Python赋值运算符不同于非赋值运算符 [英] Python assignment operator differs from non assignment

问题描述

我面临着这种奇怪的行为，我找不到任何解释.

I have face this weird behavior I can not find explications about.

MWE:

l = [1]
l += {'a': 2}
l
[1, 'a']
l + {'B': 3}
Traceback (most recent call last):
  File "<input>", line 1, in <module>
TypeError: can only concatenate list (not "dict") to list

基本上，当我 + = 时，python不会引发错误并将键附加到列表中，而当我仅计算 + 时，我得到了预期的TypeError .

Basically, when I += python does not raise an error and append the key to the list while when I only compute the + I get the expected TypeError.

注意:这是Python 3.6.10

Note: this is Python 3.6.10

推荐答案

l + = ... 实际上是在调用 object .__ iadd __(self，other)，并在 l 为可变

l += ... is actually calling object.__iadd__(self, other) and modifies the object in-place when l is mutable

原因(如@DeepSpace在他的评论中解释的那样)是，当您执行 l + = {'a':2} 时，该操作只会在适当的位置更新 l 并且仅当 l 是可变的.另一方面，未正确执行操作 l + {'a':2} ，从而导致 list + dictionary->TypeError .

The reason (as @DeepSpace explains in his comment) is that when you do l += {'a': 2} the operation updates l in place only and only if l is mutable. On the other hand, the operation l + {'a': 2} is not done in place resulting into list + dictionary -> TypeError.

(请参见此处)

l = [1]
l = l.__iadd__({'a': 2})
l
#[1, 'a']

与调用 object .__ add __(自身，其他)

l + {'B': 3}

TypeError: can only concatenate list (not "dict") to list

这篇关于Python赋值运算符不同于非赋值运算符的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！