Mocking chained methods of datetime objects in Python

This is the 4th time in a row that I’ve wasted time figuring out how to mock out a function during testing that calls the chained methods of a datetime.datetime object in the function body. So I thought I’d document it here. Consider this function:

# src.py
from __future__ import annotations

import datetime


def get_utcnow_isoformat() -> str:
    """Get UTCnow as an isoformat compliant string."""
    return datetime.datetime.utcnow().isoformat()

How’d you test it? Mocking out datetime.datetime is tricky because of its immutable nature. Third-party libraries like freezegun¹ make it easier to mock and test functions like the one above. However, it’s not too difficult to cover this simple case without any additional dependencies. Here’s one way to achieve the goal:

# src.py
from __future__ import annotations

import datetime
from unittest.mock import patch
import pytest


def get_utcnow_isoformat() -> str:
    """Get UTCnow as an isoformat compliant string."""
    return datetime.datetime.utcnow().isoformat()


@pytest.fixture
def mock_datetime():
    with patch("datetime.datetime") as m:
        # This is where the magic happens!
        m.utcnow.return_value.isoformat.return_value = (
            "2022-03-15T23:11:12.432048"
        )
        yield m


def test_get_utcnow_isoformat(mock_datetime):
    frozen_date = "2022-03-15T23:11:12.432048"
    assert get_utcnow_isoformat() == frozen_date

Here, the mock_datetime fixture function makes the output of the chained calls on the datetime object deterministic. Then I used it in the test_get_utcnow_isoformat function to get a frozen output every time the function get_utcnow_isoformat gets called. If you run the above snippet with Python, it’ll pass.

======test session starts ======
platform linux -- Python 3.10.2, pytest-7.0.1, pluggy-1.0.0
rootdir: /home/rednafi/canvas/personal/reflections
plugins: anyio-3.5.0
collected 1 item

src.py .                                              [100%]

====== 1 passed in 0.01s ======

~~~

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