Mocking default=timezone.now for unit tests

Question

I'm trying to write unit tests for a django app that does a lot of datetime operations. I have installed mock to monkey patch django's timezone.now for my tests.

While I am able to successfully mock timezone.now when it is called normally (actually calling timezone.now() in my code, I am not able to mock it for models that are created with a DateTimeField with default=timezone.now.

---

I have a User model that contains the following:

from django.utils import timezone
...
timestamp = models.DateTimeField(default=timezone.now)
modified = models.DateTimeField(default=timezone.now)
...
def save(self, *args, **kwargs):
    if kwargs.pop('modified', True):
        self.modified = timezone.now()
    super(User, self).save(*args, **kwargs)

---

My unit test looks like this:

from django.utils import timezone

def test_created(self):
    dt = datetime(2010, 1, 1, tzinfo=timezone.utc)
    with patch.object(timezone, 'now', return_value=dt):
        user = User.objects.create(username='test')
        self.assertEquals(user.modified, dt)
        self.assertEquals(user.timestamp, dt)

---

assertEquals(user.modified, dt) passes, but assertEquals(user.timestamp, dt) does not.

How can I mock timezone.now so that even default=timezone.now in my models will create the mock time?

---

Edit

I know that I could just change my unit test to pass a timestamp of my choice (probably generated by the mocked timezone.now)... Curious if there is a way that avoids that though.

Answer 1

There is another easy way to do the above thing.

import myapp.models.timezone
from unittest.mock import patch

@patch('django.utils.timezone.now')
def test_created(self, mock_timezone):
    dt = datetime(2010, 1, 1, tzinfo=timezone.utc)
    mock_timezone.return_value = dt
    user = User.objects.create(username='test')

    self.assertEquals(user.modified, dt)
    self.assertEquals(user.timestamp, dt)

This is the best way to mock timezone.now.

Answer 2

Looks like you are patching timezone in the wrong place.

Assuming your User model lives in myapp\models.py and you want to test save() in that file. The problem is that when you from django.utils import timezone at the top, it imports it from django.utils. In your test you are patching timezone locally, and it has no effect on your test, since module myapp\models.py already has a reference to the real timezone and it looks like our patching had no effect.

Try patching timezone from myapp\models.py, something like:

import myapp.models.timezone

def test_created(self):
    with patch('myapp.models.timezone') as mock_timezone:
        dt = datetime(2010, 1, 1, tzinfo=timezone.utc)
        mock_timezone.now.return_value = dt

        assert myapp.models.timezone.now() == dt

        user = User.objects.create(username='test')
        self.assertEquals(user.modified, dt)
        self.assertEquals(user.timestamp, dt)

Answer 3

I just ran into this issue myself. The problem is that models are loaded before mock has patched the timezone module, so at the time the expression default=timezone.now is evaluated, it sets the default kwarg to the real timezone.now function.

The solution is the following:

class MyModel(models.Model):
    timestamp = models.DateTimeField(default=lambda: timezone.now())

Answer 4

Here's a method you can use that doesn't require altering your non-test code. Just patch the default attributes of the fields you want to affect. For example--

field = User._meta.get_field('timestamp')
mock_now = lambda: datetime(2010, 1, 1)
with patch.object(field, 'default', new=mock_now):
    # Your code here

You can write helper functions to make this less verbose. For example, the following code--

@contextmanager
def patch_field(cls, field_name, dt):
    field = cls._meta.get_field(field_name)
    mock_now = lambda: dt
    with patch.object(field, 'default', new=mock_now):
        yield

would let you write--

with patch_field(User, 'timestamp', dt):
    # Your code here

Similarly, you can write helper context managers to patch multiple fields at once.