django alter form data in clean method

Question

I have a django form that I want to custom clean. Instead of just specifying an error message like here (Django form and field validation), I'd like to just alter the field myself. I tried severl ways, but keep running into error like cleaned_data is immutable.

So to solve this I made a copy, changed it and reassigned it to self. Is this the best way to do this? Could/should I have handled this in the view? Making a copy seems poor form but I keep running into 'immutable' road blocks. Sample code below where I simply check if the subject has '--help' at the end, and if not add it. Thanks

def clean(self):
        cleaned_data=self.cleaned_data.copy()
        subject=cleaned_data.get['subject']
        if not subject.endswith('--help'):
            cleaned_data['subject']=subject+='--help'
        self.cleaned_data=cleaned_data
        return self.cleaned_data

Answer 1

The correct way to deal with this is by using the field specific clean methods.

Whatever you return from the clean_FOO method is what the cleaned_data will be populated with by the time it gets to the clean function.

Do the following instead:

def clean_subject(self):
        data = self.cleaned_data.get('subject', '')
        if not data:
             raise forms.ValidationError("You must enter a subject")
             # if you don't want this functionality, just remove it.

        if not data.endswith('--help'):
             return data += '--help'
        return data