The code shown allows you, in GeoDjango, to reduce the number of points in your polygons. It helps reduce storage needs and makes queries run faster, at the cost of some precision. It provides a variation on the simplify() method that comes with the GEOS API, allowing you to specify a number of points instead of a distance tolerance.
It is set up as a management command so that you can run it with python manage.py. See the django docs for how to set that up.
The example shown assumes a table called CountyBorders with fields "name" and "mpoly." It should be straightforward to adapt it for your needs. Look at the first three lines of the simplify() method in particular for customization. The rest of the code is pretty generic, and it should run fast enough in a one-time batch process for most needs.
The algorithm tries to keep the 75 points that provide the most definition for the shape. Each point in a polygon defines a triangle with its immediate neighbors. If that triangle has no area (the degenerate case), it is a midpoint on the segment between its neighbors and adds no value whatsoever. This principle is extended to say that the larger the triangle, the more value the point has in defining the shape. (You can find more refined algorithms, but this code seems to work fine by visual inspection.)
For most applications, simplejson.dumps() is enough. But I’m especially fond of iterators, generators, functors (objects with a `__call__()` method) and closures, dense components that express one thought well: the structure of a tree, or the rows of a database, to be sent to the browser. The routine dumps() doesn’t understand any of those things, but with a simple addition, you can plug them into your code and be on your way without headache. Dumps() just calls JSONEncoder(), and JSONEncoder has a routine for extending its functionality.
The routine is to override a method named default() (why “default?” I have no idea) and add the object types and signatures you want to send to the browser. Normally, this exists for you to provide custom “object to JSON” handlers for your objects, but there’s nothing custom about iterators, generators, functors and closures. They are native Python objects. This snippet provides the functionality needed by JSONEncoder to correctly dereference these useful Python objects and render their contents.
(Originally posted [here](http://www.elfsternberg.com/2009/05/20/fixing-an-omission-from-djangos-simplejson-iterators-generators-functors-and-closures/) )
Decorates signals for executing only one time
Exemple usage :
from django.core.mail import EmailMultiAlternatives
from django.template.loader import render_to_string
from django.contrib.auth.models import User
@one
def user_welcome(sender, instance, created, **kwargs):
# Send a welcome email
if created == True and isinstance(instance, User):
instance.message_set.create(message=_(u"Ho, Welcome %s!" % instance))
subject, from_email, to = 'Welcome !', '[email protected]', instance.email
text_content = render_to_string('mail/welcome.html', { 'user': instance })
msg = EmailMultiAlternatives(subject, text_content, from_email, [to])
msg.send()