[Python-ideas] proposed sequence method: index_subseq()

[Jess Austin]

Recently I've repeatedly needed to check whether a particular sequence
occurred as a "subsequence" of another. I think this could be a general
requirement, so I'd like to ask if anyone else agrees. In python code, I
have the following, although I can rewrite it in C if people on this list
like it enough to add it to core:

def index_subseq(self, subseq):
    '''slice over which subseq is a sub-sequence of sequence self'''
    i = -1
    while True:
        i = self.index(subseq[0], i + 1)
        if all((a == b) for a, b in zip(subseq, itertools.chain(self[i:],
                iter(object, None)))):
            return slice(i, i + len(subseq))

(0, 1, 2, 3, 4, 5, 6, 7, 8).index_subseq((3,4,5)))       # slice(3, 6, None)
[0, 1, 2, 3, 4, 5, 6, 7, 8].index_subseq((3,4,5)))
(0, 1, 2, 3, 4).index_subseq((3,4,5)))                      # ValueError
[0, 1, 2, 3, 4].index_subseq((3,4,5)))

[This listing omits the monkeypatching into the list and tuple builtins.]

The index() method of more specialized sequences like str and bytearray
already has behavior much like what I propose for this method, since it
doesn't have to worry about elements of those sequences having elements in
turn. This method returns a slice object: I considered but decided against
returning just the initial index of the slice. As I've written it here, the
method doesn't care whether it is passed a list or a tuple as the "subseq"
argument. This could be generalized to take any iterable. If anyone has a
suggestion for a better name than "index_subseq", that would be good too.

If people think this is a good idea, I'll post a patch in the tracker.

thanks,
Jess
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