Convert camelcase string into underscored one

def camel_to_underscore(string):
    """
    Convert a CamelCased string into an underscored one
    Usage :
    >>> camel_to_underscore("ThisIsATest")
    this_is_a_test
    """
    list_string = []
    for c in string[1:]:
        if c.isupper():
            list_string.append("_" + c.lower())
        else:
            list_string.append(c)
    return string[0].lower() + ''.join(list_string)