Python unittest.mock fails when using pydantic.BaseModel

Question

I have come across this problem when making my unit tests.

from unittest.mock import AsyncMock, patch
import pydantic
# class Parent(): # Using non-pydantic class works
class Parent(pydantic.BaseModel): # Using pydantic.BaseModel makes this test fail.
 async def method(self):
 raise NotImplementedError
class Child(Parent):
 async def method(self):
 # Overriding the parent method
 return False
async def test_patching():
 child_instance = Child()
 with patch.object(child_instance, 'method', new_callable=AsyncMock) as mock_method:
 mock_method.return_value = True
 
 assert await child_instance.method() is True

This results in the error:

AttributeError: 'Child' object has no attribute 'method'

If I do not use the pydantic.BaseModel, I do not get this error.

What can I do to fix this?

• Automatico: Might look like parent/child is irrelevant.
• Automatico: Here is some relevant dialog around this, but the solution of "not using basemodel" here is in sufficient in my case. github.com/pydantic/pydantic/discussions/4537
• Automatico: The problem is equivalent of this: stackoverflow.com/questions/75804182/…

Answer 1

It seems that pydantic somehow changes the internals of the object.

The method is located under the __dict__ -object inside of the object. Rewriting the test to the below solves the issue.

The parent child relationship is not important in this case.

from unittest.mock import AsyncMock, patch
import pydantic
class Parent(pydantic.BaseModel):
 async def method(self):
 raise NotImplementedError
class Child(Parent):
 async def method(self):
 # Overriding the parent method
 return False
async def test_patching():
 child_instance = Child()
 with patch.dict(
 child_instance.__dict__,
 {"method": AsyncMock(),}
 ) as mocked_dict:
 mocked_dict["method"].return_value = True
 
 assert await child_instance.method() is True