3.11. Typing Alias

Since Python 3.12 you can use type soft-keyword to define type aliases
Since Python 3.12 PEP 695 - Type Parameter Syntax

3.11.1. Problem

>>> class List:
...     data: list
...
...     def __init__(self):
...         self.data = []
...
...     def append(self, item):
...         self.data.append(item)
...
...     def remove(self, item):
...         self.data.remove(item)

>>> class List:
...     data: list[int]
...
...     def __init__(self):
...         self.data = []
...
...     def append(self, item: int):
...         self.data.append(item)
...
...     def remove(self, item: int):
...         self.data.remove(item)

3.11.2. Type Generic

class List[T] - type generic

>>> class List[T]:
...     data: list[T]
...
...     def __init__(self):
...         self.data = []
...
...     def append(self, item: T):
...         self.data.append(item)
...
...     def remove(self, item: T):
...         self.data.remove(item)
>>>
>>>
>>> mylist: List = List()
>>> mylist.append(1)  # ok
>>> mylist.append(2.0)  # ok
>>> mylist.append('three')  # ok
>>>
>>> mylist: List[int] = List()
>>> mylist.append(1)  # ok
>>> mylist.append(2.0)  # error
>>> mylist.append('three')  # error
>>>
>>> mylist: List[int|float] = List()
>>> mylist.append(1)  # ok
>>> mylist.append(2.0)  # ok
>>> mylist.append('three')  # error
>>>
>>> mylist: List[int|float|str] = List()
>>> mylist.append(1)  # ok
>>> mylist.append(2.0)  # ok
>>> mylist.append('three')  # ok

3.11.3. Type Generic with Constraints

class List[T: (int,float)] - type generic with constraints

>>> class List[T: (int,float)]:
...     data: list[T]
...
...     def __init__(self):
...         self.data = []
...
...     def append(self, item: T):
...         self.data.append(item)
...
...     def remove(self, item: T):
...         self.data.remove(item)
>>>
>>>
>>> mylist: List = List()  # ok
>>> mylist: List[int] = List()  # ok
>>> mylist: List[int|float] = List()  # ok
>>> mylist: List[int|float|str] = List()  # error