camelCase ==> snake_case

slytherin.py

def slytherin(string):
    """Convert & return strings in camelCase to snake_case"""
    tmp = re.sub(pattern=r'(.)([A-Z][a-z]+)', repl=r'\1_\2', string=string)
    return re.sub(pattern=r'([a-z0-9])([A-Z])', repl=r'\1_\2', string=tmp)\
        .lower()