A function that returns a function that sorts according to a list of keys

props_to_sort_lambda.py

def props_to_sort_lambda(*props):
    """Return a function that sorts according to property list `props`.

    The sort is supposed to apply on a list of dictionaries with the same set of
    keys.  Argument `props` is a list of keys for the dicts.

    Example:

    xs = [{"aaa": 1, "bbb": 2, ...}, {"aaa": 2, "bbb": 3, ...}, ...]
    sorted_xs = props_to_sort_lambda("-aaa", "bbb")(xs)

    This sorts `xs` first by "bbb", and then by "aaa" but reversed. In another
    word, the result would contain the largest aaa in the beginning, and if two
    dicts have the same aaa, the one that has the smaller bbb comes first.
    """
    if len(props) == 0:
        return lambda x:x

    prop = props[0]
    reverse = False
    if prop[0] == '-':
        prop = prop[1:]
        reverse = True
    return lambda xs: sorted(props_to_sort_lambda(*(props[1:]))(xs),
                             key=lambda x:x[prop], reverse=reverse)