demo.md

$ pyright mycode_test.py
pyright 1.1.261
./mycode_test.py
  ./mycode_test.py:42:16 - error: Cannot assign member "slow_method" for type "Gizmo"
    Expression of type "() -> Literal[3]" cannot be assigned to member "slow_method" of class "Gizmo"
      Type "() -> Literal[3]" cannot be assigned to type "() -> Frob"
        Function return type "Literal[3]" is incompatible with type "Frob"
          "Literal[3]" is incompatible with "Frob" (reportGeneralTypeIssues)
1 error, 0 warnings, 0 informations 
Completed in 0.564sec
exit code: 1

mycode.py

from __future__ import annotations


def thing_to_test(gizmo: Gizmo):
  return gizmo.slow_method()


class Frob:
  pass


class Gizmo:
  def slow_method(self):
    return Frob()

mycode_test.py

import typing
from unittest.mock import Mock

import mycode


def fake_slow_method():
  return mycode.Frob()


def fake_slow_method_wrong():
  return 3


def test_a_thing():
  mocked_gizmo = typing.cast(mycode.Gizmo, Mock())
  mocked_gizmo.slow_method = fake_slow_method
  assert mycode.thing_to_test(mocked_gizmo) == 3

def test_a_thing_wrong():
  mocked_gizmo = typing.cast(mycode.Gizmo, Mock())
  mocked_gizmo.slow_method = fake_slow_method_wrong
  assert mycode.thing_to_test(mocked_gizmo) == 3

pyproject.toml

[tool.pyright]
include = ["**"]
typeCheckingMode = "strict"