Question

I need to loop over a list of objects, comparing them like this: 0 vs. 1, 1 vs. 2, 2 vs. 3, etc. (I’m using pysvn to extract a list of diffs.) I wound up just looping over an index, but I keep wondering if there’s some way to do it which is more closely idiomatic. It’s Python; shouldn’t I be using iterators in some clever way? Simply looping over the index seems pretty clear, but I wonder if there’s a more expressive or concise way to do it.

for revindex in xrange(len(dm_revisions) - 1):
    summary = \
        svn.diff_summarize(svn_path,
                          revision1=dm_revisions[revindex],
                          revision2 = dm_revisions[revindex+1])

Answer 1

This is called a sliding window. There’s an example in the itertools documentation that does it. Here’s the code:

from itertools import islice

def window(seq, n=2):
    "Returns a sliding window (of width n) over data from the iterable"
    "   s -> (s0,s1,...s[n-1]), (s1,s2,...,sn), ...                   "
    it = iter(seq)
    result = tuple(islice(it, n))
    if len(result) == n:
        yield result    
    for elem in it:
        result = result[1:] + (elem,)
        yield result

What that, you can say this:

for r1, r2 in window(dm_revisions):
    summary = svn.diff_summarize(svn_path, revision1=r1, revision2=r2)

Of course you only care about the case where n=2, so you can get away with something much simpler:

def adjacent_pairs(seq):
    it = iter(seq)
    a = it.next()
    for b in it:
        yield a, b
        a = b

for r1, r2 in adjacent_pairs(dm_revisions):
    summary = svn.diff_summarize(svn_path, revision1=r1, revision2=r2)