? Form Validation & Error Handling

? Quick Overview

Form validation ensures that user-submitted data is accurate, complete, and secure. Django provides built-in mechanisms to validate form input and handle errors efficiently.

? Key Concepts

• Automatic validation using is_valid()
• Field-level and form-level validation
• Displaying user-friendly error messages

? Syntax / Theory

Django validates form data when form.is_valid() is called. It checks required fields, data types, and custom validation rules.

? Code Example – Basic Form Validation

# Defining a basic Django form with required fields
from django import forms
class ContactForm(forms.Form):
name = forms.CharField(max_length=100, required=True)
email = forms.EmailField(required=True)
message = forms.CharField(widget=forms.Textarea, required=True)

? Code Example – Displaying Form Errors

# Django template showing form errors
{% for field in form %}
{% if field.errors %}

? Code Example – Custom Field Validation

# Custom validation for name field
def clean_name(self):
name = self.cleaned_data.get('name')
if len(name) < 3:
raise ValidationError("Name must be at least 3 characters long.")
return name

? Code Example – Non-Field Errors

# Validating multiple fields together
def clean(self):
cleaned_data = super().clean()
if cleaned_data.get('password') != cleaned_data.get('confirm_password'):
raise ValidationError("Passwords do not match.")
return cleaned_data

? Live Output / Explanation

If validation fails, Django automatically attaches error messages to the form. These can be displayed in templates to guide users in correcting their input.

? Interactive Example

Try entering invalid data below (e.g., a name shorter than 3 chars or mismatched passwords) to simulate how Django catches errors.

? Use Cases

• User registration forms
• Contact and feedback forms
• Secure data collection