How many bytes does a string have

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

import sys
sys.getsizeof(s)

# getsizeof(object, default) -> int
# Return the size of object in bytes.

But actually you need to know its represented length, so something like len(s) should be enough.