Django — Form Validation

Validation Methods

class RegistrationForm(forms.Form):
    username = forms.CharField(max_length=50)
    email = forms.EmailField()
    password = forms.CharField(widget=forms.PasswordInput)
    
    def clean_username(self):
        username = self.cleaned_data['username']
        if User.objects.filter(username=username).exists():
            raise forms.ValidationError("Username already taken")
        return username
    
    def clean_email(self):
        email = self.cleaned_data['email']
        if not email.endswith('@example.com'):
            raise forms.ValidationError("Must be example.com email")
        return email
    
    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 forms.ValidationError("Passwords don't match")
        return cleaned_data

Validation Flow

1. clean_<fieldname>() — Single field
2. clean() — Whole form
3. validate_<fieldname>() — Custom validators

Built-in Validators

from django.core.validators import (
    MinValueValidator, MaxValueValidator,
    MinLengthValidator, RegexValidator,
    URLValidator, EmailValidator
)

age = forms.IntegerField(validators=[MinValueValidator(0), MaxValueValidator(120)])

Error Handling

if form.is_valid():
    # Process valid data
else:
    # Access errors
    print(form.errors)
    print(form.errors.as_json())
    print(form.errors.as_text())

Mini Practice

1. Create a form with field validation
2. Add cross-field validation
3. Display validation errors
4. Use built-in validators

Up Next

Continue with Static Files — serving CSS, JavaScript, and images.