Why is if True slower than if 1?

True and False are not keywords in Python 2.

They must resolve at runtime. This has been changed in Python 3

Same test on Python 3:

>>> timeit.timeit('test1()',setup="from __main__ import test1", number=10000000)2.806439919999889>>> timeit.timeit('test2()',setup="from __main__ import test2", number=10000000)2.801301520000038>>> timeit.timeit('test3()',setup="from __main__ import test3", number=10000000)2.7952816800000164>>> timeit.timeit('test4()',setup="from __main__ import test4", number=10000000)2.7862537199999906

Time error is in 1%, which is acceptable.

Bytecode disassembly makes difference obvious.

>>> dis.dis(test1)  2           0 LOAD_GLOBAL              0 (True)              3 JUMP_IF_FALSE            5 (to 11)              6 POP_TOP               3           7 LOAD_CONST               1 (1)             10 RETURN_VALUE                >>   11 POP_TOP               5          12 LOAD_CONST               2 (0)             15 RETURN_VALUE                     16 LOAD_CONST               0 (None)             19 RETURN_VALUE        

As Kabie mentioned, True and False are globals in Python 2. Lots of stuff is going on to access them.

>>> dis.dis(test2)  3           0 LOAD_CONST               1 (1)              3 RETURN_VALUE        

Python compiler was able to recognize 1 as a constantly "truthy" expression and optimize redundant condition away!

>>> dis.dis(test3)  2           0 LOAD_GLOBAL              0 (True)              3 JUMP_IF_FALSE            5 (to 11)              6 POP_TOP               3           7 LOAD_GLOBAL              0 (True)             10 RETURN_VALUE                >>   11 POP_TOP               5          12 LOAD_GLOBAL              1 (False)             15 RETURN_VALUE                     16 LOAD_CONST               0 (None)             19 RETURN_VALUE        

Pretty much the same as test1, with one more LOAD_GLOBAL.

>>> dis.dis(test4)  3           0 LOAD_GLOBAL              0 (True)              3 RETURN_VALUE        

See test2. But LOAD_GLOBAL is a bit more costly than LOAD_CONST.