.. _ref-validators:

==========
Validators
==========

.. versionadded:: 1.2

Writing validators
==================

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

For example, lets write 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 then add this to your model fields via the field's ``validators``
argument::

    from django.db import models

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

Since 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])

How validators are run
======================

See the :ref:`form validation <ref-forms-validation>` for more information on
how validators are run in forms, and :ref:`Validating objects <validating-objects>`
for how they are run in models.

Built-in validators
===================

Django has a collection of callable validators for use with model and form
fields. Many of them are used internally, but they are available for use with
your own fields too. They can be used in addition to, or in lieu of custom
field.clean() methods.

``RegexValidator``
------------------

.. class:: RegexValidator(regex=None, message=None, code=None)

.. attribute:: regex=None

The regular expression to search for the provided ``value``. Raises a
``ValidationError`` if no match was found.

.. attribute:: code='invalid'

The error code to use if validation fails. Defaults to ``'invalid'``.

.. attribute:: message=None

The error message to use if ``regex`` doesn't match the provided ``value``.

``URLValidator``
----------------

.. class:: URLValidator(verify_exists=False, validator_user_agent=URL_VALIDATOR_USER_AGENT)

A ``RegexValidaor`` that ensures a value looks like a URL, and optionally
verifies that the URL actually exists. Raises an error code of ``'invalid'``
if it doesn't look like a URL, and a code of ``'invalid_link'`` if it doesn't
exist.

.. attribute:: verify_exists=False

If ``verify_exists`` is ``True``, this validator checks that the url actually
exists.

.. attribute:: validator_user_agent=URL_VALIDATOR_USER_AGENT

If ``verify_exists`` is ``True``, use validator_user_agent as the User-agent
for the request. Defaults to ``settings.URL_VALIDATOR_USER_AGENT``.

``validate_email``
------------------

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

``validate_slug``
-----------------

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

``validate_ipv4_address``
-------------------------

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

``validate_comma_separated_integer_list``
-----------------------------------------

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

``MaxValueValidator``
---------------------

.. class:: MaxValueValidator(max_value)

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

``MinValueValidator``
---------------------

.. class:: MinValueValidator(min_value)

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

``MaxLengthValidator``
----------------------

.. class:: MaxLengthValidator(max_length)

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

``MinLengthValidator``
----------------------

.. class:: MinLengthValidator(min_length)

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