Function Overloading - Alternative

func_overload.py

#!/usr/bin/env python
from string import ascii_lowercase

def sum_list(values, ignore_invalid=True):
    """Returns the sum of a list of values.
       String representations of integers ('0'-'9') are added at their
       integer value, but letters have a value between 1-26.
       (a=1, b=2, c=3, ...)
    """
    total = 0
    for value in values:
        try:
            # Try casting it to an int.
            total += int(value)
        except ValueError as e:
            """ ValueError: invalid literal for int() with base 10: 'A' """
            try:
                assert len(value) is 1
                total += (ascii_lowercase.index(value.lower()) + 1)
            except (AssertionError, ValueError):
                """ ValueError: substring not found """
                if ignore_invalid is True:
                    continue
                raise ValueError("Invalid value '%s'" % (str(value)))

    return total

_tests_ = [
    ([], 0),
    ([1,2,3], 6),
    (['A','B','C'], 6),
    (['1','2','3'], 6),
    (['a','b','c'], 6),
    (['a','!', ' '], 1),
    (['d', 'e', 3, 10, 'asdf'], 22),
    (['d', 'e', 3, 10, 'abc'], 22), # ascii_lowercase.index('abc')
    ]

if __name__ == '__main__':
    for test, expected in _tests_:
        msg = "\033[92m PASS \033[0m"
        answer = sum_list(test)

        if answer != expected:
            msg = "\033[91m FAIL \033[0m"
            msg += "+%d\n" % (expected)
            msg += "-%s\n" % (str(answer))
        print msg