Django - override form clean method

https://docs.djangoproject.com/en/dev/ref/forms/validation/

Cleaning and validating fields that depend on each other¶

Suppose we add another requirement to our contact form: if the cc_myself field is True, the subject must contain the word"help". We are performing validation on more than one field at a time, so the form's clean() method is a good spot to do this. Notice that we are talking about the clean() method on the form here, whereas earlier we were writing a clean()method on a field. It's important to keep the field and form difference clear when working out where to validate things. Fields are single data points, forms are a collection of fields.

By the time the form's clean() method is called, all the individual field clean methods will have been run (the previous two sections), so self.cleaned_data will be populated with any data that has survived so far. So you also need to remember to allow for the fact that the fields you are wanting to validate might not have survived the initial individual field checks.

There are two ways to report any errors from this step. Probably the most common method is to display the error at the top of the form. To create such an error, you can raise a ValidationError from the clean() method. For example:

class ContactForm(forms.Form):
    # Everything as before.
    ...

    def clean(self):
        cleaned_data = super(ContactForm, self).clean()
        cc_myself = cleaned_data.get("cc_myself")
        subject = cleaned_data.get("subject")

        if cc_myself and subject:
            # Only do something if both fields are valid so far.
            if "help" not in subject:
                raise forms.ValidationError("Did not send for 'help' in "
                        "the subject despite CC'ing yourself.")

        # Always return the full collection of cleaned data.
        return cleaned_data

http://stackoverflow.com/questions/2117048/django-overriding-the-clean-method-in-forms-question-about-raising-errors

override form clean method
django form overriding clean

from django.forms.util import ErrorList
def clean(self):

  if self.cleaned_data['type'].organized_by != self.cleaned_data['organized_by']:
    msg = 'The type and organization do not match.'
    self._errors['type'] = ErrorList([msg])
    del self.cleaned_data['type']

  if self.cleaned_data['start'] > self.cleaned_data['end']:
    msg = 'The start date cannot be later than the end date.'
    self._errors['start'] = ErrorList([msg])
    del self.cleaned_data['start']

  return self.cleaned_data