NameError: 名称‘ List’未定义

我真的不知道为什么这个不起作用。这是代码的重要部分(它来自一个 leetcode 挑战)。第一行将引发 NameError。

def totalFruit(self, tree: List[int]) -> int:
pass

如果我尝试先导入 List，我会得到一个错误 No module named 'List'。

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最佳答案

To be able to annotate what types your list should accept, you need to use typing.List

from typing import List

So did you import List?

Update

If you're using Python > 3.9, see @Adam.Er8's answer

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To be able to specify a list of str's in a type hint, you can use the typing package, and from typing import List (capitalized, not to be confused with the built-in list)

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Since Python 3.9, you can use built-in collection types (such as list) as generic types, instead of importing the corresponding capitalized types from typing.
This is thanks to PEP 585

So in Python 3.9 or newer, you could actually write:

def totalFruit(self, tree: list[int]) -> int: # Note list instead of List
pass

without having to import anything.

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If we define a list such as a = [1,2,3], then type(a) will return <class 'list'>, which means it will be created by built-in list.

The List is useful for annotating return types. For example, a function signature using Python3: def threeSumClosest(self, nums: List[int], target: int) -> int: from https://leetcode.com/problems/integer-to-roman/.