Adding Model String Representations

When we printed out the list of publishers, all we got was this unhelpful display that makes it difficult to tell the Publisher objects apart:

<QuerySet [<Publisher: Publisher object>, <Publisher: Publisher object>]>

We can fix this easily by adding a method called __str__() to our Publisher object. A __str__() method tells Python how to display the “string” representation of an object. You can see this in action by adding a __str__() method to the three models:

from django.db import models

class Publisher(models.Model):
name = models.CharField(max_length=30)
address = models.CharField(max_length=50)
city = models.CharField(max_length=60)
state_province = models.CharField(max_length=30)
country = models.CharField(max_length=50)
website = models.URLField()

def __str__(self):
return self.name

class Author(models.Model):
first_name = models.CharField(max_length=30)
last_name = models.CharField(max_length=40)
email = models.EmailField()

def __str__(self):
return u'%s %s' % (self.first_name, self.last_name)

class Book(models.Model):
title = models.CharField(max_length=100)
authors = models.ManyToManyField(Author)
publisher = models.ForeignKey(Publisher)
publication_date = models.DateField()

def __str__(self):
return self.title

As you can see, a __str__() method can do whatever it needs to do in order to return a representation of an object. Here, the __str__() methods for Publisher and Book simply return the object’s name and title, respectively, but the __str__() for Author is slightly more complex – it pieces together the first_name and last_name fields, separated by a space. The only requirement for __str__() is that it return a string object. If __str__() doesn’t return a string object – if it returns, say, an integer – then Python will raise a TypeError with a message like:

 TypeError: __str__ returned non-string (type int).

 For the __str__() changes to take effect, exit out of the Python shell and enter it again with python manage.py shell. (This is the simplest way to make code changes take effect.) Now the list of Publisher objects is much easier to understand:

   >>> from books.models import Publisher

>>> publisher_list = Publisher.objects.all()

>>> publisher_list

<QuerySet [<Publisher: Apress>, <Publisher: O'Reilly>]>

 Make sure any model you define has a __str__() method – not only for your own convenience when using the interactive interpreter, but also because Django uses the output of __str__() in several places when it needs to display objects. Finally, note that __str__() is a good example of adding behavior to models. A Django model describes more than the database table layout for an object; it also describes any functionality that object knows how to do. __str__() is one example of such functionality – a model knows how to display itself.

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