TypeVar in class init type hinting

Question

I am trying to use a TypeVar to indicate an init parameter as a certain type. But I am doing it wrong, or it might not even be possible.

from typing import TypeVar T=TypeVar("T") class TestClass: def __init__(self,value:T): self._value=value a = TestClass(value=10) b = TestClass(value="abc") reveal_type(a._value) reveal_type(b._value)

I was hoping the reveal type of a._value would have been int and b._value to have been string. But they are both revealed as 'T`-1'

Any help or insight appreciated!

[EDIT]

A little more expanded example. The BaseClass will be overridden and the actual type hint is provided by the overriding class.

from typing import TypeVar T=TypeVar("T") class BaseClass: def __init__(self,value): self._value = value class Class1(BaseClass): def __init__(self,value:str): super().__init__(value) class Class2(BaseClass): def __init__(self,value:int): super().__init__(value) a = Class1("A value") b = Class2(10) reveal_type(a._value) reveal_type(b._value)

What did you even mean when you wrote that code? What did you expect the type variable to do? That's not how you use type variables. — user2357112
– user2357112, Commented Nov 17, 2020 at 10:39
I basically want the _value property to take the type of the value I assign it to. But it doesn't. — sanders
– sanders, Commented Nov 17, 2020 at 10:43
You must define TestClass as generic, i.e. class TestClass(Generic[T]):. Otherwise, the conrcete type of T is only scoped to the method and lost afterwards. — MisterMiyagi
– MisterMiyagi, Commented Nov 17, 2020 at 10:55

MisterMiyagi · Accepted Answer · 2020-11-17 11:15:43Z

By default, using a TypeVar restricts its scope only to the method/function in which it is used as an annotation. In order to scope a TypeVar to the instance and all methods/attributes, declare the class as Generic.

from typing import TypeVar, Generic T=TypeVar("T") class BaseClass(Generic[T]): # Scope of `T` is the class: def __init__(self, value: T): # Providing some `T` on `__init__` self._value = value # defines the class' `T`

This allows declaring subclasses either as generic or as concrete.

class Class1(BaseClass[str]): # "is a" BaseClass where `T = str` pass # No need to repeat ``__init__`` class ClassT(BaseClass[T]): # "is a" BaseClass where `T = T'` @property def value(self) -> T: return self._value reveal_type(Class1("Hello World")._value) # Revealed type is 'builtins.str*' reveal_type(Class1(b"Uh Oh!")._value) # error: Argument 1 to "Class1" has incompatible type "bytes"; expected "str" reveal_type(ClassT(42).value) # Revealed type is 'builtins.int*'

Collectives™ on Stack Overflow

TypeVar in class init type hinting

1 Answer 1

Comments

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Related