python *运算符，使用默认值创建列表 [英] python * operator, creating list with default value

问题描述

以下工作如何?

>>> 3*[2]
>>> [2,2,2]
>>> [2]*3
>>> [2,2,2]

我知道*是位置扩展运算符.由于[2]是一个包含单个项目的列表，所以我看不到3*[2]如何扩展到任何有意义的内容，我希望使用SyntaxError，但事实并非如此.

I understand that * is the positional expansion operator. Since [2] is a list with a single item, I don't see how 3*[2] expands to anything meaningful, I'd expect a SyntaxError, but that's not the case.

我很难找到一个现有的答案，我发现所有引用*args和**kwargs来传递可变参数列表，但这并不能完全回答我的问题.

I'm having a hard time searching for an existing answer, all I find are references to *args and **kwargs for passing variadic parameter lists, which don't quite answer my question.

推荐答案

*是 序列类型文档:

* is the multiplication operator. All Python sequences support multiplication. See the Sequence Types documentation:

s * n，n * s
s 级联的

s * n, n * s
n shallow copies of s concatenated

请注意，副本是 shallow ；任何嵌套的可变类型也不会递归地复制.这可能会导致令人惊讶的结果:

Note that the copies are shallow; any nested mutable types are not recursively copied too. This can lead to suprising results:

>>> nested = [[None]] * 5
>>> nested
[[None], [None], [None], [None], [None]]
>>> nested[0].append(42)
>>> nested
[[None, 42], [None, 42], [None, 42], [None, 42], [None, 42]]

只有一个嵌套的[None]列表，被引用5次，而不是5个单独的列表对象.

There is just one nested [None] list, referenced 5 times, not 5 separate list objects.

*args和**kw变量参数语法仅在函数定义或调用中具有含义(因此callable_object(<arguments>)).它根本不适用于这里.参见 **(双星)和*(星号)用于参数?有关该语法的更多详细信息.

The *args and **kw variable argument syntax only has meaning in a function definition or in a call (so callable_object(<arguments>)). It doesn't apply here at all. See What does ** (double star) and * (star) do for parameters? for more detail on that syntax.

序列类型通过 object.__mul__()重载*运算符和 object.__rmul__() 方法(表达式中的左或右操作数)，请参见 模拟容器类型 ，以获取有关钩子序列类型通常实现的文档.

Sequence types overload the * operator through the object.__mul__() and object.__rmul__() methods (when being the left or right operand in the expression), see Emulating container types for documentation on what hooks sequence types typically implement.

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