Mathematical Captcha

When spam bots started to post comments in my blog every day I realized that I need some kind of simple anti-spam protection. I didn't saw yet real anti-spam AI and don't like image captchas. I looked for a mathematical captcha where human is asked to solve a simple mathematical calculation like 3+5=?. Unfortunately I didn't found open-source implementation and write own MathCaptchaForm. A week already I don't see spam posts.

import re
from random import randint
from django import newforms as forms

class MathCaptchaForm(forms.Form):
    Q_RE = re.compile("^(\d)\+(\d)$")
    A_RE = re.compile("^(\d+)$")
    captcha_question = forms.CharField(max_length=10, required=True,
        widget=forms.HiddenInput())
    captcha_answer = forms.CharField(max_length = 2, required=True,
        widget = forms.TextInput(attrs={'size':'2'}))

    def __init__(self, *args, **kwargs):
        super(MathCaptchaForm, self).__init__(*args, **kwargs)
        q = self.data.get('captcha_question') or self._generate_question()
        self.initial['captcha_question'] = q

    def _generate_question(self):
        return "%s+%s" % (randint(1,9), randint(1,9))

    def clean_captcha_answer(self):
        q = self.Q_RE.match(self.cleaned_data['captcha_question'])
        if not q:
            raise forms.ValidationError("Are you hacker?")
        q = q.groups()
        a = self.A_RE.match(self.cleaned_data['captcha_answer'])
        if not a:
            raise forms.ValidationError("Number is expected!")
        a = a.groups()
        if int(q[0]) + int(q[1]) != int(a[0]):
            raise forms.ValidationError("Are you human?")

Django supports forms subclassing, so to add mathematical captcha to any existing form it is enough to extend MathCaptchaForm. For example comments form used on this site is defined as bellow.

class CommentForm(MathCaptchaForm):
    """Form for editing a comment."""
    author = forms.CharField(label='Name', required=False,
        max_length=Comment._meta.get_field('author').maxlength,
        widget=TextInput(attrs={'size':20}))
    url = forms.URLField(label='URL', required=False,
        max_length=Comment._meta.get_field('url').maxlength,
        widget=TextInput(attrs={'size':24}))
    content = forms.CharField(label="Your comment",
        max_length=Comment._meta.get_field('content').maxlength,
        widget=Textarea(attrs={'cols':80, 'rows': 10})

Then it's only need to add 2 captcha's fields to your template and spam bots along with humans who was bad at school will not be able to post comments. The template may look like:

{{ comment_form.captcha_question }}
<label for="id_captcha_answer">
{% if comment_form.captcha_answer.errors %}
<em class="error">{{ comment_form.initial.captcha_question }}=</em>
{% else %}
{{ comment_form.initial.captcha_question }}=
{% endif %}
</label>
{{ comment_form.captcha_answer }}

Well, it's quite simple to write spam bot that will solve such simple mathematical captchas, but in this case we can improve current simple MathCaptchaForm with integral and differential equations.