rot two 28 29 in python

1>>> import dis
2>>> def foo(a, b):
3...     a, b = b, a
4... 
5>>> dis.dis(foo)
6  2           0 LOAD_FAST                1 (b)
7              3 LOAD_FAST                0 (a)
8              6 ROT_TWO             
9              7 STORE_FAST               0 (a)
10             10 STORE_FAST               1 (b)
11             13 LOAD_CONST               0 (None)
12             16 RETURN_VALUE