Interview question

How does Django form validation work with clean_() and clean()? Django form validation clean_() और clean() से कैसे काम करता है?

Answer

Method	Validates	When to use
clean_<fieldname>()	A single specific field	Field-specific rules (format, uniqueness)
clean()	The whole form, across multiple fields	Cross-field validation (e.g. password confirmation)

from django import forms
from django.core.exceptions import ValidationError

class SignupForm(forms.Form):
    username = forms.CharField(max_length=30)
    password = forms.CharField(widget=forms.PasswordInput)
    confirm_password = forms.CharField(widget=forms.PasswordInput)
    email = forms.EmailField()

    # FIELD-level validation - runs automatically for the 'username' field
    def clean_username(self):
        username = self.cleaned_data['username']
        if User.objects.filter(username=username).exists():
            raise ValidationError('This username is already taken')
        return username  # MUST return the (possibly modified) cleaned value

    def clean_email(self):
        email = self.cleaned_data['email']
        if not email.endswith('@company.com'):
            raise ValidationError('Must use a company email address')
        return email

    # FORM-level validation - runs AFTER all field-level clean_X methods
    def clean(self):
        cleaned_data = super().clean()
        password = cleaned_data.get('password')
        confirm_password = cleaned_data.get('confirm_password')

        if password and confirm_password and password != confirm_password:
            # attach the error to a specific field, or use None for a
            # general (non-field) error shown at the top of the form
            self.add_error('confirm_password', 'Passwords do not match')

        return cleaned_data

# Validation execution order:
# 1. Field's built-in validators (e.g. EmailField's format check)
# 2. clean_<fieldname>() for each field with one defined
# 3. Form's clean() method for cross-field validation

form = SignupForm(request.POST)
if form.is_valid():
    # all validation passed, form.cleaned_data has clean values
    pass
else:
    print(form.errors)  # dict of field -> list of error messages

Method	क्या validate करता है	कब use करें
clean_<fieldname>()	एक specific field	Field-specific rules
clean()	पूरा form, कई fields में	Cross-field validation

from django import forms
from django.core.exceptions import ValidationError

class SignupForm(forms.Form):
    username = forms.CharField(max_length=30)
    password = forms.CharField(widget=forms.PasswordInput)
    confirm_password = forms.CharField(widget=forms.PasswordInput)
    email = forms.EmailField()

    def clean_username(self):
        username = self.cleaned_data['username']
        if User.objects.filter(username=username).exists():
            raise ValidationError('यह username पहले से लिया गया है')
        return username

    def clean_email(self):
        email = self.cleaned_data['email']
        if not email.endswith('@company.com'):
            raise ValidationError('Company email address ज़रूरी है')
        return email

    def clean(self):
        cleaned_data = super().clean()
        password = cleaned_data.get('password')
        confirm_password = cleaned_data.get('confirm_password')

        if password and confirm_password and password != confirm_password:
            self.add_error('confirm_password', 'Passwords match नहीं करते')

        return cleaned_data

form = SignupForm(request.POST)
if form.is_valid():
    pass
else:
    print(form.errors)

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