Syntax+pep8 checking before committing in git

UPDATE 2012/08/13: I’ve updated the git hook code per suggestions from folks in the comments.

I write quite a bit of code in python. As a considerate python programmer I tend to try to follow PEP8 as best I can. That said, even though I’ve been working with python & pep8 for years it’s still easy to forget the little things.

While PEP8 is great while you’re working alone, it’s even more important when you’re working on code with other people. Wouldn’t it be nice if you could make sure that every piece of python code you push into your repository that other people might see is pep8 compliant? Fortunately this is very possible, and you can even add some syntax checking as well thanks to Tarek Ziade’s awesome flake8 script.

So, first of all you’re going to need to get flake8 installed. Easy enough with the help of pip:

$ sudo pip install flake8
Downloading/unpacking flake8
  Downloading flake8-1.4.tar.gz
  Running setup.py egg_info for package flake8

Installing collected packages: flake8
  Running setup.py install for flake8
    changing mode of build/scripts-2.7/flake8 from 644 to 755

    changing mode of /usr/local/bin/flake8 to 755
Successfully installed flake8
Cleaning up...

At this point you’re all set. Say you write some code and you manually want to check it for syntax errors and pep8 violations? You just run flake8 <filename> and it’ll tell you how badly you’ve done. For example, say I wrote this lovely piece of code:

#!/usr/bin/python

import sys, string, random

def random_capital_letter()
  letters = string.uppercase
    print "Capital Letter: "+random.choice(letters)
    print ---

random_capital_letter()

As you can see there are all sorts of syntax & pep8 errors in the code. It’d suck if you checked that in - noone wants to be teased for goofy looking code. So lets run it through flake8 and see what it says:

$ flake8 random-capital-letter.py
random-capital-letter.py:5: invalid syntax
def random_capital_letter()
                           ^
random-capital-letter.py:3:11: E401 multiple imports on one line
random-capital-letter.py:5:1: E302 expected 2 blank lines, found 1
random-capital-letter.py:6:3: E111 indentation is not a multiple of four
random-capital-letter.py:6:3: E113 unexpected indentation
random-capital-letter.py:7:5: E113 unexpected indentation
random-capital-letter.py:7:29: E225 missing whitespace around operator

Awesome right?

Now rather than having to remember to run flake8 against everything you write everytime you write it, how about we make it so git (and you can do this in just about any VCS I’m sure) uses flake8 to check every python file we try to commit before we commit it, and then won’t let us commit if there’s an issue? Again, this is super easy - thanks to git’s hooks.

The hook we’re interested in is called pre-commit. Hooks are just scripts, and in this case we’re going to use a shell script to call flake8 on every python file we check in. First, here’s the hook itself. You want to save this as .git/hooks/pre-commit in your repository.

#!/bin/sh
# Auto-check for pep8 so I don't check in bad code
FILES=$(git diff --cached --name-only --diff-filter=ACM | grep -e '\.py$')

if [ -n "$FILES" ]; then
    flake8 -r $FILES
fi

And thats it. Save that file, and now if you try to git commit the code from above into your repo it’ll fail with the same error that you got when you ran flake8 manually.