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

.. versionadded:: 1.2
.. module:: django.core.validators
    :synopsis: Validation utilities and base classes

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])

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

See the :doc:`form validation </ref/forms/validation>` for more information on
how validators are run in forms, and :ref:`Validating objects
<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 :doc:`ModelForm documentation </topics/forms/modelforms>`
for information on how model validation interacts with forms.

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

Django has 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.

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

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

.. attribute:: regex

The regular expression pattern to search for the provided ``value``,
or a pre-compiled regular expression. Raises a ``ValidationError``
with ``message`` and ``code`` if no match is found.

.. attribute:: message=None

The error message used by ``ValidationError`` if validation fails. If no
``message`` is specified, a generic ``"Enter a valid value"`` message is used.

.. attribute:: code=None

The error code used by ``ValidationError`` if validation fails. If ``code``
is not specified, ``"invalid"`` is used.

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

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

A ``RegexValidator`` that ensures a value looks like a URL and optionally
verifies that the URL actually exists (i.e., doesn't return a 404 status code).
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``, it uses ``validator_user_agent`` as the "User-agent"
for the request. This defaults to ``settings.URL_VALIDATOR_USER_AGENT``.

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

A ``RegexValidator`` instance that ensures a value looks like an e-mail 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``.