Counting Lines and Sum of Lines
line_count can be obtained with len(lines) instead of an enumerate.
You use rstrip (rightstrip), I think that strip is better as you (probably) don't want to count the blanks of indentation as characters.
f is the standard name for files in Python not fin
Using list comprehension and a for loop as you do is much more readable than a functional alternative, anyway, down here I show you a functional version:


:

def code_metric(file_name):
    with open(file_name) as f:
        lines = f.read().splitlines()
    char_count = sum(map(len,(map(str.strip,filter(None,lines)))))
    return len(filter(None,lines)), char_count