Why does Python code run faster in a function?

Question

def main():     for i in xrange(10**8):         pass main() 

This piece of code in Python runs in (Note: The timing is done with the time function in BASH in Linux.)

real    0m1.841s user    0m1.828s sys     0m0.012s 

However, if the for loop isn't placed within a function,

for i in xrange(10**8):     pass 

then it runs for a much longer time:

real    0m4.543s user    0m4.524s sys     0m0.012s 

Why is this?

Answer 1

Inside a function, the bytecode is:

  2           0 SETUP_LOOP              20 (to 23)               3 LOAD_GLOBAL              0 (xrange)               6 LOAD_CONST               3 (100000000)               9 CALL_FUNCTION            1              12 GET_ITER                     >>   13 FOR_ITER                 6 (to 22)              16 STORE_FAST               0 (i)    3          19 JUMP_ABSOLUTE           13         >>   22 POP_BLOCK                    >>   23 LOAD_CONST               0 (None)              26 RETURN_VALUE         

At the top level, the bytecode is:

  1           0 SETUP_LOOP              20 (to 23)               3 LOAD_NAME                0 (xrange)               6 LOAD_CONST               3 (100000000)               9 CALL_FUNCTION            1              12 GET_ITER                     >>   13 FOR_ITER                 6 (to 22)              16 STORE_NAME               1 (i)    2          19 JUMP_ABSOLUTE           13         >>   22 POP_BLOCK                    >>   23 LOAD_CONST               2 (None)              26 RETURN_VALUE         

The difference is that STORE_FAST is faster (!) than STORE_NAME. This is because in a function, i is a local but at toplevel it is a global.

To examine bytecode, use the dis module. I was able to disassemble the function directly, but to disassemble the toplevel code I had to use the compile builtin.