Python – Why is looping over range() in Python faster than using a while loop

benchmarkingperformancepython

The other day I was doing some Python benchmarking and I came across something interesting. Below are two loops that do more or less the same thing. Loop 1 takes about twice as long as loop 2 to execute.

Loop 1:

int i = 0
while i < 100000000:
  i += 1

Loop 2:

for n in range(0,100000000):
  pass

Why is the first loop so much slower? I know it's a trivial example but it's piqued my interest. Is there something special about the range() function that makes it more efficient than incrementing a variable the same way?

Best Solution

see the disassembly of python byte code, you may get a more concrete idea

use while loop:

1           0 LOAD_CONST               0 (0)
            3 STORE_NAME               0 (i)

2           6 SETUP_LOOP              28 (to 37)
      >>    9 LOAD_NAME                0 (i)              # <-
           12 LOAD_CONST               1 (100000000)      # <-
           15 COMPARE_OP               0 (<)              # <-
           18 JUMP_IF_FALSE           14 (to 35)          # <-
           21 POP_TOP                                     # <-

3          22 LOAD_NAME                0 (i)              # <-
           25 LOAD_CONST               2 (1)              # <-
           28 INPLACE_ADD                                 # <-
           29 STORE_NAME               0 (i)              # <-
           32 JUMP_ABSOLUTE            9                  # <-
      >>   35 POP_TOP
           36 POP_BLOCK

The loop body has 10 op

use range:

1           0 SETUP_LOOP              23 (to 26)
            3 LOAD_NAME                0 (range)
            6 LOAD_CONST               0 (0)
            9 LOAD_CONST               1 (100000000)
           12 CALL_FUNCTION            2
           15 GET_ITER
      >>   16 FOR_ITER                 6 (to 25)        # <-
           19 STORE_NAME               1 (n)            # <-

2          22 JUMP_ABSOLUTE           16                # <-
      >>   25 POP_BLOCK
      >>   26 LOAD_CONST               2 (None)
           29 RETURN_VALUE

The loop body has 3 op

The time to run C code is much shorter than intepretor and can be ignored.