Given two text files A,B, what is an easy way to get the line numbers of lines in B not present in A? I see there's difflib, but don't see an interface for retrieving line numbers

difflib can give you what you need. Assume:

a.txt

this 
is 
a 
bunch 
of 
lines

b.txt

this 
is 
a 
different
bunch 
of 
other
lines

code like this:

import difflibfileA = open("a.txt", "rt").readlines()
fileB = open("b.txt", "rt").readlines()d = difflib.Differ()
diffs = d.compare(fileA, fileB)
lineNum = 0for line in diffs:# split off the codecode = line[:2]# if the  line is in both files or just b, increment the line number.if code in ("  ", "+ "):lineNum += 1# if this line is only in b, print the line number and the text on the lineif code == "+ ":print "%d: %s" % (lineNum, line[2:].strip())

gives output like:

bgporter@varese ~/temp:python diffy.py 
4: different
7: other

You'll also want to look at the difflib code "? " and see how you want to handle that one.

(also, in real code you'd want to use context managers to make sure the files get closed, etc etc etc)