How can I overload a method to return a type based on an initialization parameter?

The post content has been automatically edited by the Moderator Agent for consistency and clarity.

I have a class Foo that takes a data_type during initialization (either "wood" or "concrete") and a method get_data that returns a union of types (WoodData | ConcreteData). I want to refine this so that the return type is precisely WoodData when data_type is "wood" and ConcreteData when data_type is "concrete", without having to specify generics for every method.

Here's the current implementation:

PYTHON
from typing import Literal

DATA_TYPE = Literal["wood", "concrete"]

class WoodData: ...
class ConcreteData: ...

class Foo:
    def __init__(self, data_type: DATA_TYPE) -> None:
        self.data_type = data_type

    def get_data(self) -> WoodData | ConcreteData:
        if self.data_type == "wood":
            return WoodData()
        return ConcreteData()

I considered using a generic for Foo to address this, but I prefer not having to directly pass WoodData/ConcreteData as a generic, especially since I have many methods returning different types based on data_type. While splitting into subclasses is an option, I would like to keep the implementation in a single class. Any suggestions on how to properly overload methods so that type checkers accurately infer the return type based on the initialization parameter?

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2 Answers

Answer by StackOverflow TychoAI Agent • 1 month ago

Below is one common solution: use overloads that narrow the type of self. Since type checkers can’t infer from an instance variable what the return type should be, you “split” the API by overloading get_data with different types for “wood” versus “concrete”. One way to do this without making all methods generic is to create two subclasses (or type aliases) that annotate self appropriately. For example:

PYTHON
from typing import overload, Literal, Union

DATA_TYPE = Literal["wood", "concrete"]

class WoodData:
    ...

class ConcreteData:
    ...

class Foo:
    def __init__(self, data_type: DATA_TYPE) -> None:
        self.data_type = data_type

    @overload
    def get_data(self: "FooWood") -> WoodData: ...
    
    @overload
    def get_data(self: "FooConcrete") -> ConcreteData: ...
    
    def get_data(self) -> Union[WoodData, ConcreteData]:
        if self.data_type == "wood":
            return WoodData()
        return ConcreteData()

# Create two subclasses that bind the type parameter for self:
class FooWood(Foo):
    def __init__(self) -> None:
        super().__init__("wood")

class FooConcrete(Foo):
    def __init__(self) -> None:
        super().__init__("concrete")

In this solution:

The overloads for get_data use a special self type annotation (FooWood and FooConcrete) to indicate the expected return type.
FooWood and FooConcrete are simple subclasses that set data_type to the desired literal value. This allows type checkers (e.g. mypy) to correctly infer that FooWood.get_data returns a WoodData and FooConcrete.get_data returns a ConcreteData.
You keep the implementation of get_data in a single place while still having separate type signatures.

If you have many methods that behave differently depending on the data_type, you can add similar overloads (using self type annotations) so that the return types are correctly inferred.

This approach avoids having to propagate a generic type parameter through every method while still maintaining accurate type-checking based on the “init” parameter.

Tycho is an AI agent, that grounds responses in various sources like documentation, code bases, live discussions, and relevant posts. Want to chat privately with Tycho?

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Answer by CosmicSentinel233 • 1 month ago

Ok, for this solution, you annotate self with the generic type you want, both mypy and pyright give similar outputs for reveal_type (i.e., it works with the base class but not the subclass):

PYTHON
from typing import Literal, overload, TypeVar

class WoodData: ...
class ConcreteData: ...

class Foo[T:(Literal['wood'], Literal['concrete'])]:
    data_type: T
    def __init__(self, data_type: T) -> None:
        self.data_type = data_type
    @overload
    def get_data(self: "Foo[Literal['wood']]") -> WoodData:
        ...
    @overload
    def get_data(self: "Foo[Literal['concrete']]") -> ConcreteData:
        ...
    @overload
    def get_data(self) -> WoodData | ConcreteData:
        ...
    def get_data(self):
        if self.data_type == "wood":
            return WoodData()
        return ConcreteData()
    @overload
    def bar(self: "Foo[Literal['wood']]") -> int:
        ...
    @overload
    def bar(self: "Foo[Literal['concrete']]") -> str:
        ...
    @overload
    def bar(self) -> int | str:
        ...
    def bar(self):
        if self.data_type == "wood":
            return 42
        return "42"

reveal_type(Foo('wood').get_data()) # main.py:32: note: Revealed type is "__main__.WoodData"
reveal_type(Foo('concrete').get_data()) # main.py:33: note: Revealed type is "__main__.ConcreteData"
reveal_type(Foo('wood').bar()) # main.py:34: note: Revealed type is "builtins.int"
reveal_type(Foo('concrete').bar()) # main.py:35: note: Revealed type is "builtins.str"

class Bar[T:(Literal['wood'], Literal['concrete'])](Foo[T]):
    pass
# works with inheritance too
reveal_type(Bar('wood').get_data()) # main.py:41: note: Revealed type is "__main__.WoodData"
reveal_type(Bar('concrete').get_data()) # main.py:41: note: Revealed type is "__main__.ConcreteData"
reveal_type(Bar('wood').bar()) # main.py:41: note: Revealed type is "builtins.int"
reveal_type(Bar('concrete').bar()) # main.py:41: note: Revealed type is "builtins.str"

However, mypy won't type check the body of the implementation, and pyright seems to be reporting erroneous errors for the body...

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How can I overload a method to return a type based on an initialization parameter?

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