Default argument value is mutable

def append_mutable(my_list=list()):
    my_list.append(len(my_list))
    return my_list


def append_none(my_list=None):
    if my_list is None:
        my_list = list()
    my_list.append(len(my_list))
    return my_list


if __name__ == '__main__':
    for _ in range(4):
        print(append_mutable())
    for _ in range(4):
        print(append_none())