Direct assignment to local variables different for Python 2 and 3

gistfile1.txt

import sys

def foo(op, val):
    num = 1;
    exec('num = num ' + op + ' val;');
    print("INSIDE - num");
    print(num);
    return num;

if len(sys.argv) != 2:
    op = '+';
else:
    op = sys.argv[1];

bar = 6;
print("BEFORE - bar");
print(bar);
bar = foo(op, bar);
print("AFTER - bar");
print(bar);