Writing validators

A validator is a callable that takes a value and raises a ValidationError if it doesn’t meet some criteria. Validators can be useful for re-using validation logic between different types of fields.

For example, here’s a validator that only allows even numbers:

from django.core.exceptions import ValidationError

def validate_even(value):
    if value % 2 != 0:
        raise ValidationError(u'%s is not an even number' % value)

You can add this to a model field via the field’s validators argument:

from django.db import models

class MyModel(models.Model):
    even_field = models.IntegerField(validators=[validate_even])

Because values are converted to Python before validators are run, you can even use the same validator with forms:

from django import forms

class MyForm(forms.Form):
    even_field = forms.IntegerField(validators=[validate_even])

You can also use a class with a __call__() method for more complex or configurable validators. RegexValidator, for example, uses this technique. If a class-based validator is used in the validators model field option, you should make sure it is serializable by the migration framework by adding deconstruct() and __eq__() methods.

How validators are run

See the form validation for more information on how validators are run in forms, and Validating objects for how they’re run in models. Note that validators will not be run automatically when you save a model, but if you are using a ModelForm, it will run your validators on any fields that are included in your form. See the ModelForm documentation for information on how model validation interacts with forms.

Built-in validators

The django.core.validators module contains a collection of callable validators for use with model and form fields. They’re used internally but are available for use with your own fields, too. They can be used in addition to, or in lieu of custom field.clean() methods.


class RegexValidator([regex=None, message=None, code=None, inverse_match=None, flags=0])[source]
  • regex – If not None, overrides regex. Can be a regular expression string or a pre-compiled regular expression.
  • message – If not None, overrides message.
  • code – If not None, overrides code.
  • inverse_match – If not None, overrides inverse_match.
  • flags – If not None, overrides flags. In that case, regex must be a regular expression string, or TypeError is raised.

The regular expression pattern to search for the provided value, or a pre-compiled regular expression. By default, raises a ValidationError with message and code if a match is not found. That standard behavior can be reversed by setting inverse_match to True, in which case the ValidationError is raised when a match is found. By default, matches any string (including an empty string).


The error message used by ValidationError if validation fails. Defaults to "Enter a valid value".


The error code used by ValidationError if validation fails. Defaults to "invalid".

New in Django 1.7.

The match mode for regex. Defaults to False.

New in Django 1.7.

The flags used when compiling the regular expression string regex. If regex is a pre-compiled regular expression, and flags is overridden, TypeError is raised. Defaults to 0.


class URLValidator([schemes=None, regex=None, message=None, code=None])[source]

A RegexValidator that ensures a value looks like a URL, and raises an error code of 'invalid' if it doesn’t. In addition to the optional arguments of its parent RegexValidator class, URLValidator accepts an extra optional attribute:


URL/URI scheme list to validate against. If not provided, the default list is ['http', 'https', 'ftp', 'ftps']. As a reference, the IANA Web site provides a full list of valid URI schemes.

Changed in Django 1.7:

The optional schemes attribute was added.



An EmailValidator instance that ensures a value looks like an email address.



A RegexValidator instance that ensures a value consists of only letters, numbers, underscores or hyphens.



A RegexValidator instance that ensures a value looks like an IPv4 address.



Uses django.utils.ipv6 to check the validity of an IPv6 address.



Uses both validate_ipv4_address and validate_ipv6_address to ensure a value is either a valid IPv4 or IPv6 address.



A RegexValidator instance that ensures a value is a comma-separated list of integers.


class MaxValueValidator(max_value)[source]

Raises a ValidationError with a code of 'max_value' if value is greater than max_value.


class MinValueValidator(min_value)[source]

Raises a ValidationError with a code of 'min_value' if value is less than min_value.


class MaxLengthValidator(max_length)[source]

Raises a ValidationError with a code of 'max_length' if the length of value is greater than max_length.


class MinLengthValidator(min_length)[source]

Raises a ValidationError with a code of 'min_length' if the length of value is less than min_length.