That time_machine starts ticking after the first call

I was surprised to learn today that this test passes:

import time_machine
import datetime

def test_ticking_behaviour():
    with time_machine.travel("2020-06-10T14:00:00", tick=True):
        # Let some time pass.
        time.sleep(1)
        # Time should now be around 14:00:01, right? Wrong!
        assert datetime.datetime.now() == datetime.datetime(2020, 6, 10, 14, 0, 0)

This is because time_machine.travel starts ticking after the first call to the system clock, not from when the context manager is entered. This is documented but is quite counter intuitive.

Antipattern: using time_machine to set auto_now_add=True Django field values

I stumbled upon this when debugging a failing test. The test was using time_machine to specify a value for a Django model field declared with auto_now_add=True. Something like this:

import time_machine
import datetime

# Import some factory-boy factory classes.
from . import factories

def test_frob_creation():
    with time_machine.travel("2020-06-10T14:00:00"):
        # Frob's have a `created_at` field declared with `auto_now_add=True`.
        # Since this is the first system clock call in this `time_machine`
        # block, the `created_at` field will be set to *exactly* the above time.
        frob = factories.FrobFactory()

    assert frob.created_at == datetime.datetime(2020, 6, 10, 14, 0, 0)

This test was passing until I wrapped one of the FrobFactory fields with factory.LazyFunction to ensure the system clock was only called during test execution time (not test collection). This caused the test to fail as (the equivalent of) frob.created_at now had a microsecond component.

This was because the first system clock call was now in computing a factory field value, and hence some time had ticked by before the model was saved (and the created_at value computed).

I wouldn’t recommend using time_machine to set auto_now_add=True fields in this way — it’s better to pass them inexplicitly. But, if you do, ensure you pass tick=False to avoid this problem.