[Python] mypy: argument of method incompatible with supertype

Look at the example code (mypy_test.py):

import typing

class Base:
def fun(self, a: str):
pass

SomeType = typing.NewType('SomeType', str)

class Derived(Base):
def fun(self, a: SomeType):
pass


now mypy complains:

mypy mypy_test.py
mypy_test.py:10: error: Argument 1 of "fun" incompatible with supertype "Base"


In that case, how can I work with class hierarchy and be type safe?

Software versions:

mypy 0.650 Python 3.7.1

What I tried:

import typing

class Base:
def fun(self, a: typing.Type[str]):
pass

SomeType = typing.NewType('SomeType', str)

class Derived(Base):
def fun(self, a: SomeType):
pass


But it didn't help.

One user commented: "Looks like you cannot narrow down accepted types in overridden method?"

But in that case, if I use the broadest type possible (typing.Any) in a base class signature it shoudn't work either. But it does:

import typing

class Base:
def fun(self, a: typing.Any):
pass

SomeType = typing.NewType('SomeType', str)

class Derived(Base):
def fun(self, a: SomeType):
pass


No complains from mypy with the code directly above.

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