~$python -mtimeit "'a' + 'b' + 'c' + 'd'"
10000000 loops, best of 3: 0.026 usec per loop
~$python -mtimeit "''.join(('a','b','c','d'))"
10000000 loops, best of 3: 0.197 usec per loop
10000000 loops, best of 3: 0.026 usec per loop
~$python -mtimeit "''.join(('a','b','c','d'))"
10000000 loops, best of 3: 0.197 usec per loop
$ python -mtimeit "'aaaaaaaaaaaaaaa' + 'bbbbbbbbbbbbbbb' + 'ccccccccccccccc' + 'ddddddddddddddd'"
1000000 loops, best of 3: 0.224 usec per loop
$ python -mtimeit "''.join(('aaaaaaaaaaaaaaa','bbbbbbbbbbbbbbb','ccccccccccccccc','ddddddddddddddd'))"
10000000 loops, best of 3: 0.201 usec per loop
This recommendation is to go with .join() by default:
In performance sensitive parts of the library, the ''.join() form should be used instead. This will ensure that concatenation occurs in linear time across various implementations. — http://www.python.org/dev/peps/pep-0008/
despite the fact it might not be better in CPython.
For a fair test, add elements of an array vs. joining them.