Implementing a text based captcha in Pylons

One of the must have elements on any html form as an anti-spam measure is a captcha, and the captcha technique that has worked pretty well for me is text based. So here’s how to implement one.

Create a controller and name it captcha

 ~$ paster controller captcha

Add this function to your file

def makeCaptcha(lang):
      label = {
          1 : 'one',
          2 : 'two',
          3 : 'three',
          4 : 'four',
          5 : 'five',
          6 : 'six',
          7 : 'seven',
          8 : 'eight',
          9 : 'nine',
          10 : 'ten'
      num1 = random.randint(1,10)
      num2 = random.randint(1,10)
      res = num1 + num2
      session['captcha'] = res
      return [label[num1],label[num2]]

On the templates folder create an html file with an example form

<form method="post" action="${url(controller='captcha',action='contact')}" id="contact-frm">
      capt = h.makeCaptcha()
      c.captcha =  u"What is the result of the sum %s and %s?." % (capt[0],capt[1])
    <span>${u"As an  anti spam measure, please answer the following math question."}</span><br />
    <label for="captcha">${c.captcha}</label>
    <!-- we set a maxlength of 2 characters on this input field because the answer should be numeric -->

Save it and call it captcha.html

Add the following to captcha controller

I decided to create a validation schema using formencode:

import formencode
from formencode import variabledecode
from formencode import validators
from formencode.validators import Invalid, FancyValidator
#this class will validate the captcha value entered by the user
class CaptchaValidator(formencode.FancyValidator):
    def _to_python(self,values,state):
        if session.get('captcha') != int(values.get('captcha')):
          raise formencode.Invalid(u"The math answer is incorrect",values, state)
          return values
class NewInquiry(formencode.Schema):
  allow_extra_fields = True
  filter_extra_field = True
  captcha = formencode.validators.String(
    not_empty = True,
    messages = {
      'empty' : 'You need to answer the math question.'
  # chain the captcha validator
  chained_validators = [CaptchaValidator()]
# our contact view
def contact(self):
      if request.method == 'POST':
          values = dict(request.params)
          schema = NewInquiry()
          results = schema.to_python(values)
        except Invalid, e:
          #raise error if something went wrong
          #send to contacts
      return render('captcha.html')

And that’s pretty much it. The only drawback I can find is that isn’t very intuitive at times since users my try to answer with words instead of numeric values. That’s why I added a maxlength of 2 characters to the input field.

But you could easily implement a Javascript validation function to notify the user to type numeric values instead of characters before she submits the form.