[Python] How to test the concrete method of an abstract class without manual subclassing?

I have an abstract Python class with a concrete method:

class AbstractFoo(abc.ABC):
def append_something(self, text: str) -> str:
return text + self.create_something(len(text))

@abc.abstractmethod
def create_something(self, number: int) -> str:
raise NotImplementedError()

# IMPORTANT: many other abstract methods irrelevant for the test omitted here


Given that there are many abstract methods, I'm looking for a way to test the concrete method without having to manually subclass and implement all these methods. I was hoping to use a mock instance of the class somehow.

• 
  First failed attempt:
  
  # Without this, `AbstractFoo` cannot be instantiated.
  AbstractFoo.__abstractmethods__ = frozenset()
  
  mock_foo = unittest.mock.Mock(wraps=AbstractFoo())
  mock_foo.create_something.return_value = "bar"
  
  assert mock_foo.append_something("foo") == "foobar"
  mock_foo.create_something.assert_called_once_with(3)
  
  
  Even though I try to mock the create_something method return value, the NotImplementedError from AbstractFoo.create_something is raised when mock_foo.append_something("foo") is called.
  
  
• 
  Second failed attempt
  
  class MockFoo(AbstractFoo):
  def create_something(self, number: int) -> str:
  return "bar"
  
  MockFoo.__abstractmethods__ = frozenset()
  
  mock_foo = unittest.mock.Mock(wraps=MockFoo())
  mock_foo.create_something.side_effect = lambda *args, **kwargs: unittest.mock.DEFAULT
  
  assert mock_foo.append_something("foo") == "foobar"
  mock_foo.create_something.assert_called_once_with(3)
  
  
  This time I get:
  
  AssertionError: Expected 'create_something' to be called once. Called 0 times.

Questions:

1. 
   In both of my failed attempts, the mock_foo.create_something mock does not seem to be used. Why's that?
   
   
2. 
   How can I test the append_something method of AbstractFoo without having to manually implement all the abstract methods of AbstractFoo in a dummy subclass?
   

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