32. Callable defaults

This example describes Django version 0.96. For the current example, go here.

You can pass callable objects as the default parameter to a field. When the object is created without an explicit value passed in, Django will call the method to determine the default value.

This example uses datetime.datetime.now as the default for the pub_date field.

Model source code

from django.db import models
from datetime import datetime

class Article(models.Model):
    headline = models.CharField(maxlength=100, default='Default headline')
    pub_date = models.DateTimeField(default=datetime.now)

    def __str__(self):
        return self.headline

Sample API usage

This sample code assumes the above model has been saved in a file mysite/models.py.

>>> from mysite.models import Article

>>> from datetime import datetime

# No articles are in the system yet.
>>> Article.objects.all()
[]

# Create an Article.
>>> a = Article(id=None)

# Grab the current datetime it should be very close to the default that just
# got saved as a.pub_date
>>> now = datetime.now()

# Save it into the database. You have to call save() explicitly.
>>> a.save()

# Now it has an ID. Note it's a long integer, as designated by the trailing "L".
>>> a.id
1L

# Access database columns via Python attributes.
>>> a.headline
'Default headline'

# make sure the two dates are sufficiently close
>>> d = now - a.pub_date
>>> d.seconds < 5
True