Arg/Kwarg rename passes liskov substitution rule

[JasonGilholme]

• Are you reporting a bug, or opening a feature request?

Reporting a bug

from typing import *


class Base(object):

    def fn(self, arg, kwarg=None):
        # type: (int, Optional[str]) -> None
        return


# Adding a kwarg passes - I think this correct
class Extension(Base):

    def fn(self, arg, kwarg=None, anotherKwarg=None):
        # type: (int, Optional[str], Optional[str]) -> None
        return


# Removing a kwarg errors - I think this correct
class RemovedKwargsExtension(Base):

    def fn(self, arg):
        # type: (int) -> None
        return


# Renaming args or kwargs passes - I expected this to error, at least for the kwarg rename.
class RenamedArgsExtension(Base):

    def fn(self, renamedArg, renamedKwarg=None):
        # type: (int, Optional[str]) -> None
        return

• What is the actual behavior/output?
  RenamedArgsExtension passes type checking

• What is the behavior/output you expect?
  I think RenamedArgsExtension should throw an error as the kwarg name has changed which will break the runtime. To a lesser degree the rename of the arg could also be considered an error, but that depends on if you're allowing the implicit kwarg -> positional arg usage.

• What are the versions of mypy and Python you are using?
  mypy 0.761 running on python 3.6.4

• What are the mypy flags you are using? (For example --strict-optional)
  mypy --strict /path/to/file.py

[JelleZijlstra]

This is a duplicate of #1013.