Checking One Field

Part of the Forms section of Coddy's Django journey. Lesson 10 of 20.

The form accepts an event dated last year. Django's built-in checks know about types and limits, not about the rules of your site. A rule of your own about one field goes in a method named clean_ plus the field's name. A sign-up form that keeps the name admin for the staff:

class SignupForm(forms.Form):
    username = forms.CharField(max_length=30)

    def clean_username(self):
        username = self.cleaned_data["username"]
        if username.lower() == "admin":
            raise forms.ValidationError("That name is reserved.")
        return username

is_valid() calls it after the field's own checks, so the value is already converted: a DateField gives a date. Raising forms.ValidationError refuses the field, and the message appears next to it.

The method must return the value: what it returns lands in cleaned_data. Without the return, the value becomes None.

Challenge

Easy

In EventForm, refuse a date before today with the message The date cannot be in the past. An event today is fine.

Add an event dated yesterday, typed like 2026-10-30: you see The date cannot be in the past.

Try it yourself

from django import forms


class EventForm(forms.Form):
    title = forms.CharField(max_length=100)
    date = forms.DateField()
    place = forms.CharField(max_length=100)
    description = forms.CharField(widget=forms.Textarea, required=False)
    capacity = forms.IntegerField(min_value=1)