efficiently checking that string consists of one character in Python

Question

What is an efficient way to check that a string s in Python consists of just one character, say \'A\'? Something like all_equal(s, \'A\')

Answer 1

Interesting answers so far. Here's another:

flag = True
for c in 'AAAAAAAfAAAA':
    if not c == 'A': 
        flag = False
        break

The only advantage I can think of to mine is that it doesn't need to traverse the entire string if it finds an inconsistent character.