# django.utils.timezone

def now():
    return datetime.utcnow().replace(tzinfo=utc)

# django.contrib.auth.models

class User:
    date_joined = models.DateTimeField(_('date joined'), default=timezone.now)

# django.db.models.fields

class DateTimeField:
    def to_python(self, value):
        if isinstance(value, datetime.datetime):
            ...

# test case 1

# TypeError: cannot set 'utcnow' attribute of immutable type 'datetime.datetime'
@patch('datetime.datetime.utcnow', MagicMock())  
def test_():
    ...

# test case 2

class DatetimeMock(datetime.datetime):
    @classmethod
    def utcnow(cls):
        ...

# E       - [{'date_joined': DatetimeMock(2022, 7, 1, 0, 0, tzinfo=<UTC>),
# E       ?                  ^       ^^^^             ^  ^
# E       
# E       + [{'date_joined': datetime.datetime(2022, 7, 1, 16, 47, 39, 294114, tzinfo=<UTC>),
# E       ?                  ^       ^^^^^^^^^             ^^  ^^ ++++++++++++
@patch('datetime.datetime.utcnow', DatetimeMock)  
def test_():
    ...

# test case 3

mock = MagicMock(wraps=datetime.datetime)
mock.utcnow.return_value = NOW

# TypeError: Mixer (authenticate.UserInvite): isinstance() arg 2 must be a type, a tuple of types, or a union
@patch('datetime.datetime', mock)
def test_():
    ...