Python 3.6: Signature of {method} incompatible with super type {Class}

Question

While trying to update my code to be PEP-484 compliant (I'm using mypy 0.610) I've ran into the following report:

$ mypy mymodule --strict-optional --ignore-missing-imports --disallow-untyped-calls --python-version 3.6

myfile.py:154: error: Signature of "deliver" incompatible with supertype "MyClass"

MyClass:

from abc import abstractmethod

from typing import Any


class MyClass(object):

@abstractmethod
def deliver(self, *args: Any, **kwargs: Any) -> bool:
    raise NotImplementedError

myfile.py:

class MyImplementation(MyClass):

[...]

    def deliver(self, source_path: str,
                dest_branches: list,
                commit_msg: str = None,
                exclude_files: list = None) -> bool:

        [...]

        return True

I'm definitely doing something wrong here, but I can't quite understand what :)

Any pointers would be much appreciated.

Answer 1

@abstractmethod
def deliver(self, *args: Any, **kwargs: Any) -> bool:
    raise NotImplementedError

This declaration doesn't mean subclasses can give deliver any signature they want. Subclass deliver methods must be ready to accept any arguments the superclass deliver method would accept, so your subclass deliver has to be ready to accept arbitrary positional or keyword arguments:

# omitting annotations
def deliver(self, *args, **kwargs):
    ...

Your subclass's deliver does not have that signature.

---

If all subclasses are supposed to have the same deliver signature you've written for MyImplementation, then you should give MyClass.deliver the same signature too. If your subclasses are going to have different deliver signatures, maybe this method shouldn't really be in the superclass, or maybe you need to rethink your class hierarchy, or give them the same signature.