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?

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
– Automatico, Commented Sep 29 at 15:52
The problem is equivalent of this: stackoverflow.com/questions/75804182/… — Automatico
– Automatico, Commented Sep 29 at 16:00

Automatico · Accepted Answer · 2025-09-29 16:14:03Z

0

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

answered Sep 29 at 16:14

Automatico

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