在Python中，为什么list(None)是一个错误，但[None]不是一个错误? [英] In Python, why is list(None) an error but [None] is not?

问题描述

将None传递给Python的list构造函数是TypeError:

Passing None to Python's list constructor is a TypeError:

>>> l = list(None)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'NoneType' object is not iterable

但是使用方括号来实例化list是可以的；将None与内置函数一起使用也可以:

But using brackets to instantiate a list is fine; using None with the built-in functions is also fine:

>>> l = [None]
>>> l.append(None)
>>> l
[None, None]

我的理解是list()和[]是创建新list的等效方法.我想念什么?

My understanding was that list() and [] were equivalent ways of creating a new list. What am I missing?

推荐答案

它们与不等价. list()尝试遍历其参数，使每个对象成为一个单独的列表项，而列表文字[]恰好采用它给出的内容.比较:

They are not equivalent. list() attempts to iterate over its argument, making each object a separate list item, whereas a list literal [] just takes exactly what it's given. Compare:

>>> list("foo")
['f', 'o', 'o']
>>> ["foo"]
['foo']

您无法遍历None，因此会出现错误.使用任何不可迭代的参数，您将得到相同的结果:

You can't iterate over None, hence the error. You will get the same with any non-iterable argument:

>>> list(1)
Traceback (most recent call last):
  File "<pyshell#14>", line 1, in <module>
    list(1)
TypeError: 'int' object is not iterable

要使用list()获得a = [None]的等价物，您必须将其传递给单个对象，例如，

To get the equivalent of a = [None] using list(), you would have to pass it an iterable with a single object in, e.g.

a = list([None])

尽管这将创建列表文字，然后将其复制，所以在这个琐碎的示例中并不是很有用！

although this will create the list literal and then copy it, so is not terribly useful in this trivial example!

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