Sort items in a list by multiple fields

multisort.py

from operator import itemgetter

def multisort(vals, sort_spec, in_place=False):
    comparers = []
    for i, spec in enumerate(sort_spec):
        if isinstance(spec, tuple):
            key, polarity = spec
        elif isinstance(spec, (str, unicode)):
            key = spec.replace("-", "", 1)
            polarity = 1
            if spec[0] == "-":
                polarity = -1
        else:
            key = i
            polarity = spec

        if callable(key):
            key_fn = key
        else:
            key_fn = itemgetter(key)

        comparers.append((key_fn, polarity))

    def _compare_fn(left, right):
        for func, polarity in comparers:
            result = cmp(func(left), func(right)) * polarity
            if result:
                return result
        return 0

    if in_place:
        vals.sort(cmp=_compare_fn)
        return vals

    return sorted(vals, _compare_fn)