TypeVarsAttempt-#1.py

TypeVarsAttempt-#1.py

from dataclasses import dataclass
from typing import TypeVar

T = TypeVar('T', bound=B) # "B" is not defined

class A:
    def __init__(self, a1): ...

@dataclass
class BBuilder:
    foo: int

    def build(self) -> T: # TypeVar "T" appears only once in generic function signature  Use "Unknown" instead
        a1_i_dont_want_to_be_changeable = 123


        class B(A):
            def __init__(self, a2):
                super().__init__(a1=a1_i_dont_want_to_be_changeable)

        # also tried writing `T = TypeVar('T', bound=B)` here in attempt 3

        return B # Expression of type "type[B]" cannot be assigned to return type "T@build" Type "type[B]" cannot be assigned to type "T@build"

my_b_builder: BBuilder = BBuilder(foo=456)
BBuilt: T = my_b_builder.build() # Type variable "T" has no meaning in this context
my_b = BBuilt(a2='foo') # my_b: WhatTypeHintDoIPutHere