Python: Faster Way

Re:Python: Faster Way by jwl on Tuesday December 30, 2014 @05:42AM (#8814036)

Could someone please explain #Test 11. How is using %s instead of %d double as fast even though it is the same thing is being calculated? I always use %d because it seems like the prooper way to do it.

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Re:Python: Faster Way by nikital on Tuesday December 30, 2014 @07:04AM (#8814242)

I looked at the source of 2.7:

The function PyString_Format (Objects/stringobject.c) does the formatting for the % operator. For %s it calls _PyObject_Str which in turn calls "str()" on the object. For %d it calls formatint (located in the same file).

The str() implementation for ints is in int_to_decimal_string (Objects/intobject.c) and it's incredibly simple:

    do {
        *--p = '0' + (char)(absn % 10);
        absn /= 10;
    } while (absn);

The code for formatint is way more complex, and it contains two call to the native snprintf:

        PyOS_snprintf(fmt, sizeof(fmt), "%s%%%s.%dl%c",
                      sign, (flags&F_ALT) ? "#" : "",
                      prec, type);
        // ...
        PyOS_snprintf(buf, buflen, fmt, -x);

The native snprintf is heavier because it handles precision, zero-padding and stuff like that.

I believe this is why %d is slower. %s is a straight "devide-by-10-and-subtract" loop while %d are two library calls to the full-blown sprintf. However I didn't actually profile the code because I don't have a debug build, so I might be completely wrong.

Edit: Link to code: https://github.com/python/cpython/blob/a5c7a4257507a77699a1a...

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