Well done! I did not expect this to work with empty lists as well. :D More

You can change this: "for i in range(len(data)): x = data[i]" directly to "for c in data: x = c" More

You can replace if digit == 0 or upper == 0 or lower == 0: like this if 0 in (digit, upper, lower): More

Try to assert checkio('ABC###abc') == False More

The comment should be inside the function it comments on. Also, if you feel you need to explain you one-letter-length variables, then maybe use longer and more descriptive names to begin with. More

But why import collections and not from collections import counter when this is the only item you use from there? More

You can combine your two if-statements into one with the "and", like you did in the second line anyway. Also, if you have a pattern like this: if check_something and something_else: return True else: return False You can cut it short into th More