这个链分配如何工作? [英] How does this chain assignment work?

问题描述

考虑下面的代码；这是一种糟糕的编程习惯.我想知道为什么结果列表 A 是 [1, 1, 3] 而不是 [1, 2, 1].从Java的角度来看，结果应该是[1, 2, 1].谁能解释为什么这个结果是这样的?

Consider the following code; it is a bad programming practice. I am wondering why the resulting list A is [1, 1, 3] rather than [1, 2, 1]. From the view of Java, the result should be [1, 2, 1]. Can anyone explain why this result is what it is?

A = [1, 2, 3]
t = 2
t = A[t] = A.count(3)

求值后，A是[1, 1, 3]，t是1.

我的 Python 版本是 3.3.

My Python version is 3.3.

推荐答案

在第 3 行，您有一个链式作业

On line 3 you have a chained assignment

t = A[t] = A.count(3)

t = A.count(3) 首先评估 - t 设置为 A.count(3) 的返回值，其中在这种情况下是 1.然后将t(=1)处A的成员设置为A.count(3)的返回值，仍然是1.

t = A.count(3) is evaluated first – t set to the return value of A.count(3) which in this case is 1. Then the member of A at index t(=1) is set to the return value of A.count(3), which is still 1.

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