Imagine we've launched our feedback form, and the e-mails have started tumbling in. There's just one problem: some of the submitted messages are just one or two words, which isn't long enough for us to make sense of. We decide to adopt a new validation policy: four words or more, please.

There are a number of ways to hook custom validation into a Django form. If our rule is something we will reuse again and again, we can create a custom field type. Most custom validations are one-off affairs, though, and can be tied directly to the Form class. We want additional validation on the message field, so we add a clean_message() method to our Form class:

from django import forms 
 
class ContactForm(forms.Form): 
    subject = forms.CharField(max_length=100) 
    email = forms.EmailField(required=False) 
    message = forms.CharField(widget=forms.Textarea) 
 
    def clean_message(self):

        message = self.cleaned_data['message...