I am not able to patch datetime.utcnow correctly and I am wondering what could be wrong. The code below illustrates the idea:

# In a file called 'my_module.py'
from datetime import datetime
from pydantic import BaseModel, Field

class Pet(BaseModel):
    ts: datetime = Field(default_factory=datetime.utcnow)

And the corresponding test code is:

from datetime import datetime
from unittest.mock import patch
from my_module import Pet

@patch("my_module.datetime")
def test_pet(mocked):
    timestamp = datetime(2000, 10, 10, 21, 50)
    mocked.utcnow.return_value = timestamp
    pet = Pet()
    assert pet.ts == timestamp
    # pet.ts is getting current date and time :/

When I run pytest, the pet.ts does not get assigned the patched timestamp. Instead it is getting the current date and time. I am wondering what could be wrong/missing in my setup. Would anyone have an idea?

I am using the following tools:

Python   3.9.8
pydantic 1.9.0
pytest   6.25