) # check files here...

For reference, would the user need to provide a value for tmpdir? This is meant to be automated such that it would look at the staged changes of the currently checked out branch.
Yes, the user would need to provide a value for $tmpdir. You can see this in the Python example; which uses tempfile.TemporaryDirectory to create (and clean up) a temporary directory. In a shell script you could use mktemp -d to create the directory, and then arrange to clean it up afterwards using e.g. an EXIT trap.

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