Nathan Ostgard

When I have Django URL patterns like:

urlpatterns = patterns('',
  (r'^(?P<foo_id>\w+)/$', 'myproject.myapp.views.show'),
  (r'^(?P<foo_id>\w+)/edit/$', 'myproject.myapp.views.edit'),
  (r'^(?P<foo_id>\w+)/delete/$', 'myproject.myapp.views.delete'),
)

I always end up with views like:

def show(self, foo_id):
  foo = get_object_or_404(Foo, id=foo_id)
  ...

def edit(self, foo_id):
  foo = get_object_or_404(Foo, id=foo_id)
  ...

def delete(self, foo_id):
  foo = get_object_or_404(Foo, id=foo_id)
  ...

Having the same few lines at the top of every function in makes me feel dirty. You can clean this up with a Middleware class, replacing foo_id with the actual object before calling the view.

myproject/myapp/middleware.py looks like this:

from django.shortcuts import get_object_or_404
from someapp.models import Foo

class FindObjects:
  def process_view(self, request, view_func, view_args, view_kwargs):
    if 'foo_id' in view_kwargs:
      view_kwargs['foo'] = get_object_or_404(Foo, id=view_kwargs['foo_id'])
      del view_kwargs['foo_id']

I include it in settings.py:

MIDDLEWARE_CLASSES = (
  ...
  'myproject.myapp.middleware.FindObjects',
)

And now my views look like this:

def show(request, foo):
  ... yay! do stuff with foo! ...

Very useful for views with many parameters, or views with multiple optional parameters. In one project, I have views which process data for three different situations: data in a metropolitan area, data in a city, or data in a city within a metro. My middleware for that looks like this:

from django.shortcuts import get_object_or_404
from phuce.metros.models import City, Metro

class FindObjects:
  def process_view(self, request, view_func, view_args, view_kwargs):
    city = metro = None
    if 'metro_link' in view_kwargs:
      metro = get_object_or_404(Metro, link=view_kwargs['metro_link'])
      del view_kwargs['metro_link']
      view_kwargs['metro'] = metro
    if 'city_link' in view_kwargs:
      if metro:
        city = get_object_or_404(City, metro=metro, \
                                 metro_link=view_kwargs['city_link'])
      else:
        city = get_object_or_404(City, link=view_kwargs['city_link'])
      del view_kwargs['city_link']
      view_kwargs['city'] = city

It’s much tidier without that monster in my views.

There’s no way in Django (that I’ve found) to render a field’s name in a template. This means you end up with <th>Field Name</th> all over your templates. Why hello there, DRY violation!

The fields are stored in model._meta.fields, but templates don’t allow you to access variables which start with an underscore. I’ve got two little utility functions I wrote for myself to generate a dict of labels I can use in my templates:

def get_labels_for(model, cap=True, esc=True):
  from django.template.defaultfilters import capfirst
  from django.utils.html import escape
  labels = {}
  for field in model._meta.fields:
    label = field.verbose_name
    if cap:
      label = capfirst(label)
    if esc:
      label = escape(label)
    labels[field.name] = label
  return labels

def with_labels(context, cap=True, esc=True):
  from django.db.models import Model
  result = context.copy()
  for k, v in context.iteritems():
    if isinstance(v, Model):
      result[k + '_labels'] = get_labels_for(v, cap, esc)
    elif hasattr(v, '__getitem__') and len(v) > 0:
      if isinstance(v[0], Model):
        result[k + '_labels'] = get_labels_for(v[0], cap, esc)
  return result

The parameters:

• model can be a model class or a model instance.
• If cap is True, the first letter of each label will be capitalized.
• If esc is True, the labels will be escaped for HTML.

So, in your view:

def some_view(request, foo_id):
  foo = get_object_or_404(Foo, id=foo_id)
  context = {'foo': foo, 'foo_labels': get_labels_for(foo)}
  return render_to_response('foo.html', context)

And in your template:

{{ foo_labels.bar }}: {{ foo.bar }}

with_labels works the same way, except you can just surround your context with it:

def some_view(request, foo_id):
  foo = get_object_or_404(Foo, id=foo_id)
  bars = Bars.objects.all()
  context = {'foo': foo, 'bars': bars}
  return render_to_response('foo.html', with_labels(context))

It will detect the models and lists of models in the context and add foo_labels and bars_labels to the context.

When most pages in a site require authentication, decorating all the views with @login_required can be annoying. You can reverse the default behavior by creating a custom middleware class:

import urllib
from django.conf import settings
from django.contrib.auth import REDIRECT_FIELD_NAME
from django.http import HttpResponseRedirect

def allow_anonymous(view_func):
  view_func.allow_anonymous = True
  return view_func

class RequireLogin:
  def process_view(self, request, view_func, view_args, view_kwargs):
    if request.path != settings.LOGIN_URL and \
      not request.user.is_authenticated() and \
      not getattr(view_func, 'allow_anonymous', False):
      url = '%s?%s=%s' % (settings.LOGIN_URL, REDIRECT_FIELD_NAME, \
                          urllib.quote(request.get_full_path()))
      return HttpResponseRedirect(url)

That’s an ugly block of code but it’s not too complex. allow_anonymous is a function decorator, like login_required. It just tags the function to tell the middleware that authentication isn’t required. The RequireLogin class verifies that the user is logged in. If not, and if the function is not decorated with allow_anonymous, it redirects to settings.LOGIN_URL.

So put that code in a file in your project – let’s say, yourproject/yourapp/middleware.py. Then open settings.py and add "yourproject.yourapp.middleware.RequireLogin" to MIDDLEWARE_CLASSES. This tells Django about your new middleware class, and RequireLogin.process_view will be called any time a view is about to be rendered.

Now you can use it in your view:

from yourproject.yourapp.middleware import allow_anonymous

def some_private_view(request):
  # won't be accessible unless user is logged in
  return HttpResponse('Hello, user!')

@allow_anonymous
def some_public_view(request):
  return HttpResponse('Hello, world!')

If all is working correctly, some_private_view should ask for a login, but some_public_view will allow viewing without it.

Update: I’ve updated the code to fix a bug when using django.contrib.auth.views.login for your login view. As you can’t mark this view function with @allow_anonymous, it would infinitely redirect back to it. Oops! Thanks for pointing it out, Phil.

Simon Willison linked to an article which argues:

Warnings cause us to lose our work, to mistrust our computers, and to blame ourselves. A simple but foolproof design methodology solves the problem: “Never use a warning when you mean undo.” And when a user is deleting their work, you always mean undo.

The post spawned a discussion on undo techniques for Django. I decided to implement one method and post the results here. It only offers undo for deleting, and not for editing. Other than that, I like it.

How it works

It’s a pretty simple concept: add a trashed_at field to your model, with the default value of None. When delete() is called on an object, if trashed_at is None, set it to the current time but don’t delete it. If it’s not None, actually delete it from the database.

Continue reading...

Sometimes users want to bring text from an editor like Word into your web forms. You will often find nasty little characters hiding in the text, like ’\u2022’ (a.k.a. the notorious bullet). These characters will normally throw errors if you try to convert them to ASCII:

>>> u'\u2022'.encode('ascii')
Traceback (most recent call last):
  File "<console>", line 1, in ?
UnicodeEncodeError: 'ascii' codec can't encode ... (yadda yadda)

To sanitize these strings and make them XML/HTML safe:

>>> u'\u2022'.encode('ascii', 'xmlcharrefreplace')
'•'

It translates the invalid characters into their XML equivalents. Woo! You can also use 'ignore' or 'replace' (replaces with ?):

>>> u'\u2022'.encode('ascii', 'ignore')
''
>>> u'\u2022'.encode('ascii', 'replace')
'?'

If you’re getting nasty Unicode errors from your templates in Django now that they’ve merged the Unicode branch, this might help as a quick fix.

Foo.objects.get(keywords__contains='bar') will make your database cry when you run it against a 30 million row table. At times it becomes necessary to segment a large table into many smaller tables.

Say you have these models:

class City(models.Model):
  city = models.CharField(maxlength=255)
  state = models.USStateField()

class Listings(models.Model):
  name = models.CharField(maxlength=100)
  street = models.CharField(maxlength=100)
  city = models.ForeignKey(City)
  zip = models.CharField(maxlength=10)
  keywords = models.CharField(maxlength=255)

The listings table has gotten huge. Users only search within a single city, so you want to break the listings table into a table for each city. Instead of yourapp_listing, you now have yourapp_listing_1, yourapp_listing_2, etc. All the listings are in the matching table for their city.

I couldn’t find a documented way to make db_table dynamic in Django. So, how do you get Django to use the right table when you’re querying for listings? Here’s how I did it:

class ListingManager(models.Manager):
  def get_table_for(self, city):
    '''
    someapp_listing if city == None
    someapp_listing_012 if city.id == 12
    '''
    table = '_'.join((self.model._meta.app_label,
                      self.model._meta.module_name))
    if city:
      table += '_%03d' % city.id
    return table

  def in_city(self, city):
    self.city = city
    self.model._meta.db_table = self.get_table_for(city)
    return self

class Listing(models.Model):
  city = models.ForeignKey(City)
  objects = ListingManager()

  def delete(self):
    Listing.objects.in_city(self.city)
    super(Listing, self).delete()

  def save(self):
    Listing.objects.in_city(self.city)
    super(Listing, self).save()

It’s simple to use:

>>> city = City.objects.get(id=2)
>>> Listings.objects.in_city(city).all()

Edit 2007/08: Simplified the code. When I first wrote this I replaced db_table with a subclass of str that called ListingManager.get_table_for. It was overkill, and didn’t always work as expected (e.g., 'foo' + Listing._meta.db_table didn’t work).

Did a bit of running around today to get Django sending email via Gmail. It’s simple once you figure it out.

If you’re running 0.96, upgrade to the latest development version or apply the patch from ticket #2897. 0.96 does not support TLS, which Gmail requires. Then add the appropriate values to settings.py:

EMAIL_USE_TLS = True
EMAIL_HOST = 'smtp.gmail.com'
EMAIL_HOST_USER = 'youremail@gmail.com'
EMAIL_HOST_PASSWORD = 'yourpassword'
EMAIL_PORT = 587

You can use the shell to test it:

>>> from django.core.mail import send_mail
>>> send_mail('Test', 'This is a test', to = ['youremail@somewhere.com'])

Edit: Bryan commented that send_mail is deprecated. Use EmailMessage instead:

>>> from django.core.mail import EmailMessage
>>> email = EmailMessage('Hello', 'World', to = ['youremail@somewhere.com'])
>>> email.send()