efficiently checking that string consists of one character in Python

Try using the built-in function all:

all(c == 'A' for c in s)

You could convert to a set and check there is only one member:

len(set("AAAAAAAA"))

If you need to check if all the characters in the string are same and is equal to a given character, you need to remove all duplicates and check if the final result equals the single character.

>>> set("AAAAA") == set("A")
True

In case you desire to find if there is any duplicate, just check the length

>>> len(set("AAAAA")) == 1
True

>>> s = 'AAAAAAAAAAAAAAAAAAA'
>>> s.count(s[0]) == len(s)
True

This doesn’t short circuit. A version which does short-circuit would be:

>>> all(x == s[0] for x in s)
True

However, I have a feeling that due the the optimized C implementation, the non-short circuiting version will probably perform better on some strings (depending on size, etc)

Here’s a simple timeit script to test some of the other options posted:

import timeit
import re

def test_regex(s,regex=re.compile(r'^(.)1*$')):
    return bool(regex.match(s))

def test_all(s):
    return all(x == s[0] for x in s)

def test_count(s):
    return s.count(s[0]) == len(s)

def test_set(s):
    return len(set(s)) == 1

def test_replace(s):
    return not s.replace(s[0],'')

def test_translate(s):
    return not s.translate(None,s[0])

def test_strmul(s):
    return s == s[0]*len(s)

tests = ('test_all','test_count','test_set','test_replace','test_translate','test_strmul','test_regex')

print "WITH ALL EQUAL"
for test in tests:
    print test, timeit.timeit('%s(s)'%test,'from __main__ import %s; s="AAAAAAAAAAAAAAAAA"'%test)
    if globals()[test]("AAAAAAAAAAAAAAAAA") != True:
        print globals()[test]("AAAAAAAAAAAAAAAAA")
        raise AssertionError

print
print "WITH FIRST NON-EQUAL"
for test in tests:
    print test, timeit.timeit('%s(s)'%test,'from __main__ import %s; s="FAAAAAAAAAAAAAAAA"'%test)
    if globals()[test]("FAAAAAAAAAAAAAAAA") != False:
        print globals()[test]("FAAAAAAAAAAAAAAAA")
        raise AssertionError

On my machine (OS-X 10.5.8, core2duo, python2.7.3) with these contrived (short) strings, str.count smokes set and all, and beats str.replace by a little, but is edged out by str.translate and strmul is currently in the lead by a good margin:

WITH ALL EQUAL
test_all 5.83863711357
test_count 0.947771072388
test_set 2.01028490067
test_replace 1.24682998657
test_translate 0.941282987595
test_strmul 0.629556179047
test_regex 2.52913498878

WITH FIRST NON-EQUAL
test_all 2.41147494316
test_count 0.942595005035
test_set 2.00480484962
test_replace 0.960338115692
test_translate 0.924381017685
test_strmul 0.622269153595
test_regex 1.36632800102

The timings could be slightly (or even significantly?) different between different systems and with different strings, so that would be worth looking into with an actual string you’re planning on passing.

Eventually, if you hit the best case for all enough, and your strings are long enough, you might want to consider that one. It’s a better algorithm … I would avoid the set solution though as I don’t see any case where it could possibly beat out the count solution.

If memory could be an issue, you’ll need to avoid str.translate, str.replace and strmul as those create a second string, but this isn’t usually a concern these days.

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.

not len("AAAAAAAAA".replace('A', ''))

Adding another solution to this problem

>>> not "AAAAAA".translate(None,"A")
True