Why can't List contain multiple types?


You can mix types inside tuples or lists. Why can’t you specify that in typing hints?

>>> from typing import Tuple, List
>>> t = ('a', 1)
>>> l = ['a', 1]

>>> t2: Tuple[str, int] = ('a', 1)
>>> l2: List[str, int] = ['a', 1]

TypeError: Too many parameters for typing.List; actual 2, expected 1
Asked By: Chris



You could use a Union, but generally, if you can avoid it, lists should be homogenous instead of heterogeneous:

from typing import List, Union
lst: List[Union[str, int]] = [1, 'a']

myp, at least, will accept this just fine.

This means though that your list accessors will return a Union type, often necessitating handling different possible types in any downstream functions. Accepting unions is generally less problematic.

Answered By: juanpa.arrivillaga

In type theory, a list is a homogenous structure containing values of one type. As such, List only takes a single type, and every element of that list has to have that type.

However, type theory also provides sum types, which you can think of as a wrapper around exactly one value selected from some fixed set of types. A sum type is supported by typing.Union. To specify that a list is a mix of int and str values, use

List[Union[str, int]]

as the type hint.

By contrast, a tuple is an example of a product type, a type consisting of a fixed set of types, and whose values are a collection of values, one from each type in the product type. Tuple[int,int,int], Tuple[str,int] and Tuple[int,str] are all distinct types, distinguished both by the number of types in the product and the order in which they appear.

Answered By: chepner

The answer of using a List and a Union works in some situations, but is prone to causing bugs and is not best pracitce. There is a reason why List doesn’t allow multiple types. Let me explain.

Incorrect way

The incorrect way of doing this (like some other answers have written) is this way.

from typing import Union, List
List[Union[str, int]]

There are two issues.

  1. You cannot repeat types multiple times
  2. Type hinting can become inaccurate when the list is updated or changed.

While it might solve it short term, it is not the correct way to use type hinting in python.

Correct way 1: Homogenous lists

This is where you use a list. A list is mutable, so if you add typing, and then change the list, then the typing isn’t accurate anymore. You should therefore never use List for different types.

from typing import List
my_list = List[str]

Correct way 2: Non-homogenous tuples

If you actually need to type hint a list, then please use a Tuple. A tuple is immutable and you can therefore be sure that the tuple has the correct types within.

from typing import Tuple
my_list = Tuple[str, int, int, str, int]
Answered By: Hultan
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