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Django form validation with clean methods

Priya Sharma Jan 2026
1 tab 
from django import forms
from .models import Event


class EventForm(forms.ModelForm):
    class Meta:
        model = Event
        fields = ['title', 'description', 'start_date', 'end_date', 'capacity']

    def clean_capacity(self):
        capacity = self.cleaned_data.get('capacity')
        if capacity and capacity < 1:
            raise forms.ValidationError('Capacity must be at least 1')
        return capacity

    def clean(self):
        cleaned_data = super().clean()
        start_date = cleaned_data.get('start_date')
        end_date = cleaned_data.get('end_date')

        if start_date and end_date:
            if end_date <= start_date:
                self.add_error(
                    'end_date',
                    'End date must be after start date'
                )

        return cleaned_data
1 file · python Explain with highlit

I use clean_<fieldname>() to validate individual fields and clean() to validate field combinations. Raising ValidationError shows the message to the user near the appropriate field. For cross-field validation (like 'end date must be after start date'), I override clean() and attach errors with add_error(). I keep validation logic in forms rather than models because it's often context-dependent. Using cleaned_data ensures I'm working with validated, type-converted values. This pattern keeps forms self-contained and easy to test.

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