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I want to create a pytest with a fake utcnow, but also I need to preserve the functionality of all other datetime methods. Simple example here:

import datetime as dt


class FakeTime(dt.datetime):
 fake_time = None

 @classmethod
 def utcnow(cls):
 return cls.fake_time


def str_2_time(str_dt: str) -> dt.datetime:
 """Shortcut to do convert the string to datetime"""
 return dt.datetime.strptime(str_dt, '%Y-%m-%d %H:%M')


def test_patch_datetime():
 for utc_time in ['2019-01-01 10:00', '2019-02-01 13:00', '2019-03-01 16:00']:
 FakeTime.fake_time = str_2_time(utc_time)
 dt.datetime = FakeTime
 assert dt.datetime.utcnow() == str_2_time(utc_time)

Is this the right way?

The method str_2_time just need to show that all other methods of the datetime works fine.

According to this, subclassing datetime.datetime seems the way to go.

There is no use for the str_2_time method though. You can easily inline this, or even simpler, just use the datetime.datetime constructor:

def test_patch_datetime():
 for utc_time in [
 dt.datetime(2019, 1, 1, 10),
 dt.datetime(2019, 2, 1, 13),
 dt.datetime(2019, 3, 1, 16),
 ]:
 FakeTime.fake_time = utc_time
 dt.datetime = FakeTime
 assert dt.datetime.utcnow() == utc_time

You should be aware that this can have side effects in other parts of your code, so it might be needed to replace it back with the original class after the test method:

def test_patch_datetime():
 datetime_orig = dt.datetime

 utc_times = [
 dt.datetime(2019, 1, 1, 10),
 dt.datetime(2019, 2, 1, 13),
 dt.datetime(2019, 3, 1, 16),
 ]
 for utc_time in utc_times:
 FakeTime.fake_time = utc_time
 dt.datetime = FakeTime
 assert dt.datetime.utcnow() == utc_time
 dt.datetime = datetime_orig
 # print(dt.datetime.utcnow())
 assert dt.datetime.utcnow() > max(utc_times)

Usually, I do:

1. Separate module, for example utils.py, that contains:

from datetime import datetime

def get_utcnow() -> datetime:
 return datetime.utcnow()

2. Use this function everywhere in my code.


3. Add the mocking fixture in tests/conftest.py:

from datetime import datetime, timedelta

import pytest

from .. import utils

@pytest.fixture
def mock_utcnow(monkeypatch):
 now = datetime.min

 def wrapped(delta=0.0):
 when = now + timedelta(delta)
 monkeypatch.setattr(utils, "get_utcnow", lambda: when)
 return when

 return wrapped

4. Now it's easy to use it in your tests:

def test(mock_utcnow):
 now = mock_utcnow()
 new_now = mock_utcnow(0.1)

Additionally, with this fixture you can set the returning value with desired offset.

Hope it helps.