变量交换是否保证在python中是原子的？

Question

参考以下链接：http : //docs.python.org/faq/library.html#what-
kinds-of-global-value-mutation-are-thread-
safe

我想知道以下情况：

(x, y) = (y, x)

在cPython中将保证是原子的。（x和y都是python变量）

Answer 1

让我们来看看：

>>> x = 1
>>> y = 2
>>> def swap_xy():
...   global x, y
...   (x, y) = (y, x)
... 
>>> dis.dis(swap_xy)
  3           0 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