NameError: name 'List' is not defined
Question:
I’m really unsure why this isn’t working. Here is the important part of the code (it’s from a leetcode challenge).
The first line throws the NameError.
def totalFruit(self, tree: List[int]) -> int:
pass
If I try importing List first I get an error No module named 'List'. I’m using Python 3.7.3 from Anaconda.
Answers:
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
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)
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.
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/.
I’m really unsure why this isn’t working. Here is the important part of the code (it’s from a leetcode challenge).
The first line throws the NameError.
def totalFruit(self, tree: List[int]) -> int:
pass
If I try importing List first I get an error No module named 'List'. I’m using Python 3.7.3 from Anaconda.
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
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)
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.
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/.