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Incompatible types in assignment (x: SomeMessage = Message) #97

DaniGuardiola opened this issue Oct 10, 2019 · 3 comments


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@DaniGuardiola DaniGuardiola commented Oct 10, 2019

I have the following code:

from proto.example_pb2 import SomeMessage
from google.protobuf.message import Message

def request(...) -> Message:

result: SomeMessage = request(

Request will return a protobuf message which can vary depending on the input parameters. This is giving the following mypy error:

error: Incompatible types in assignment (expression has type "Message", variable has type "SomeMessage")

Now, I understand why this is happening: Message is not assignable to SomeMessage. It would be valid the other way around (SomeMessage -> Message) because SomeMessage is a subclass of Message.

The reasoning behind this code is that while request can return any message, when I use it I already know which specific message it will return, and I want to be able to type it like this. I just don't know how to go about it.

The only solution I thought about is to use @overload with some Literal types to set the return of every message type, as it really only depends on the value a string parameter, but it seems a bit overkill. I have some other situations in my code where this happens (even without protobuf) so what I'm really asking is for help on what pattern I should be using in these kind of situations, because I'm all out of ideas.

Can anyone help me figure this out?

Thanks :)


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@nipunn1313 nipunn1313 commented Oct 10, 2019

There exists a cast method in the typing module which might allow you to force a conversion like this.

Generally speaking, a single method that could return a variety of types based on the input - is not a strongly typed method - so you will have to cast. The input string is “stringly typed” encoding the type in a way that bypasses the type system.

One pattern might be to have N typed methods (either with overload like you mention - or even N methods with different names that don’t require arguments).


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@nipunn1313 nipunn1313 commented Oct 10, 2019

You may be able to ask a similar question in the mypy repo to get more eyes on it for possible alternatives.


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@ikonst ikonst commented Oct 11, 2019

If you know which message each kind of request returns, this sounds very much like RPC services. In such case, I would expect your RPC service (defined in Protobuf) to result in a generated Python typestub to the effect of:

class MyService:
   def foo(my_request: MyRequest) -> MyResult: ...
   def bar(...) -> ...: ...


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