# Django 1.8.18.dev20170123152445 documentation

# Validators

## 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
    from django.utils.translation import ugettext_lazy as _

    def validate_even(value):
        if value % 2 != 0:
            raise ValidationError(
                _('%(value)s is not an even number'),
                params={'value': 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.

### `RegexValidator`

class `RegexValidator`(regex=None, message=None, code=None, inverse_match=None, flags=0)

  Parameters: | 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.

  `regex`

    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).

  `message`

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

  `code`

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

  `inverse_match`

    The match mode for `regex`. Defaults to `False`.

  `flags`

    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.

### `EmailValidator`

class `EmailValidator`(message=None, code=None, whitelist=None)

  Parameters: | message – If not `None`, overrides `message`. code – If not `None`, overrides `code`. whitelist – If not `None`, overrides `whitelist`.

  `message`

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

  `code`

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

  `whitelist`

    Whitelist of email domains to allow. By default, a regular expression (the `domain_regex` attribute) is used to validate whatever appears after the @ sign. However, if that string appears in the whitelist, this validation is bypassed. If not provided, the default whitelist is `['localhost']`. Other domains that don’t contain a dot won’t pass validation, so you’d need to whitelist them as necessary.

### `URLValidator`

class `URLValidator`(schemes=None, regex=None, message=None, code=None)

  A `RegexValidator` that ensures a value looks like a URL, and raises an error code of `'invalid'` if it doesn’t.

  Loopback addresses and reserved IP spaces are considered valid. Literal IPv6 addresses (RFC 2732) and unicode domains are both supported.

  In addition to the optional arguments of its parent `RegexValidator` class, `URLValidator` accepts an extra optional attribute:

  `schemes`

    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.

  The optional `schemes` attribute was added.

  Support for IPv6 addresses, unicode domains, and URLs containing authentication data was added.

### `validate_email`

`validate_email`

  An `EmailValidator` instance without any customizations.

### `validate_slug`

`validate_slug`

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

### `validate_ipv4_address`

`validate_ipv4_address`

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

### `validate_ipv6_address`

`validate_ipv6_address`

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

### `validate_ipv46_address`

`validate_ipv46_address`

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

### `validate_comma_separated_integer_list`

`validate_comma_separated_integer_list`

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

### `MaxValueValidator`

class `MaxValueValidator`(max_value, message=None)

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

  The `message` parameter was added.

### `MinValueValidator`

class `MinValueValidator`(min_value, message=None)

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

  The `message` parameter was added.

### `MaxLengthValidator`

class `MaxLengthValidator`(max_length, message=None)

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

  The `message` parameter was added.

### `MinLengthValidator`

class `MinLengthValidator`(min_length, message=None)

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

  The `message` parameter was added.