str performance in python

Question

While profiling a piece of python code (python 2.6 up to 3.2), I discovered that the str method to convert an object (in my case an integer) to a string is almost an order of magnitude slower than using string formatting.

Here is the benchmark

>>> from timeit import Timer >>> Timer('str(100000)').timeit() 0.3145311339386332 >>> Timer('"%s"%100000').timeit() 0.03803517023435887 

Does anyone know why this is the case? Am I missing something?

Answer 1

'%s' % 100000 is evaluated by the compiler and is equivalent to a constant at run-time.

>>> import dis >>> dis.dis(lambda: str(100000))   8           0 LOAD_GLOBAL              0 (str)               3 LOAD_CONST               1 (100000)               6 CALL_FUNCTION            1               9 RETURN_VALUE         >>> dis.dis(lambda: '%s' % 100000)   9           0 LOAD_CONST               3 ('100000')               3 RETURN_VALUE         

% with a run-time expression is not (significantly) faster than str:

>>> Timer('str(x)', 'x=100').timeit() 0.25641703605651855 >>> Timer('"%s" % x', 'x=100').timeit() 0.2169809341430664 

Do note that str is still slightly slower, as @DietrichEpp said, this is because str involves lookup and function call operations, while % compiles to a single immediate bytecode:

>>> dis.dis(lambda x: str(x))   9           0 LOAD_GLOBAL              0 (str)               3 LOAD_FAST                0 (x)               6 CALL_FUNCTION            1               9 RETURN_VALUE         >>> dis.dis(lambda x: '%s' % x)  10           0 LOAD_CONST               1 ('%s')               3 LOAD_FAST                0 (x)               6 BINARY_MODULO                      7 RETURN_VALUE         

Of course the above is true for the system I tested on (CPython 2.7); other implementations may differ.