The long story in a short sentence is that you cannot subclass or customize CommentForm in todays version without getting your hands dirty.

I recently enabled Captcha verification to comments on this site to prevent spammers from filling my database with non-public comments. Tor Brede Vekterli has written about how to do this by using signals, but for technical reasons I’d rather not go down that road. So I landed on the next possible strategy, namely subclassing CommentForm.

My subclass is a standard Django form:

class CaptchaCommentForm(CommentForm):
    captcha = forms.CharField(max_length=20, label='Enter this word')

    def clean_captcha(self):
        # verify self.cleaned_data['captcha'] here, details omitted,
        # raise forms.ValidationError if verification fails

Since I wanted to control the way this field is displayed by adding an image I also edited my form template. In my comments/form.html template I added a check to include the captcha image:

{% for field in form %}
    {% ifequal field.name "captcha" %}
        do whatever it takes to display the custom field.
    {% else %}
        display regular comment form field
    {% endifequal %}
{% endfor %}

As I mentioned above the comments framework is not really designed to be extended or customized in this way. To make the framework use my CaptchaCommentForm instead of its own CommentForm I resorted to monkeypatching!

The framework uses a single function, comments.get_form, whenever it needs a fresh CommentForm instance. What I wanted is for that function to return my CaptchaCommentForm instead. Here is what I did to my urls.py:

from django.contrib import comments
from forms import CaptchaCommentForm

def override_get_form():
    return CaptchaCommentForm()
comments.get_form = override_get_form

Thanks to the dynamic features of Python we can simply replace the original function with our own that returns the correct instance. Not pretty, but it works.