How do I write a single-file Django application?

You might want to consider Simon Willison's library:

• djng—a Django powered microframework

From the readme:

djng is a micro-framework that depends on a macro-framework (Django).

My definition of a micro-framework: something that lets you create an entire Python web application in a single module:

import djng

def index(request):
    return djng.Response('Hello, world')

if __name__ == '__main__':
    djng.serve(index, '0.0.0.0', 8888)

[...]

Getting started with Django can be pretty easy too. Here's a 10-line single-file Django webapp:

import os
from django.conf.urls.defaults import patterns
from django.http import HttpResponse
filepath, extension = os.path.splitext(__file__)
ROOT_URLCONF = os.path.basename(filepath)

def yoohoo(request):
    return HttpResponse('Yoohoo!')

urlpatterns = patterns('', (r'^hello/$', yoohoo))

Check out my blog post Minimal Django for details.