If it’s a Python 2.x str, get its len. If it’s a Python 3.x str (or a Python 2.x unicode), first encode to bytes (or a str, respectively) using your preferred encoding ('utf-8' is a good choice) and then get the len of the encoded bytes/str object.
For example, ASCII characters use 1 byte each:
>>> len("hello".encode("utf8"))
5
whereas Chinese ones use 3 bytes each:
>>> len("你好".encode("utf8"))
6