序列拆包是原子的吗? [英] Is sequence unpacking atomic?

问题描述

序列解包是原子的吗?例如:

Is sequence unpacking atomic? e.g.:

(a, b) = (c, d)

我对它的印象不是.

我的意思是在多线程上下文中使用原子性，即整个语句是否像原子一样是不可分割的.

I meant atomicity in the context of multi-threading, i.e. whether the entire statement is indivisible, as atoms used to be.

推荐答案

这是一项操作；在应用左手赋值之前先评估右手表达式:

It is one operation; the right-hand expression is evaluated before the left-hand assignment is applied:

>>> a, b = 10, 20
>>> a, b
(10, 20)
>>> b, a = a, b
>>> a, b
(20, 10)
>>> a, b = a*b, a/b
>>> a, b
(200, 2)

或者，如果您正在谈论多线程环境，则该分配是 not 原子的；解释器使用单个操作码评估一个元组分配，但是使用单独的操作码将结果存储到每个受影响的变量中:

Or, if you are talking about multi-threaded environments, then the assignment is not atomic; the interpreter evaluates a tuple assignment with a single opcode, but uses separate opcodes to then store the results into each affected variable:

>>> def t(self): a,b=20,20
... 
>>> dis.dis(t)
  1           0 LOAD_CONST               2 ((20, 20))
              3 UNPACK_SEQUENCE          2
              6 STORE_FAST               1 (a)
              9 STORE_FAST               2 (b)
             12 LOAD_CONST               0 (None)
             15 RETURN_VALUE        

但是，正常分配总是至少是两个操作码(一个用于右侧表达式，一个用于存储结果)，因此通常在python 中使用分配不是原子的.序列拆包没有什么不同.

However, normal assigment is always going to be at least two opcodes (one for the right-hand expression, one for storing the result), so in python in general assigment is not atomic. Sequence unpacking is no different.

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