Nicer dynamic forms in django

I used to make dynamic forms for Django in very bad way, I'm happy to admit that now as I've improved my process.

Basically the solution is to use type() as I'm sure many of you know. If your doing that already there isn't much for you here. If your messing around with 'self.fields["name"]' in your forms then read on.

Lets take a simple use case; a quiz system. You can think of it like who wants to be a millionaire. We will have a Question with four possible answers. So two models...

from django.db import models

class Question(models.Model):
    test = models.CharField(max_length=128)

class Answer(models.Model):
    question = models.ForeignKey(Question)
    test = models.CharField(max_length=20)
    is_correct = models.BooleanField(default=False)

Basically we want a form that stores both the question and the answers and checks the answer is a valid choice. You could do it this way.

from django import forms

class QuizForm(forms.Form):

    def __init__(self,question, *args, **kwargs):
        super(QuizForm, self).__init__(*args, **kwargs)

        self.fields['question'] = forms.IntegerField(widget= \
        self.fields['answers'] = forms.ModelChoiceField(queryset= \

Now in this example that's actually not too bad. It's still a bit hacky as we tap into the fields dict after calling the parents constructor. I have a variant of this where I moved the field generation out of the __init__ but that doesn't really change much - just gives you the option of calling it later. Remember this is a very simple example, what if you need to generate a class based on 20 parameters?

So how can we solve this with type()? Well lets step back a minute and quickly refresh ourselves with the builtin function type(). There are two ways to use this function the first is by calling type(object) and the type of that object is returned. The second is to create classes at runtime by using type() as a class constructor.

First lets look at an example that has exactly with the same result as the previous example.

def quiz_form_factory(question):

    properties = {
        'question' : forms.IntegerField(widget=forms.HiddenInput, \
        'answers' : forms.ModelChoiceField(queryset= \

    return type('QuizForm', (forms.Form,), properties)

And there we go, it's as simple as that. When using type() to construct classes it takes three parameters type(class_name, base_classes_tuple, properties_dict); the name of the class, the base classes it inherits from and the properties the created class will have.

Besides the above implementation the usage is slightly different.

# start with a random question
question = Question.objects.all().order_by('?')[0]

# method A
quiz_form = QuizForm(question)
# or with POST data
quiz_form = QuizForm(question, request.POST)

# method B
QuizForm = quiz_form_factory(question)
quiz_form = QuizForm()
# or with POST data
quiz_form = QuizForm(request.POST)

Again, perhaps I should have chosen a more complex example as the the first method may not look to bad and requires less code but its not nice. Using this approach basically class initialisation with the preparation and modification of said class. The second is much clearer as you explicitly generate a class then create an instance of it.

The main advantage for me is the clarify this gives you and the code used to make it dynamic is clear and better structured. It's also worth mentioning that with a type() constructed form it behaves exactly like a regular form after creation where with method A the developer needs to pass in the question instance each time and be aware of this requirement and how it varies from a typical Django style form.

Thanks for reading. You should follow me on Twitter.

Do you have any feedback or comments? The best place for discussion is on Reddit or Hacker News. Otherwise, email me.