Analyzing Git Repositories in Pandas

Posted by Erik M. Bray on Fri 03 January 2014

Although the git log command is extremely powerful for use in exploring a git repository's history, I found that for some kinds of analysis it may be easier to dump git log output into a Pandas DataFrame and work with it in Python.

In particular what motivated this was that I was trying to recreate the graph of commit activity over time that GitHub shows on its "Contributors" graph page. For a large project like Astropy that looks something like this:

Astropy commit activity over time

This graph is nice enough on its own for use in a web browser, but I wanted to make my own replication of it using matplotlib for my own use. There do exist a number of software packages already that will generate statistics and plots from git repositories. I would link to some, but honestly I didn't bother even trying any of them since the plots they made were damned ugly. I figured I only had a few limited use cases I was interested in so it would be easier to do myself and probably look a little better.

My first attempt actually did take advantage of Pandas from the start. This was the first time I had really attempted to do much with Pandas for my own use (aside from working on small tutorials to give to other people). So one thing I did know was that I was working with a time series and that Pandas might include some tools that would make my life easier. That said I am still learning Pandas myself, and some of my solutions may be ugly and hacky.

I assumed, to start with, that I would sample the commit activity on a weekly basis rather than daily. git log supports --since and --until arguments that allow returning a sample of the log in a specific date range. But I needed a quick, easy way to generate a list of dates separated by one week. I could have done this easily enough in pure Python using timedeltas, but since I already had Pandas installed my first attempt used its built-in date_range function:

>>> from pandas import date_range
>>> from pandas
>>> date_range('2011-07-25', '2014-01-03', freq='W')
<class 'pandas.tseries.index.DatetimeIndex'>
[2011-07-31 00:00:00, ..., 2013-12-29 00:00:00]
Length: 77, Freq: W-SUN, Timezone: None

As you can see, a "gotcha" in Pandas is that the 'W' frequency for week actually defaults to weeks beginning on Sunday. A reasonable default, to be sure, but this cuts off dates in my range before the first Sunday in the range. There are a few possible workarounds for this, but for my case I just shifted the start day to the first Sunday before the actual start day. There is a better way I later discovered to do this using the period_range function but I will leave that as an exercise.

My full solution involved looping over the dates in my date range and making repeated calls to git log, a process with really more overhead than necessary but it was fine for a first attempt:

from datetime import datetime
from pandas import date_range
import shlex
import subprocess as sp

# I just looked up and hard-coded the start date of the repository in
# this version
start = datetime(year=2011, month=7, day=24)
end =
rng = date_range(start, end, freq='W')

# This will list each commit in the given range on one line each (excluding
# merge commits).  This means I can count commits by simply counting lines
# of output
cmd = 'git log --oneline --since=%s --until=%s --no-merges'

def count_commits(since, until):
    p = sp.Popen(shlex.split(cmd % (since, until)), stdout=sp.PIPE)
    return len(p.stdout.readlines())

commits_by_week = []
for idx in xrange(len(rng) - 1):
    since = rng[idx]
    since_iso =
    until_iso = rng[idx + 1].date().isoformat()
    commits_by_week.append((since, count_commits(since_iso, until_iso)))

# Special case for the last week in the range
since = rng[-1]
since_iso =
until_iso = end.isoformat()
commits_by_week.append((since, count_commits(since_iso, until_iso)))

I plopped this script directly into my git repository and ran it. I've ommitted that version of the plotting code, but it came out with something like this (after taking a rather long time to make all those git log calls):

First attempt at a commit activity graph

If you compare that to the version from GitHub you can see that while there are similar periods of activity and inactivity, the two plots look rather different. In particular, my version has several large spikes that are absent from the GitHub version. At first I thought I might try sampling by day instead of week, but that only made the output even spikier, and all those git log calls were taking inordinately long.

After just a little further exploration of the GitHub graphs and comparison with my actual git log I realized the major difference: GitHub's time series is using the "author date" of the commits rather than the "committer date" to produce its graph. All git commits actually carry (at least) two dates. As this article by Alex Peattie explains, the "author is the person who originally wrote the work, whereas the committer is the person who last applied the work." And further, git log "uses commit dates when given a --since option." So my time series was based on commit dates, rather than author dates. That largely explains the big spikes—it turns out they largely correspond to activity just before making a release of Astropy, when many pull requests were being merged. When the PRs are merged their commits get a commit date same as when they are merged.

In fact, in general, the git log for a large project with many contributors is very much not in chronological order of when the commits were first created. Rather, because of the way git log works, the commits it lists will generally be in topological order. It starts by looking at your HEAD commit and then just walking back through its parents. This has nothing to do with the actual chronological order in which the commits were written by their original authors.

My first attempt at solving this problem was to use a git log command with a custom format string returning only the author dates (as well as the hashes though they aren't particularly needed):

git log --no-merges --date=short --pretty='format:%ad %H'

This displays the author dates fine, but they are still in topological order. When trying to use this with --since and --until the returned commits are still based in the commit dates and not somehow magically sorted and grouped by author date. I realized at this point that there was no way I could rely git log to prepare the data in the way I wanted. It makes much more sense at this point to use Pandas.

In addition to making the code vastly simpler, it only required one call to git log and was much faster. You can use a custom format with git log to produce any and all data you want from git in a tsv or csv format and load it directly into a Pandas DataFrame. For example, in addition to the author date, I also wanted the author, and for the heck of it the hash:

import io
from pandas import read_csv

cmd = "git log --no-merges --date=short --pretty='format:%ad\t%H\t%aN'"
p = sp.Popen(shlex.split(cmd), stdout=sp.PIPE)
# p.wait() will likely just hang if the log is long enough because the
# stdout buffer will fill up
stdout, _ = p.communicate()

table = read_csv(io.StringIO(stdout.decode('utf-8')), sep='\t',
                 names=['date', 'hash', 'author'], index_col=0,

The index_col=0 and parse_dates=True indicate that the first column ('date') should be treated as the index for the table, and that its values should be parsed as dates. The .sort() call ensures that the returned table is sorted in ascending order by date.

Now the commits are indexed by their author date, and for many applications that would be fine. But I still wanted to try grouping by weekly periods for use in my plot instead. There are probably many ways to do this, but the one I settled on was to convert to a DataFrame indexed by periods instead of dates:

table = table.to_period(freq='W')

Then it was easy to generate counts of commits in each period using a simple combination of groupby and aggregate:

commits_per_period = table.hash.groupby(level=0).aggregate(len)

I specically chose to aggregate over the hash column. I could have just as easily used author. This gives me a table mapping periods to the number of commits in each period. I'm not sure if this next step is necessary, but I could convert this to a list of datetimes and associate array of commit counts for use with matplotlib:

dates = [ for p in commits_per_period.index]
ncommits = commits_per_period.values

The result looks like this:

Second attempt at a commit activity graph

It still has a few sharper spikes and other small differences from the GitHub version, but I think is close enough now. There could be small differences in how they're sampling the data, and additional smoothing that I'm not doing. It doesn't matter—this is still an accurate representation of the actual activity of developers working on the project.

This is of course just scratching the surface. With git commit histories loaded into Pandas there's no end to the interesting analysis that could be done. For example, I later went on to add a plot of the number of committers to the project over time. That's still pretty basic though, and as I am not a statistician I can only imagine the data one could pull out of a git repo in this way.

The full code, along with the plotting code is here: with thanks to Olga Botvinnik for providing prettier plots with prettyplotlib.

tags: git, pandas, python

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