Explicit assignment and `+=` yield different results if the index produces side effects

mindfuck.py

#!/usr/bin/env python
# -*- coding: utf-8 -*-

from itertools import count

a = [1, 10]
b = [1, 10]

c1 = count(0).next
c2 = count(0).next

a[c1()] = a[c1()] + 1
b[c2()] += 1

print a  # [1, 2]
print b  # [2, 10]