列表扩展了奇怪的行为 [英] List extending strange behaviour

问题描述

在Python(2.7)中发现了从未有过的有趣的东西.

Found interesting thing in Python (2.7) that never mentioned before.

此:

a = []
a += "a"

有效，结果是:

>>> a
>>> ["a"]

但是

a = []
a = a + "a"

给予

>>> TypeError: can only concatenate list (not "str") to list

有人可以解释为什么吗?感谢您的回答.

Can someone explain why? Thanks for your answers.

推荐答案

Python区分+和+=运算符，并为它们提供了单独的钩子. __add__ 和

Python distinguishes between the + and += operators and provides separate hooks for these; __add__ and __iadd__. The list() type simply provides a different implementation for the latter.

列表单独实现这些方法更有效； __add__必须返回一个全新的列表，而__iadd__可以仅扩展self然后返回self.

It is more efficient for lists to implement these separately; __add__ has to return a completely new list, while __iadd__ can just extend self then return self.

在C代码中，__iadd__由 list_inplace_concat() ，它简单地调用listextend()，或者在python代码中调用[].extend().后者根据设计采用 any 序列.

In the C code, __iadd__ is implemented by list_inplace_concat(), which simply calls listextend(), or, in python code, [].extend(). The latter takes any sequence, by design.

另一方面，在C中由

The __add__ method on the other hand, represented in C by list_concat, only takes a list as input, probably for efficiency's sake; it can loop directly over the internal C array and copy items over to the new list.

最后，__iadd__接受任何序列的原因是因为 PEP 203 (增强添加提案)已实现，对于列表而言，仅重用.extend()方法是最简单的.

In conclusion, the reason __iadd__ accepts any sequence is because when PEP 203 (the Augmented Add proposal) was implemented, for lists it was simplest just to reuse the .extend() method.

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