How to create list field in django

Question

How do I create a ListField in Django (Python) like the ListProperty property in Google App Engine (Python)? My data is a list like this : 3,4,5,6,7,8.

What property do I have to define and how would I fetch values from it?

Answer 1

Revisiting this with a ListField type you can use. But it makes a few of assumptions, such as the fact that you're not storing complex types in your list. For this reason I used ast.literal_eval() to enforce that only simple, built-in types can be stored as members in a ListField:

from django.db import models import ast  class ListField(models.TextField):     __metaclass__ = models.SubfieldBase     description = "Stores a python list"      def __init__(self, *args, **kwargs):         super(ListField, self).__init__(*args, **kwargs)      def to_python(self, value):         if not value:             value = []          if isinstance(value, list):             return value          return ast.literal_eval(value)      def get_prep_value(self, value):         if value is None:             return value          return unicode(value)      def value_to_string(self, obj):         value = self._get_val_from_obj(obj)         return self.get_db_prep_value(value)  class Dummy(models.Model):     mylist = ListField() 

Taking it for a spin:

>>> from foo.models import Dummy, ListField >>> d = Dummy() >>> d.mylist [] >>> d.mylist = [3,4,5,6,7,8] >>> d.mylist [3, 4, 5, 6, 7, 8] >>> f = ListField() >>> f.get_prep_value(d.numbers) u'[3, 4, 5, 6, 7, 8]' 

There you have it that a list is stored in the database as a unicode string, and when pulled back out it is run through ast.literal_eval().

Previously I suggested this solution from this blog post about Custom Fields in Django:

An alternative to the CommaSeparatedIntegerField, it allows you to store any separated values. You can also optionally specify a token parameter.

from django.db import models  class SeparatedValuesField(models.TextField):     __metaclass__ = models.SubfieldBase      def __init__(self, *args, **kwargs):         self.token = kwargs.pop('token', ',')         super(SeparatedValuesField, self).__init__(*args, **kwargs)      def to_python(self, value):         if not value: return         if isinstance(value, list):             return value         return value.split(self.token)      def get_db_prep_value(self, value):         if not value: return         assert(isinstance(value, list) or isinstance(value, tuple))         return self.token.join([unicode(s) for s in value])      def value_to_string(self, obj):         value = self._get_val_from_obj(obj)         return self.get_db_prep_value(value) 

Answer 2

Try using a CommaSeparatedIntegerField which is documented here: http://docs.djangoproject.com/en/1.2/ref/models/fields/#commaseparatedintegerfield