Here's a the problem we want to solve: we want to find if the string s contains a string consisting of exactly $n$ "a"s followed by one "b". ($n+1$ "a"s followed by one "b" would count, and $n$ "a"s followed by "bb" won't count either.)

The approach we'll take is defining the state variable cur_run_a (Note: it's just a regular integer. We just call it a state variable), and make sure that it's up-to-date at every iteration.

def n_as_plus_b(s, n):
    cur_run_a = 0 #the length of the current run of a's
    
    for i in range(len(s)):
        if s[i] == "a":
            cur_run_a += 1
        elif s[i] == "b":
            if cur_run_a == n:
                if i == len(s)-1:
                    return True
                if s[i+1] != "b":
                    return True
            else:
                cur_run_a = 0
        else:
            cur_run_a = 0
    
    return False