python bytecode

bytecode

def fib(n):
    a, b = 0, 1
    for _ in range(n):
        a, b = b, a + b
    return a
    
Then, using dis.dis(fib)
CPython bytecode:

  2           0 LOAD_CONST               3 ((0, 1))
              2 UNPACK_SEQUENCE          2
              4 STORE_FAST               1 (a)
              6 STORE_FAST               2 (b)

  3           8 SETUP_LOOP              30 (to 40)
             10 LOAD_GLOBAL              0 (range)
             12 LOAD_FAST                0 (n)
             14 CALL_FUNCTION            1
             16 GET_ITER
        >>   18 FOR_ITER                18 (to 38)
             20 STORE_FAST               3 (_)

  4          22 LOAD_FAST                2 (b)
             24 LOAD_FAST                1 (a)
             26 LOAD_FAST                2 (b)
             28 BINARY_ADD
             30 ROT_TWO
             32 STORE_FAST               1 (a)
             34 STORE_FAST               2 (b)
             36 JUMP_ABSOLUTE           18
        >>   38 POP_BLOCK

  5     >>   40 LOAD_FAST                1 (a)
             42 RETURN_VALUE

PyPy bytecode:

  2           0 LOAD_CONST               1 (0)
              3 LOAD_CONST               2 (1)
              6 STORE_FAST               1 (b)
              9 STORE_FAST               2 (a)

  3          12 SETUP_LOOP              36 (to 51)
             15 LOAD_GLOBAL              0 (range)
             18 LOAD_FAST                0 (n)
             21 CALL_FUNCTION            1 (1 positional, 0 keyword pair)
             24 GET_ITER
        >>   25 FOR_ITER                22 (to 50)
             28 STORE_FAST               3 (_)

  4          31 LOAD_FAST                1 (b)
             34 LOAD_FAST                2 (a)
             37 LOAD_FAST                1 (b)
             40 BINARY_ADD
             41 STORE_FAST               1 (b)
             44 STORE_FAST               2 (a)
             47 JUMP_ABSOLUTE           25
        >>   50 POP_BLOCK

  5     >>   51 LOAD_FAST                2 (a)
             54 RETURN_VALUE