using recaptcha-client

comment_form.html

<form action="/blog/add-comment/{{ post.slug }}" method="post">{% csrf_token %}
    <script type="text/javascript"
        src="http://www.google.com/recaptcha/api/challenge?k=<insert_public_key>">
    </script>
    <noscript>
        <iframe src="http://www.google.com/recaptcha/api/noscript?k=<insert_public_key>"
            height="300" width="500" frameborder="0"></iframe><br>
        <textarea name="recaptcha_challenge_field" rows="3" cols="40">
        </textarea>
        <input type="hidden" name="recaptcha_response_field"
            value="manual_challenge">
    </noscript>
    <p><label for="nameField">Name: </label> <input type="text" name="username_field" id="nameField" size="40" maxlength="50"/></p>
    <p><label for="locationField">URL: </label> <input type="text" name="user_url_field" id="locationField" size="40" maxlength="120"/></p>
    <p>
        <label for="bodyField">Comment: </label> <br />
        <textarea name="body_field" id="bodyField" rows="8" cols="70"></textarea>
    </p>
    <input type="submit" value="Submit" />
</form>

pipinstallrecaptcha-client

pip install recaptcha-client

settings_file_modification.py

RECAPTCHA_PRIVATE_KEY = '<private_key_provided_by_reCAPTCHA>'

urls.py

url(r'^add-comment/(?P<slug>[^\.]+)', 'blog.views.add_comment'),

views.py

from recaptcha.client import captcha
from django.shortcuts import redirect, get_object_or_404
from django.http import HttpResponseBadRequest
from django.conf import settings
from blog.models import Post, Comment

def add_comment(request, slug):
    if request.method == 'POST':
        response = captcha.submit(
            request.POST['recaptcha_challenge_field'],
            request.POST['recaptcha_response_field'],
            settings.RECAPTCHA_PRIVATE_KEY,
            request.META["REMOTE_ADDR"])
        if response.is_valid:
            post = get_object_or_404(Post, slug=slug)
            post.comment_set.create(
                username = request.POST['username_field'],
                user_url = request.POST['user_url_field'],
                body = request.POST['body_field']
            )
            return redirect('view-post', slug=slug)
        return redirect('view-post', slug=slug)
    else:
        return HttpResponseBadRequest()