misc.py

def sorted_alphanumerically(strings, key=None):
    """Sort alphabetically, except treat strings of digits as numbers.
    If key is not None, it must return a string.

    >>> sorted_alphanumerically(['r1', 'r10', 'r2'])
    ['r1', 'r2', 'r10']
    >>> sorted_alphanumerically(['r1', 'R10', 'r2'])
    ['R10', 'r1', 'r2']
    >>> sorted_alphanumerically(['r1', 'R10', 'r2'], key=str.lower)
    ['r1', 'r2', 'R10']

    """
    if key is None:
        key = lambda x: x

    def split(s):
        """
        >>> split('what has 18 legs and catches flies?')
        ['what has ', 18, ' legs and catches flies?']
        """
        return [int(value) if index % 2 else value for index, value in enumerate(re.split(r'([0-9]+)', s))]

    return sorted(strings, key=lambda elt: split(key(elt)))