Validation

Validate data with built-in rules and custom clean methods.

Syntaxdef clean_<field>(self): ...

Each field validates its own type automatically. Add custom rules with a clean_<field>() method (for one field) or clean() (for cross-field checks). Raise ValidationError to reject input.

Errors are collected and displayed next to the offending field.

Example

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)

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

When to use it

A developer adds a clean_email method to reject email addresses from a blocked domain list.
A form validates that the end_date is always after the start_date in the cross-field clean() method.
A developer uses validators=[MinLengthValidator(8)] on a password field to enforce a minimum length rule.

More examples

Field-level validation method

Uses a clean_<fieldname>() method to reject reserved usernames with a custom error message.

from django import forms

class RegistrationForm(forms.Form):
    username = forms.CharField()

    def clean_username(self):
        value = self.cleaned_data["username"]
        if value.lower() in ("admin", "root", "superuser"):
            raise forms.ValidationError("This username is reserved.")
        return value

Cross-field validation in clean()

Overrides clean() to compare two fields and raise an error if the end date is not after the start date.

class EventForm(forms.Form):
    start_date = forms.DateField()
    end_date = forms.DateField()

    def clean(self):
        cleaned = super().clean()
        start = cleaned.get("start_date")
        end = cleaned.get("end_date")
        if start and end and end <= start:
            raise forms.ValidationError("End date must be after start date.")
        return cleaned

Reusable validator function

Defines a standalone validator function and attaches it to a field via the validators list.

from django.core.exceptions import ValidationError

def no_spaces(value):
    if " " in value:
        raise ValidationError("Value may not contain spaces.")

class TagForm(forms.Form):
    name = forms.CharField(validators=[no_spaces])

Discussion

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