Custom Form Validation

from django.core.exceptions import ValidationError


class SignupForm(forms.Form):
    username = forms.CharField(max_length=50)

    def clean_username(self):
        username = self.cleaned_data['username']
        if ' ' in username:
            raise ValidationError('Username cannot contain spaces.')
        return username

def clean(self):
    cleaned_data = super().clean()
    password = cleaned_data.get('password')
    confirm = cleaned_data.get('confirm_password')
    if password and confirm and password != confirm:
        raise ValidationError('Passwords do not match.')
    return cleaned_data

<form method="post">
  {% csrf_token %}
  {{ form.as_p }}
  <button type="submit">Sign Up</button>
</form>

1. Naming a clean method wrong (e.g. clean_Email instead of clean_email) so Django never calls it.
2. Forgetting to return the cleaned value at the end of a clean_<field> method, silently wiping that field's data.
3. Raising a plain Exception instead of django.core.exceptions.ValidationError, which Django can't attach to the form's errors.

• A clean_<fieldname> method validates one specific field and must return the cleaned value.
• Overriding clean() (no field name) validates relationships between multiple fields at once.
• Raise django.core.exceptions.ValidationError to reject a value with a custom error message.