In Python, how can I naturally sort a list of alphanumeric strings such that alpha characters sort ahead of numeric characters?

Question

This is a fun little challenge that confronted me recently. I\'ll provide my answer below, but I\'m curious to see whether there are more elegant or efficient solutions.

Answer 1

Here's one that works for the bonus test too:

def mykey(s):
    lst = re.findall(r'(\d+)|(\D+)', s)
    return [(0,a.lower()) if a else (1,int(n)) for n, a in lst]\
        + [a for n, a in lst if a]\
        + [len(n) for n, a in lst if n]

def mysort(lst):
    return sorted(lst, key=mykey)

With this type of pattern, re.findall breaks the string to a list of tuples, e.g.

>>> re.findall(r'(\d+)|(\D+)', 'ab12cd')
[('', 'ab'), ('12', ''), ('', 'cd')]