Django: timezone.now vs timezone.now()

Question

I am in the process of upgrading my project from Django 1.8.2 to 1.9.7 and I'm getting this warning:

WARNINGS: my_app.my_model.date_available: (fields.W161) Fixed default value provided. HINT: It seems you set a fixed date / time / datetime value as default for this field. This may not be what you want.  If you want to have the current date as default, use `django.utils.timezone.now 

Here's the line from my_app/models.py:

from django.utils import timezone ... class my_model(models.Model):     ...     datetime_released = models.DateTimeField(default=timezone.now() ) 

If I remove the parentheses and instead use:

datetime_released = models.DateTimeField(default=timezone.now ) 

The Django warning goes away. What's the difference between the two?

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In another area of my project I am using timezone.now() in a queryset filter:

def date_available(self):         return self.filter(date_available__lte = timezone.now()) 

Here, if I remove the parentheses, an error is thrown:

TypeError: expected string or buffer

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I can get both of these two work by adding/removing the parenthesis as required, but what is the difference between timezone.now() and timezone.now and why do they cause warnings/errors in these cases?

Answer 1

In python everything is an object, including functions. This means you can affect a function to a variable:

>>> from django.utils import timezone >>> foo = timezone.now >>> foo <function django.utils.timezone.now> >>> foo() datetime.datetime(2016, 7, 7, 9, 11, 6, 489063) 

A function is a callable object:

>>> callable(foo) True >>> callable(foo()) False 

When default receives a callable, the callable is called each time a default value is requested.

On the other hand, when you call timezone.now() prior to setting default, the value is given and fixed. As a reminder, the following line is executed only once at server start up, since it is a class attribute:

    datetime_released = models.DateTimeField(default=timezone.now()) 

and thus timezone.now() is executed only once. Passing a callable timezone.now makes it possible to recalculate the value whenever it needs to be.

Answer 2

The difference is that timezone.now is a callable that gets executed at runtime, while timezone.now() returns the output of that function.

For the models.DateTimeField, you need to use the callable. Better still, just set auto_now_add which does this for you:

datetime_released = models.DateTimeField(auto_now_add=True) 

The filter on the other hand does not accept a callable - it requires a value. Hence you must evaluate timezone.now() when passing this as an argument to the filter.