Tracking down unused variables with Pyflakes and git bisect

I was working on a Python project when I noticed a variable that was defined but never used. Does this indicate that some important code was accidentally deleted? Or was there just a minor oversight during refactoring?

To answer this question, I wanted to see the Git commit which removed the last use of this variable. This sounds like a job for git bisect. And because Pyflakes can detect unused variables, the whole process is completely automatic.

I made a guess that the mystery_variable became unused sometime within the past two weeks. Then I told Git to consider a commit "good" if Pyflakes's list of complaints does not mention mystery_variable.

$ git bisect start master master@{'2 weeks ago'}
Already on 'master'
Bisecting: 150 revisions left to test after this (roughly 7 steps)
[066327622129dbe863f6e2fc4746ff9e869bd049] Synthesize gravity

$ git bisect run bash -c '! pyflakes foo.py | grep mystery_variable'
running bash -c ! pyflakes foo.py | grep mystery_variable
Bisecting: 75 revisions left to test after this (roughly 6 steps)
[d3a5665eff478cccfb86d994a8fc289446325fbf] Model object components
running bash -c ! pyflakes foo.py | grep mystery_variable
Bisecting: 37 revisions left to test after this (roughly 5 steps)
[6ddcfbf27a1a4548acf972a6b817e485743f6bd9] Time-compress simulator clock
...

running bash -c ! pyflakes foo.py | grep mystery_variable
9c2b2f006207ae9f274f9182efeb3e009d18ed04 is the first bad commit
commit 9c2b2f006207ae9f274f9182efeb3e009d18ed04
Author: Ben Bitdiddle <bitdiddle@example.com>
Date:   Fri Sep 14 01:38:31 2012 -0400

    Reticulate splines

Now I can examine this commit and see what happened.