让我们来看看：
>>> x = 1>>> y = 2>>> def swap_xy():... global x, y... (x, y) = (y, x)... >>> dis.dis(swap_xy) 30 LOAD_GLOBAL 0 (y) 3 LOAD_GLOBAL 1 (x) 6 ROT_TWO 7 STORE_GLOBAL 1 (x) 10 STORE_GLOBAL 0 (y) 13 LOAD_ConST 0 (None) 16 RETURN_VALUE
它不会出现，他们是原子：x的和y的值可以被另一个线程之间改变
LOAD_GLOBAL字节码，之前或之后
ROT_TWO，和之间的
STORE_GLOBAL字节码。
如果要原子交换两个变量，则需要一个锁或一个互斥锁。
对于那些需要经验证明的人：
>>> def swap_xy_repeatedly():... while 1:... swap_xy()... if x == y:... # If all swaps are atomic, there will never be a time when x == y.... # (of course, this depends on "if x == y" being atomic, which it isn't;... # but if "if x == y" isn't atomic, what hope have we for the more complex... # "x, y = y, x"?)... print 'non-atomic swap detected'... break... >>> t1 = threading.Thread(target=swap_xy_repeatedly)>>> t2 = threading.Thread(target=swap_xy_repeatedly)>>> t1.start()>>> t2.start()>>> non-atomic swap detected
