What’s the best place for python classes in a Django project

It's natural for model classes to contain methods to operate on the model. If I have a Book model, with a method book.get_noun_count(), that's where it belongs--I don't want to have to write "get_noun_count(book)", unless the method actually intrinsically belongs with some other package. (It might--for example, if I have a package for accessing Amazon's API with "get_amazon_product_id(book)".)

I cringed when Django's documentation suggested putting models in a single file, and I took a few minutes from the very beginning to figure out how to split it into a proper subpackage.

site/models/__init__.py
site/models/book.py

__init__.py looks like:

from .book import Book

so I can still write "from site.models import Book".

The following is only required for versions prior to Django 1.7, see https://code.djangoproject.com/ticket/3591

The only trick is that you need to explicitly set each model's application, due to a bug in Django: it assumes that the application name is the third-to-last entry in the model path. "site.models.Book" results in "site", which is correct; "site.models.book.Book" makes it think the application name is "models". This is a pretty nasty hack on Django's part; it should probably search the list of installed applications for a prefix match.

class Book(models.Model):
    class Meta: app_label = "site"

You could probably use a base class or metaclass to generalize this, but I haven't bothered with that yet.

I have some code that fetches an image off the web and stores it in a model. The important bits are:

from django.core.files import File  # you need this somewhere
import urllib


# The following actually resides in a method of my model

result = urllib.urlretrieve(image_url) # image_url is a URL to an image

# self.photo is the ImageField
self.photo.save(
    os.path.basename(self.url),
    File(open(result[0], 'rb'))
    )

self.save()

That's a bit confusing because it's pulled out of my model and a bit out of context, but the important parts are:

• The image pulled from the web is not stored in the upload_to folder, it is instead stored as a tempfile by urllib.urlretrieve() and later discarded.
• The ImageField.save() method takes a filename (the os.path.basename bit) and a django.core.files.File object.

Let me know if you have questions or need clarification.

Edit: for the sake of clarity, here is the model (minus any required import statements):

class CachedImage(models.Model):
    url = models.CharField(max_length=255, unique=True)
    photo = models.ImageField(upload_to=photo_path, blank=True)

    def cache(self):
        """Store image locally if we have a URL"""

        if self.url and not self.photo:
            result = urllib.urlretrieve(self.url)
            self.photo.save(
                    os.path.basename(self.url),
                    File(open(result[0], 'rb'))
                    )
            self.save()