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A custom merge driver for git, for automatically merging my 'log.txt' file in a simplistic way.
#!/bin/env python
This is a custom merge driver for git. It should be called from git
with a stanza in .git.config like this:
[merge "mergelog"]
name = A custom merge driver for my log.txt file.
driver = ~/scripts/mergelog %O %A %B %L
recursive = binary
To make git use the custom merge driver you also need to put this in
log.txt merge=mergelog
This tells git to use the 'mergelog' merge driver to merge files called log.txt.
import sys, difflib
class DiffError(Exception):
def __init__(self,value):
self.value = value
def __str__(self):
return repr(self.value)
def get_difference(one,two):
"""Return those lines of text from the beginning of list `two` that
are not present at the beginning of list `one`.
one The base text, a list of strings.
two The modified text, a list of strings.
`one` and `two` should be the same except that `two` may begin with
some lines that are not present in `one`. If the diff of `one` and
`two` produces any other results (if in producing `two`some text was
subtracted from `one`, if text was added to `one` elsewhere than at
the top of the file, or if more than one hunk of text was added to
`one`) an assertion will be raised.
d = difflib.Differ()
diff =,two)
mode = 0
added_lines = []
for line in diff:
if mode == 0:
if line.startswith('+ '):
if line.startswith(' '):
mode = 1
raise DiffError(diff)
elif mode == 1:
if not line.startswith(' '):
raise DiffError(diff)
return added_lines
# The arguments from git are the names of temporary files that
# hold the contents of the different versions of the log.txt
# file.
# The version of the file from the common ancestor of the two branches.
# This constitutes the 'base' version of the file.
ancestor = open(sys.argv[1],'r').readlines()
# The version of the file at the HEAD of the current branch.
# The result of the merge should be left in this file by overwriting it.
current = open(sys.argv[2],'r').readlines()
# The version of the file at the HEAD of the other branch.
other = open(sys.argv[3],'r').readlines()
# The merge algorithm is as follows:
# Append any text that was added to the beginning of the file in the
# other branch to the beginning of the current branch's copy of the file.
# If the other branch contains changes other than adding text at the
# beginning of the file then fail.
ancestor_to_other = get_difference(ancestor,other)
except DiffError, d:
print ''.join(difflib.unified_diff(ancestor,other))
print "The following text will be appended to the top of the file:"
print ''.join(ancestor_to_other)
f = open(sys.argv[2],'w')
# Exit with zero status if the merge went cleanly, non-zero otherwise.
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