What's the computational cost of count operation on strings Python?

For example:

'hello'.count('e')

Is this O(n)? I'm guessing the way it works is it scans 'hello' and increments a counter each time the letter 'e' is seen. How can I know this without guessing? I tried reading the source code here, but got stuck upon finding this:

def count(s, *args):
    """count(s, sub[, start[,end]]) -> int

    Return the number of occurrences of substring sub in string
    s[start:end].  Optional arguments start and end are
    interpreted as in slice notation.

    """
    return s.count(*args)

Where can I read about what's executed in s.count(*args)?

edit: I understand what *args does in the context of Python functions.