作业的右侧总是在作业之前进行评估吗? [英] Is the right-hand side of an assignment always evaluated before the assignment?

问题描述

这是一个代码片段.

x = {}
x[1] = len(x)

print x
{1: 0}

这个定义清楚吗?也就是说，可以 x == {1: 1} 代替吗?

Is this well defined? That is, could x == {1: 1} instead?

因为我记得 C++ '98 中的等效程序(如果我们使用 std::map)具有未定义的行为.用VS编译器和G++编译时，程序的输出是不同的.

Because I remember that an equivalent program in C++ '98 (if we use std::map) has undefined behaviour. The output of the program was different when compiled with VS compiler and G++.

推荐答案

正如我在评论中提到的，这个测试用例可以简化为:

As I mentioned in a comment, this test case can be reduced to:

x = {}
x[1] = len(x)

问题就变成了，是x[1] == 0，还是x[1] == 1?

The question then becomes, is x[1] == 0, or is x[1] == 1?

Python 从左到右计算表达式.请注意，在评估赋值时，右侧先于左侧进行评估.

Python evaluates expressions from left to right. Notice that while evaluating an assignment, the right-hand side is evaluated before the left-hand side.

在以下几行中，表达式将按照其后缀的算术顺序进行计算:

In the following lines, expressions will be evaluated in the arithmetic order of their suffixes:

expr3, expr4 = expr1, expr2

因此...

len(x) 将在我们执行 x[1] 之前被完全计算，所以 x[1] == 0 和这个定义明确.

Therefore...

len(x) will be fully computed before we do x[1], so x[1] == 0 and this is well defined.

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