列表上的 Python append() 与 + 运算符，为什么这些会给出不同的结果?

问题描述

为什么这两个操作(append() 和 +)给出不同的结果?

>>>c = [1, 2, 3]>>>C[1, 2, 3]>>>c += c>>>C[1, 2, 3, 1, 2, 3]>>>c = [1, 2, 3]>>>c.append(c)>>>C[1, 2, 3, [...]]>>>

在最后一种情况下，实际上是无限递归.c[-1] 和 c 是一样的.为什么和+操作不一样?

解决方案

解释为什么":

+ 操作添加了array 元素到原始数组.array.append 操作将数组(或任何对象)插入到原始数组的末尾，这导致在那个位置引用 self(因此是无限递归).

这里的区别在于 + 操作在添加数组时是特定的(它像其他人一样重载，请参阅 本章关于序列)通过连接元素.然而，append 方法确实按照您的要求执行:将对象(数组或任何其他对象)添加到右侧，而不是获取其元素.

另一种选择

使用 extend() 如果你想使用一个类似于 + 操作符的函数(正如其他人在这里展示的那样).做相反的事情是不明智的:尝试用 + 运算符模拟列表的追加(请参阅我的 较早的链接 原因).

小历史

为了好玩，一点历史:诞生1993 年 2 月 Python 中的数组模块.你可能会感到惊讶，但数组是在序列和列表出现之后添加的.

Why do these two operations (append() resp. +) give different results?

>>> c = [1, 2, 3]
>>> c
[1, 2, 3]
>>> c += c
>>> c
[1, 2, 3, 1, 2, 3]
>>> c = [1, 2, 3]
>>> c.append(c)
>>> c
[1, 2, 3, [...]]
>>>

In the last case there's actually an infinite recursion. c[-1] and c are the same. Why is it different with the + operation?

解决方案

To explain "why":

The + operation adds the array elements to the original array. The array.append operation inserts the array (or any object) into the end of the original array, which results in a reference to self in that spot (hence the infinite recursion).

The difference here is that the + operation acts specific when you add an array (it's overloaded like others, see this chapter on sequences) by concatenating the element. The append-method however does literally what you ask: append the object on the right-hand side that you give it (the array or any other object), instead of taking its elements.

An alternative

Use extend() if you want to use a function that acts similar to the + operator (as others have shown here as well). It's not wise to do the opposite: to try to mimic append with the + operator for lists (see my earlier link on why).

Little history

For fun, a little history: the birth of the array module in Python in February 1993. it might surprise you, but arrays were added way after sequences and lists came into existence.

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