Several Python sequence-related functions I find myself using over and over again and tire of reimplementing. :) These probably all originally came from StackOverflow answers of one sort or another.

more_itertools.py

from itertools import tee, izip, izip_longest


def pairwise_overlapping(iterable, n=2):
    """
    Given a sequence, returns a list of pairs/tuples of length n, each comprised of sequential values
    from the sequence.  Each tuple starts with the subsequent value from the original sequence.
    
    >>> s = 1, 2, 3, 4, 5
    >>> list(pairwise_overlapping(s))
    [(1, 2), (2, 3), (3, 4), (4, 5)]
    >>> list(pairwise_overlapping(s, 3))
    [(1, 2, 3), (2, 3, 4), (3, 4, 5)]
    """
    seqs = tee(iterable, n)
    for advance_count, seq in enumerate(seqs, start=0):
        for _ in range(advance_count):
            next(seq, None)
    return izip(*seqs)


def pairwise_exclusive(iterable):
    """
    Given a sequence, returns a list of pairs, each comprised of sequential values from the sequence.
    If a given pair consists of items n and n+1, the next will start with item n + 2.
    
    The final item in a sequence with an odd length will be discarded.  (To preserve it as (n, None)
    use izip_longest instead of izip).

    >>> s = 1, 2, 3, 4, 5
    >>> list(pairwise_exclusive(s))
    [(1, 2), (3, 4)]
    >>> s = 1, 2, 3, 4, 5, 6
    >>> list(pairwise_exclusive(s))
    [(1, 2), (3, 4), (5, 6)]
    """
    a = iter(iterable)
    return izip(a, a)


def chunks(iterable, chunk_size, fillvalue=None):
    """
    The same as pairwise_exclusive but will divide sequences into "chunks" of any size.  Preserves all
    values in original sequence, padding final tuples with fillvalue if necessary.
    
    chunks(iterable, 2) is exactly equivalent to pairwise_exclusive using izip_longest instead of izip.

    >>> s = 1, 2, 3, 4, 5
    >>> list(chunks(s, 2))
    [(1, 2), (3, 4), (5, None)]
    >>> list(chunks(s, 3))
    [(1, 2, 3), (4, 5, None)]
    """
    return izip_longest(*[iter(iterable)] * chunk_size, fillvalue=fillvalue)