title: conditionally validate a form field in django
body: <p>I have a textfield that renders in a django form.  This text field can have different types of data like ip addresses, urls, regular text, etc.  Depending on a previous drop down's input in the form, how do I conditionally validate the text field?</p>
  
  <p>For example if I choose ip address in the drop down, how do I then conditionally throw a validation error when input besides when an ip address is entered in the text field?</p>