变量交换是否保证在 python 中是原子的?

问题描述

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

我想知道以下是否:

(x, y) = (y, x)

将在 cPython 中保证原子性.(x 和 y 都是 python 变量)

解决方案

一起来看看:

>>>x = 1>>>y = 2>>>def swap_xy():... 全局 x, y... (x, y) = (y, x)...>>>dis.dis(swap_xy)3 0 LOAD_GLOBAL 0 (y)3 LOAD_GLOBAL 1 (x)6 ROT_TWO7 STORE_GLOBAL 1 (x)10 STORE_GLOBAL 0 (y)13 LOAD_CONST 0 (无)16 RETURN_VALUE

它们似乎不是原子的:x 和 y 的值可以被 LOAD_GLOBAL 字节码之间的另一个线程改变，在 ROT_TWO 之前或之后, 以及 STORE_GLOBAL 字节码之间.

如果你想原子地交换两个变量，你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():...而1:... swap_xy()...如果 x == y:... # 如果所有交换都是原子的，那么永远不会有 x == y 的时候.... # (当然，这取决于if x == y"是原子的，它不是；... # 但是如果if x == y"不是原子的，我们对更复杂的东西有什么希望... # "x, y = y, x"?)...打印检测到非原子交换"... 休息...>>>t1 = threading.Thread(target=swap_xy_repeatedly)>>>t2 = threading.Thread(target=swap_xy_repeatedly)>>>t1.开始()>>>t2.start()>>>检测到非原子交换

With reference to the following link: http://docs.python.org/faq/library.html#what-kinds-of-global-value-mutation-are-thread-safe

I wanted to know if the following:

(x, y) = (y, x)

will be guaranteed atomic in cPython. (x and y are both python variables)

解决方案

Let's see:

>>> 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

It doesn't appear that they're atomic: the values of x and y could be changed by another thread between the LOAD_GLOBAL bytecodes, before or after the ROT_TWO, and between the STORE_GLOBAL bytecodes.

If you want to swap two variables atomically, you'll need a lock or a mutex.

For those desiring empirical proof:

>>> 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

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