Validation and errors

Validation happens at multiple layers: HTML5 client hints, Django form fields, model clean() methods, and database constraints. Never trust the client alone.

Form-level validation

def clean_title(self):
    title = self.cleaned_data["title"]
    if "spam" in title.lower():
        raise ValidationError("Title not allowed")
    return title

Displaying errors

{{ form.non_field_errors }} for form-wide issues; {{ form.title.errors }} per field. Re-render the form with errors on failed POST—preserve user input.

Important interview questions and answers

1. Q: clean vs clean_fieldname?
   A: Field cleaners run per field; clean() validates cross-field rules (e.g. password match).
2. Q: Model validation on save?
   A: Call full_clean() before save in custom code—ModelForm does this automatically.
3. Q: ValidationError in models?
   A: Raise in clean() methods—surfaces in forms and admin.

Self-check

1. Why re-render the form after validation fails?
2. Where add cross-field validation?

Tip: Re-display bound forms on validation failure so users do not retype everything—pass the same form instance back to the template.