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Some decorators and classes to make working with django projects easier.

Project Description

django-drapes is a small library that aims to ease authorization and user input verification. Most of the functionality is packed into decorators intended for applying to views, hence the name django-drapes. The decorators:

  • verify: Validate and convert values passed to a controller
  • require: Check for permissions
  • verify_post: Validate and process POST requests
  • render_with: Render a dictionary with a template or json

There are also two template tags which can be used in combination with these decorators:



verify is a decorator that turns values passed to the controller into a more usable form (such as models), and throws suitable exceptions when this does not work. The conversions are specified as keyword arguments with a validator matching the name of the controller argument. The validators have to implement the formencode validator interface.

Here is a simple example:

from django_drapes import verify
import formencode

def controller(request, int_arg):
    return 'Argument is %d' % int_arg

The controller receives int_arg as an integer, obviating the need to convert in the controller.

The values for the conversions are searched in the arguments for the controller function, and additionally the GET parameters if the request is a GET. This causes a mismatch between the url definition and the function signature, since one can’t specify get parameters in a url entry, and a controller normally has to look up a GET parameter in request.GET. Because of this mismatch, in case you want to verify a GET parameter, you should include this parameter as a keyword argument in the controller signature.

The most frequently done conversion is selecting a model with a unique field. django-drapes has a built in validator for this kind of conversion, called ModelValidator. It can be used as follows:

from django.db import models
from django_drapes import verify, ModelValidator

class Project(models.Model):
    slug = models.SlugField(unique=True)

def controller(request, item):
    return "Item's slug is %s" % item.slug

An advanced feature implemented by ModelValidator is looking up a model by multiple keys. In order to do this, you should initialize ModelValidator with a list of strings as get_by. These strings should be in the form model_field=view_arg, matching arguments to a view to fields on a model. For example, let’s assume that we have a project where users can create items identified by slugs. Items belonging to different users can have the same slug, and the page for such an item is identified by the name of the user and the slug of the item. In that case, drapes decorators can be used as follows:

@verify(owner=ModelValidator(User, get_by='username'))
@verify(item=ModelValidator(Project, get_by=['slug=item','owner=owner']))
def view_item(request, owner, item):
    return dict(item=item)

This case also demonstrates Mixing the decorators.


require checks permissions on an incoming request to a controller. Just like validate, it accepts keyword arguments with key referring either to user (accessed through request.user) or the positional or keyword arguments of a view function. Value must be a string corresponding to the permission. What the permission refers to is determined in the following order:

  • An attribute of the object
  • A method of the object that does not require any arguments
  • A method of the model permission (a subclass of ModelPermission; see below) that accepts a user as argument.

Here is a very simple example:

from django.db import models
from django_drapes import verify, ModelValidator
import formencode

class Thing(models.Model):
    slug = models.SlugField(unique=True)
    published = models.BooleanField(default=False)

@verify(item=ModelValidator(Thing, get_by=slug))
def controller(request, thing):
    return "This thing's slug is %s" % item.slug

Permissions can be added to models by subclassing the ModelPermission class, and setting a model as the class attribute:

from django.db import models
from django.shortcuts import render
from django_drapes import (verify,

class Thing(models.Model):
    slug = models.SlugField()

class ThingPermissions(ModelPermission):
    model = Thing
    def can_view(self, user):
        return user.username == 'horst'

def controller(request, thing):
    return render(request, 'thing.htm', dict(thing=thing))

The only person who can view this item is the one named horst. The default selector used by ModelValidator is model id; this can be overriden using the get_by argument, as seen above.


verify_post is a decorator for easing the workflow with form input. The aim is to split the handling of user input through forms into the presentation of empty or erronuous forms, and the processing of a valid form.

There are two ways to use verify_post. The first is the simple case, where the same entry point to an app should display a form for GET, and also process it when it gets POSTed. In this case, verify_post.single should be used. This factory method accepts two positional arguments: the form used to verify the POST, and the handler to call if the form validates:

from django import forms
from django_drapes import verify_post
from django.http import HttpResponseRedirect
from django.shortcuts import render_to_response
#we are assuming the models exist somewhere
from .models import Thing
from django_drapes import (verify,

class ThingForm(forms.Form):
    name = forms.CharField(required=True, min_length=4)

def create_thing(request, item, form):
    thing = Thing(['name'])
    return HttpResponseRedirect(thing.get_absolute_url())

@verify_post.single(ThingForm, create_thing)
def controller(request, item, invalid_form=None):
    return render_to_response('form_template.html',

Some notes on this example. When you are handling single forms, the controller must have a keyword argument invalid_form. If the form does not validate, it is passed on to the controller through this argument. The handler of the correct form, in this case create_thing, must have the same signature as the controller, except for invalid_form, which is replaced with form in the signature of the correct handler.

If you want to use the same entry point to show and validate forms of different kinds, you should use verify_post.multi. This method accepts a list of form options specified with keyword arguments which are the names of the forms on the page. The form options have to be tuples specifying the form for validation and the valid form handler. Here is an example:

from django import forms
from django_drapes import verify_post
from .models import Thing, Organism

class ThingForm(forms.Form):
    name = forms.CharField(required=True, min_length=4)
    drape_form_name = forms.CharField(required=True,

class OrganismForm(forms.Form):
    genus = forms.CharField(required=True, min_length=10)
    drape_form_name = forms.CharField(required=True,

def create_thing(request, form):

def create_organism(request, form):

@verify_post.multi(thing_form=(EntityForm, create_entity),
                   organism_form=(OrganismsForm, create_organism))
def controller(request, item, invalid_form=None):
    return render_to_response('form_template.html',

As it can be seen in this example, the hidden field drape_form_name of a form has to match the keyword argument to verify_post which specifies how that form should be handled.

One complication for which I couldn’t come up with a decent solution is form validation with a user. In some cases, it is necessary to to initialize a form class with a user; an example is when a value has to be unique per user. In these cases, you have to set the keyword argument pass_user to True for verify_post.single, and a three-element tuple whose last element is True to verify_post.multi. Let me know in case you have a better solution.


render_with turns dictionary return values into rendered templates. It requires a string as argument, signifying either a template path or json. render_with then calls django.shortcuts.render with the dictionary-like return value of the controller, and the template name:

def controller(request):
    return dict(message='Hello world')

The default template can be overriden by setting a ‘template’ key in the return dictionary to the desired template name. render_with also respects return values which are subclasses of HttpResponse (e.g. HttpResponseRedirect). If you want to return something else from your controller, do not use this decorator.

Mixing the decorators

Any number of these decorators can be applied to the same controller. The following is posible:

@verify_post.single(ThingForm, create_thing)
def controller(request, model_inst):
    return model_inst.message

The principle here is that if a decorator depends on the conversions of another, it should come after it.

Template tags

django-drapes comes with two template tags which make it possible to refer to permission classes, and to render pieces of html from a model. These tags are if_allowed and modelview.


if_allowed is a tag which conditionally renders content based on the outcome of a permission applied to a user. Let’s have an example for a change. Model and permissions:

from django.db import models
from django_drapes import ModelPermission

class Thing(models.Model):
    slug = models.SlugField(unique=True)

class ThingPermissions(ModelPermission):
    model = Thing

    def can_view(self, user):
        return user.username == 'horst'

And then in the template which gets rendered with a user and a thing, you can do the following:

{% load wherever_you_put_the_tags %}
{% if_allowed user can_view thing %}
{% else %}
    For horst's eyes only
{% end_if_allowed %}

If your username is not horst, you will see ‘For horst’s eyes only’.


The other template tag is a helper called modelview. In order to insert markup representing an aspect of a model, you can subclass ModelView, and set its class attribute model to a django model. Attributes of this model can later be referred to in a template using the modelview template tag:

from django.db import models
from django.template.loader import get_template
from django.template import Context
from django_drapes import ModelView

class Thing(models.Model):
    slug = models.SlugField(unique=True)

class ThingView(ModelView):
    model = MockModel

    def some_view(self, arg1, arg2=None):
        template = get_template('thing_some_view.html')
        #do stuff with arg1 and arg2 ...
        return template.render(Context(dict(thing=self)))

It is advised to use template.render here, since this way you don’t get a response with the full HTTP headers. A nice feature of this template tag is that it will pass on any arguments you are calling it with to the view function.

If you want to get the output of a model view outside of a template, you can use the view function named just v to get the ModelView for a model instance:

from django_drapes import verify, ModelValidator, v
from .models import Thing

def just_some_view(request, thing):
    return v(thing).some_view()

Registering the template tags

Since django-drapes is not organized as an app, both of these tags have to be manually registered to be used in templates. You can do this by creating a templatetags folder in one of your project apps, and then including the following in a file there:

from django import template
from django_drapes import model_permission, modelview
register = template.Library()
register.tag('if_allowed', model_permission)
register.tag('modelview', modelview)

You are free to change the names of the tags, of course.

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