django/docs/ref/forms/fields.txt

===========
Form fields
===========

.. module:: django.forms.fields
   :synopsis: Django's built-in form fields.

.. currentmodule:: django.forms

.. class:: Field(**kwargs)

When you create a ``Form`` class, the most important part is defining the
fields of the form. Each field has custom validation logic, along with a few
other hooks.

.. method:: Field.clean(value)

Although the primary way you'll use ``Field`` classes is in ``Form`` classes,
you can also instantiate them and use them directly to get a better idea of
how they work. Each ``Field`` instance has a ``clean()`` method, which takes
a single argument and either raises a ``django.forms.ValidationError``
exception or returns the clean value::

    >>> from django import forms
    >>> f = forms.EmailField()
    >>> f.clean('foo@example.com')
    u'foo@example.com'
    >>> f.clean('invalid email address')
    Traceback (most recent call last):
    ...
    ValidationError: [u'Enter a valid email address.']

.. _core-field-arguments:

Core field arguments
--------------------

Each ``Field`` class constructor takes at least these arguments. Some
``Field`` classes take additional, field-specific arguments, but the following
should *always* be accepted:

``required``
~~~~~~~~~~~~

.. attribute:: Field.required

By default, each ``Field`` class assumes the value is required, so if you pass
an empty value -- either ``None`` or the empty string (``""``) -- then
``clean()`` will raise a ``ValidationError`` exception::

    >>> f = forms.CharField()
    >>> f.clean('foo')
    u'foo'
    >>> f.clean('')
    Traceback (most recent call last):
    ...
    ValidationError: [u'This field is required.']
    >>> f.clean(None)
    Traceback (most recent call last):
    ...
    ValidationError: [u'This field is required.']
    >>> f.clean(' ')
    u' '
    >>> f.clean(0)
    u'0'
    >>> f.clean(True)
    u'True'
    >>> f.clean(False)
    u'False'

To specify that a field is *not* required, pass ``required=False`` to the
``Field`` constructor::

    >>> f = forms.CharField(required=False)
    >>> f.clean('foo')
    u'foo'
    >>> f.clean('')
    u''
    >>> f.clean(None)
    u''
    >>> f.clean(0)
    u'0'
    >>> f.clean(True)
    u'True'
    >>> f.clean(False)
    u'False'

If a ``Field`` has ``required=False`` and you pass ``clean()`` an empty value,
then ``clean()`` will return a *normalized* empty value rather than raising
``ValidationError``. For ``CharField``, this will be a Unicode empty string.
For other ``Field`` classes, it might be ``None``. (This varies from field to
field.)

``label``
~~~~~~~~~

.. attribute:: Field.label

The ``label`` argument lets you specify the "human-friendly" label for this
field. This is used when the ``Field`` is displayed in a ``Form``.

As explained in "Outputting forms as HTML" above, the default label for a
``Field`` is generated from the field name by converting all underscores to
spaces and upper-casing the first letter. Specify ``label`` if that default
behavior doesn't result in an adequate label.

Here's a full example ``Form`` that implements ``label`` for two of its fields.
We've specified ``auto_id=False`` to simplify the output::

    >>> class CommentForm(forms.Form):
    ...     name = forms.CharField(label='Your name')
    ...     url = forms.URLField(label='Your Web site', required=False)
    ...     comment = forms.CharField()
    >>> f = CommentForm(auto_id=False)
    >>> print(f)
    <tr><th>Your name:</th><td><input type="text" name="name" /></td></tr>
    <tr><th>Your Web site:</th><td><input type="url" name="url" /></td></tr>
    <tr><th>Comment:</th><td><input type="text" name="comment" /></td></tr>

``initial``
~~~~~~~~~~~

.. attribute:: Field.initial

The ``initial`` argument lets you specify the initial value to use when
rendering this ``Field`` in an unbound ``Form``.

To specify dynamic initial data, see the :attr:`Form.initial` parameter.

The use-case for this is when you want to display an "empty" form in which a
field is initialized to a particular value. For example::

    >>> class CommentForm(forms.Form):
    ...     name = forms.CharField(initial='Your name')
    ...     url = forms.URLField(initial='http://')
    ...     comment = forms.CharField()
    >>> f = CommentForm(auto_id=False)
    >>> print(f)
    <tr><th>Name:</th><td><input type="text" name="name" value="Your name" /></td></tr>
    <tr><th>Url:</th><td><input type="url" name="url" value="http://" /></td></tr>
    <tr><th>Comment:</th><td><input type="text" name="comment" /></td></tr>

You may be thinking, why not just pass a dictionary of the initial values as
data when displaying the form? Well, if you do that, you'll trigger validation,
and the HTML output will include any validation errors::

    >>> class CommentForm(forms.Form):
    ...     name = forms.CharField()
    ...     url = forms.URLField()
    ...     comment = forms.CharField()
    >>> default_data = {'name': 'Your name', 'url': 'http://'}
    >>> f = CommentForm(default_data, auto_id=False)
    >>> print(f)
    <tr><th>Name:</th><td><input type="text" name="name" value="Your name" /></td></tr>
    <tr><th>Url:</th><td><ul class="errorlist"><li>Enter a valid URL.</li></ul><input type="url" name="url" value="http://" /></td></tr>
    <tr><th>Comment:</th><td><ul class="errorlist"><li>This field is required.</li></ul><input type="text" name="comment" /></td></tr>

This is why ``initial`` values are only displayed for unbound forms. For bound
forms, the HTML output will use the bound data.

Also note that ``initial`` values are *not* used as "fallback" data in
validation if a particular field's value is not given. ``initial`` values are
*only* intended for initial form display::

    >>> class CommentForm(forms.Form):
    ...     name = forms.CharField(initial='Your name')
    ...     url = forms.URLField(initial='http://')
    ...     comment = forms.CharField()
    >>> data = {'name': '', 'url': '', 'comment': 'Foo'}
    >>> f = CommentForm(data)
    >>> f.is_valid()
    False
    # The form does *not* fall back to using the initial values.
    >>> f.errors
    {'url': [u'This field is required.'], 'name': [u'This field is required.']}

Instead of a constant, you can also pass any callable::

    >>> import datetime
    >>> class DateForm(forms.Form):
    ...     day = forms.DateField(initial=datetime.date.today)
    >>> print(DateForm())
    <tr><th>Day:</th><td><input type="text" name="day" value="12/23/2008" /><td></tr>

The callable will be evaluated only when the unbound form is displayed, not when it is defined.

``widget``
~~~~~~~~~~

.. attribute:: Field.widget

The ``widget`` argument lets you specify a ``Widget`` class to use when
rendering this ``Field``. See :doc:`/ref/forms/widgets` for more information.

``help_text``
~~~~~~~~~~~~~

.. attribute:: Field.help_text

The ``help_text`` argument lets you specify descriptive text for this
``Field``. If you provide ``help_text``, it will be displayed next to the
``Field`` when the ``Field`` is rendered by one of the convenience ``Form``
methods (e.g., ``as_ul()``).

Here's a full example ``Form`` that implements ``help_text`` for two of its
fields. We've specified ``auto_id=False`` to simplify the output::

    >>> class HelpTextContactForm(forms.Form):
    ...     subject = forms.CharField(max_length=100, help_text='100 characters max.')
    ...     message = forms.CharField()
    ...     sender = forms.EmailField(help_text='A valid email address, please.')
    ...     cc_myself = forms.BooleanField(required=False)
    >>> f = HelpTextContactForm(auto_id=False)
    >>> print(f.as_table())
    <tr><th>Subject:</th><td><input type="text" name="subject" maxlength="100" /><br /><span class="helptext">100 characters max.</span></td></tr>
    <tr><th>Message:</th><td><input type="text" name="message" /></td></tr>
    <tr><th>Sender:</th><td><input type="email" name="sender" /><br />A valid email address, please.</td></tr>
    <tr><th>Cc myself:</th><td><input type="checkbox" name="cc_myself" /></td></tr>
    >>> print(f.as_ul()))
    <li>Subject: <input type="text" name="subject" maxlength="100" /> <span class="helptext">100 characters max.</span></li>
    <li>Message: <input type="text" name="message" /></li>
    <li>Sender: <input type="email" name="sender" /> A valid email address, please.</li>
    <li>Cc myself: <input type="checkbox" name="cc_myself" /></li>
    >>> print(f.as_p())
    <p>Subject: <input type="text" name="subject" maxlength="100" /> <span class="helptext">100 characters max.</span></p>
    <p>Message: <input type="text" name="message" /></p>
    <p>Sender: <input type="email" name="sender" /> A valid email address, please.</p>
    <p>Cc myself: <input type="checkbox" name="cc_myself" /></p>

``error_messages``
~~~~~~~~~~~~~~~~~~

.. attribute:: Field.error_messages

The ``error_messages`` argument lets you override the default messages that the
field will raise. Pass in a dictionary with keys matching the error messages you
want to override. For example, here is the default error message::

    >>> generic = forms.CharField()
    >>> generic.clean('')
    Traceback (most recent call last):
      ...
    ValidationError: [u'This field is required.']

And here is a custom error message::

    >>> name = forms.CharField(error_messages={'required': 'Please enter your name'})
    >>> name.clean('')
    Traceback (most recent call last):
      ...
    ValidationError: [u'Please enter your name']

In the `built-in Field classes`_ section below, each ``Field`` defines the
error message keys it uses.

``validators``
~~~~~~~~~~~~~~

.. attribute:: Field.validators

The ``validators`` argument lets you provide a list of validation functions
for this field.

See the :doc:`validators documentation </ref/validators>` for more information.

``localize``
~~~~~~~~~~~~

.. attribute:: Field.localize

The ``localize`` argument enables the localization of form data, input as well
as the rendered output.

See the :ref:`format localization <format-localization>` documentation for
more information.

.. _built-in fields:

Built-in ``Field`` classes
--------------------------

Naturally, the ``forms`` library comes with a set of ``Field`` classes that
represent common validation needs. This section documents each built-in field.

For each field, we describe the default widget used if you don't specify
``widget``. We also specify the value returned when you provide an empty value
(see the section on ``required`` above to understand what that means).

``BooleanField``
~~~~~~~~~~~~~~~~

.. class:: BooleanField(**kwargs)

    * Default widget: :class:`CheckboxInput`
    * Empty value: ``False``
    * Normalizes to: A Python ``True`` or ``False`` value.
    * Validates that the value is ``True`` (e.g. the check box is checked) if
      the field has ``required=True``.
    * Error message keys: ``required``

    .. note::

        Since all ``Field`` subclasses have ``required=True`` by default, the
        validation condition here is important. If you want to include a boolean
        in your form that can be either ``True`` or ``False`` (e.g. a checked or
        unchecked checkbox), you must remember to pass in ``required=False`` when
        creating the ``BooleanField``.

``CharField``
~~~~~~~~~~~~~

.. class:: CharField(**kwargs)

    * Default widget: :class:`TextInput`
    * Empty value: ``''`` (an empty string)
    * Normalizes to: A Unicode object.
    * Validates ``max_length`` or ``min_length``, if they are provided.
      Otherwise, all inputs are valid.
    * Error message keys: ``required``, ``max_length``, ``min_length``

    Has two optional arguments for validation:

    .. attribute:: max_length
    .. attribute:: min_length

    If provided, these arguments ensure that the string is at most or at least
    the given length.

``ChoiceField``
~~~~~~~~~~~~~~~

.. class:: ChoiceField(**kwargs)

    * Default widget: :class:`Select`
    * Empty value: ``''`` (an empty string)
    * Normalizes to: A Unicode object.
    * Validates that the given value exists in the list of choices.
    * Error message keys: ``required``, ``invalid_choice``

    The ``invalid_choice`` error message may contain ``%(value)s``, which will be
    replaced with the selected choice.

    Takes one extra required argument:

    .. attribute:: choices

        An iterable (e.g., a list or tuple) of 2-tuples to use as choices for this
        field. This argument accepts the same formats as the ``choices`` argument
        to a model field. See the :ref:`model field reference documentation on
        choices <field-choices>` for more details.

``TypedChoiceField``
~~~~~~~~~~~~~~~~~~~~

.. class:: TypedChoiceField(**kwargs)

    Just like a :class:`ChoiceField`, except :class:`TypedChoiceField` takes two
    extra arguments, ``coerce`` and ``empty_value``.

    * Default widget: :class:`Select`
    * Empty value: Whatever you've given as ``empty_value``
    * Normalizes to: A value of the type provided by the ``coerce`` argument.
    * Validates that the given value exists in the list of choices and can be
      coerced.
    * Error message keys: ``required``, ``invalid_choice``

    Takes extra arguments:

    .. attribute:: coerce

        A function that takes one argument and returns a coerced value. Examples
        include the built-in ``int``, ``float``, ``bool`` and other types. Defaults
        to an identity function.

    .. attribute:: empty_value

        The value to use to represent "empty." Defaults to the empty string;
        ``None`` is another common choice here. Note that this value will not be
        coerced by the function given in the ``coerce`` argument, so choose it
        accordingly.

``DateField``
~~~~~~~~~~~~~

.. class:: DateField(**kwargs)

    * Default widget: :class:`DateInput`
    * Empty value: ``None``
    * Normalizes to: A Python ``datetime.date`` object.
    * Validates that the given value is either a ``datetime.date``,
      ``datetime.datetime`` or string formatted in a particular date format.
    * Error message keys: ``required``, ``invalid``

    Takes one optional argument:

    .. attribute:: input_formats

        A list of formats used to attempt to convert a string to a valid
        ``datetime.date`` object.

    If no ``input_formats`` argument is provided, the default input formats are::

        ['%Y-%m-%d',       # '2006-10-25'
        '%m/%d/%Y',       # '10/25/2006'
        '%m/%d/%y']       # '10/25/06'

    Additionally, if you specify :setting:`USE_L10N=False<USE_L10N>` in your settings, the
    following will also be included in the default input formats::

        ['%b %d %Y',      # 'Oct 25 2006'
        '%b %d, %Y',      # 'Oct 25, 2006'
        '%d %b %Y',       # '25 Oct 2006'
        '%d %b, %Y',      # '25 Oct, 2006'
        '%B %d %Y',       # 'October 25 2006'
        '%B %d, %Y',      # 'October 25, 2006'
        '%d %B %Y',       # '25 October 2006'
        '%d %B, %Y']      # '25 October, 2006'

    See also :ref:`format localization <format-localization>`.

``DateTimeField``
~~~~~~~~~~~~~~~~~

.. class:: DateTimeField(**kwargs)

    * Default widget: :class:`DateTimeInput`
    * Empty value: ``None``
    * Normalizes to: A Python ``datetime.datetime`` object.
    * Validates that the given value is either a ``datetime.datetime``,
      ``datetime.date`` or string formatted in a particular datetime format.
    * Error message keys: ``required``, ``invalid``

    Takes one optional argument:

    .. attribute:: input_formats

        A list of formats used to attempt to convert a string to a valid
        ``datetime.datetime`` object.

    If no ``input_formats`` argument is provided, the default input formats are::

        ['%Y-%m-%d %H:%M:%S',    # '2006-10-25 14:30:59'
        '%Y-%m-%d %H:%M',        # '2006-10-25 14:30'
        '%Y-%m-%d',              # '2006-10-25'
        '%m/%d/%Y %H:%M:%S',     # '10/25/2006 14:30:59'
        '%m/%d/%Y %H:%M',        # '10/25/2006 14:30'
        '%m/%d/%Y',              # '10/25/2006'
        '%m/%d/%y %H:%M:%S',     # '10/25/06 14:30:59'
        '%m/%d/%y %H:%M',        # '10/25/06 14:30'
        '%m/%d/%y']              # '10/25/06'

    See also :ref:`format localization <format-localization>`.

``DecimalField``
~~~~~~~~~~~~~~~~

.. class:: DecimalField(**kwargs)

    * Default widget: :class:`NumberInput` when :attr:`Field.localize` is
      ``False``, else :class:`TextInput`.
    * Empty value: ``None``
    * Normalizes to: A Python ``decimal``.
    * Validates that the given value is a decimal. Leading and trailing
      whitespace is ignored.
    * Error message keys: ``required``, ``invalid``, ``max_value``,
      ``min_value``, ``max_digits``, ``max_decimal_places``,
      ``max_whole_digits``

    The ``max_value`` and ``min_value`` error messages may contain
    ``%(limit_value)s``, which will be substituted by the appropriate limit.

    .. versionchanged:: 1.6

        Similarly, the ``max_digits``, ``max_decimal_places`` and
        ``max_whole_digits`` error messages may contain ``%(max)s``.

    Takes four optional arguments:

    .. attribute:: max_value
    .. attribute:: min_value

        These control the range of values permitted in the field, and should be
        given as ``decimal.Decimal`` values.

    .. attribute:: max_digits

        The maximum number of digits (those before the decimal point plus those
        after the decimal point, with leading zeros stripped) permitted in the
        value.

    .. attribute:: decimal_places

        The maximum number of decimal places permitted.

``EmailField``
~~~~~~~~~~~~~~

.. class:: EmailField(**kwargs)

    * Default widget: :class:`EmailInput`
    * Empty value: ``''`` (an empty string)
    * Normalizes to: A Unicode object.
    * Validates that the given value is a valid email address, using a
      moderately complex regular expression.
    * Error message keys: ``required``, ``invalid``

    Has two optional arguments for validation, ``max_length`` and ``min_length``.
    If provided, these arguments ensure that the string is at most or at least the
    given length.

``FileField``
~~~~~~~~~~~~~

.. class:: FileField(**kwargs)

    * Default widget: :class:`ClearableFileInput`
    * Empty value: ``None``
    * Normalizes to: An ``UploadedFile`` object that wraps the file content
      and file name into a single object.
    * Can validate that non-empty file data has been bound to the form.
    * Error message keys: ``required``, ``invalid``, ``missing``, ``empty``,
      ``max_length``

    Has two optional arguments for validation, ``max_length`` and
    ``allow_empty_file``. If provided, these ensure that the file name is at
    most the given length, and that validation will succeed even if the file
    content is empty.

    To learn more about the ``UploadedFile`` object, see the :doc:`file uploads
    documentation </topics/http/file-uploads>`.

    When you use a ``FileField`` in a form, you must also remember to
    :ref:`bind the file data to the form <binding-uploaded-files>`.

    The ``max_length`` error refers to the length of the filename. In the error
    message for that key, ``%(max)d`` will be replaced with the maximum filename
    length and ``%(length)d`` will be replaced with the current filename length.

``FilePathField``
~~~~~~~~~~~~~~~~~

.. class:: FilePathField(**kwargs)

    * Default widget: :class:`Select`
    * Empty value: ``None``
    * Normalizes to: A unicode object
    * Validates that the selected choice exists in the list of choices.
    * Error message keys: ``required``, ``invalid_choice``

    The field allows choosing from files inside a certain directory. It takes three
    extra arguments; only ``path`` is required:

    .. attribute:: path

        The absolute path to the directory whose contents you want listed. This
        directory must exist.

    .. attribute:: recursive

        If ``False`` (the default) only the direct contents of ``path`` will be
        offered as choices. If ``True``, the directory will be descended into
        recursively and all descendants will be listed as choices.

    .. attribute:: match

        A regular expression pattern; only files with names matching this expression
        will be allowed as choices.

    .. attribute:: allow_files

        .. versionadded:: 1.5

        Optional.  Either ``True`` or ``False``.  Default is ``True``.  Specifies
        whether files in the specified location should be included.  Either this or
        :attr:`allow_folders` must be ``True``.

    .. attribute:: allow_folders

        .. versionadded:: 1.5

        Optional.  Either ``True`` or ``False``.  Default is ``False``.  Specifies
        whether folders in the specified location should be included.  Either this or
        :attr:`allow_files` must be ``True``.


``FloatField``
~~~~~~~~~~~~~~

.. class:: FloatField(**kwargs)

    * Default widget: :class:`NumberInput` when :attr:`Field.localize` is
      ``False``, else :class:`TextInput`.
    * Empty value: ``None``
    * Normalizes to: A Python float.
    * Validates that the given value is an float. Leading and trailing
      whitespace is allowed, as in Python's ``float()`` function.
    * Error message keys: ``required``, ``invalid``, ``max_value``,
      ``min_value``

    Takes two optional arguments for validation, ``max_value`` and ``min_value``.
    These control the range of values permitted in the field.

``ImageField``
~~~~~~~~~~~~~~

.. class:: ImageField(**kwargs)

    * Default widget: :class:`ClearableFileInput`
    * Empty value: ``None``
    * Normalizes to: An ``UploadedFile`` object that wraps the file content
      and file name into a single object.
    * Validates that file data has been bound to the form, and that the
      file is of an image format understood by Pillow/PIL.
    * Error message keys: ``required``, ``invalid``, ``missing``, ``empty``,
      ``invalid_image``

    Using an ``ImageField`` requires that either `Pillow`_ (recommended) or the
    `Python Imaging Library`_ (PIL) are installed and supports the image
    formats you use. If you encounter a ``corrupt image`` error when you
    upload an image, it usually means either Pillow or PIL
    doesn't understand its format. To fix this, install the appropriate
    library and reinstall Pillow or PIL.

    When you use an ``ImageField`` on a form, you must also remember to
    :ref:`bind the file data to the form <binding-uploaded-files>`.

.. _Pillow: http://python-imaging.github.io/Pillow/
.. _Python Imaging Library: http://www.pythonware.com/products/pil/

``IntegerField``
~~~~~~~~~~~~~~~~

.. class:: IntegerField(**kwargs)

    * Default widget: :class:`NumberInput` when :attr:`Field.localize` is
      ``False``, else :class:`TextInput`.
    * Empty value: ``None``
    * Normalizes to: A Python integer or long integer.
    * Validates that the given value is an integer. Leading and trailing
      whitespace is allowed, as in Python's ``int()`` function.
    * Error message keys: ``required``, ``invalid``, ``max_value``,
      ``min_value``

    The ``max_value`` and ``min_value`` error messages may contain
    ``%(limit_value)s``, which will be substituted by the appropriate limit.

    Takes two optional arguments for validation:

    .. attribute:: max_value
    .. attribute:: min_value

    These control the range of values permitted in the field.

``IPAddressField``
~~~~~~~~~~~~~~~~~~

.. class:: IPAddressField(**kwargs)

    * Default widget: :class:`TextInput`
    * Empty value: ``''`` (an empty string)
    * Normalizes to: A Unicode object.
    * Validates that the given value is a valid IPv4 address, using a regular
      expression.
    * Error message keys: ``required``, ``invalid``

``GenericIPAddressField``
~~~~~~~~~~~~~~~~~~~~~~~~~

.. class:: GenericIPAddressField(**kwargs)

    A field containing either an IPv4 or an IPv6 address.

    * Default widget: :class:`TextInput`
    * Empty value: ``''`` (an empty string)
    * Normalizes to: A Unicode object. IPv6 addresses are
      normalized as described below.
    * Validates that the given value is a valid IP address.
    * Error message keys: ``required``, ``invalid``

    The IPv6 address normalization follows :rfc:`4291#section-2.2` section 2.2,
    including using the IPv4 format suggested in paragraph 3 of that section, like
    ``::ffff:192.0.2.0``. For example, ``2001:0::0:01`` would be normalized to
    ``2001::1``, and ``::ffff:0a0a:0a0a`` to ``::ffff:10.10.10.10``. All characters
    are converted to lowercase.

    Takes two optional arguments:

    .. attribute:: protocol

        Limits valid inputs to the specified protocol.
        Accepted values are ``both`` (default), ``IPv4``
        or ``IPv6``. Matching is case insensitive.

    .. attribute:: unpack_ipv4

        Unpacks IPv4 mapped addresses like ``::ffff:192.0.2.1``.
        If this option is enabled that address would be unpacked to
        ``192.0.2.1``. Default is disabled. Can only be used
        when ``protocol`` is set to ``'both'``.

``MultipleChoiceField``
~~~~~~~~~~~~~~~~~~~~~~~

.. class:: MultipleChoiceField(**kwargs)

    * Default widget: :class:`SelectMultiple`
    * Empty value: ``[]`` (an empty list)
    * Normalizes to: A list of Unicode objects.
    * Validates that every value in the given list of values exists in the list
      of choices.
    * Error message keys: ``required``, ``invalid_choice``, ``invalid_list``

    The ``invalid_choice`` error message may contain ``%(value)s``, which will be
    replaced with the selected choice.

    Takes one extra required argument, ``choices``, as for ``ChoiceField``.

``TypedMultipleChoiceField``
~~~~~~~~~~~~~~~~~~~~~~~~~~~~

.. class:: TypedMultipleChoiceField(**kwargs)

    Just like a :class:`MultipleChoiceField`, except :class:`TypedMultipleChoiceField`
    takes two extra arguments, ``coerce`` and ``empty_value``.

    * Default widget: :class:`SelectMultiple`
    * Empty value: Whatever you've given as ``empty_value``
    * Normalizes to: A list of values of the type provided by the ``coerce``
      argument.
    * Validates that the given values exists in the list of choices and can be
      coerced.
    * Error message keys: ``required``, ``invalid_choice``

    The ``invalid_choice`` error message may contain ``%(value)s``, which will be
    replaced with the selected choice.

    Takes two extra arguments, ``coerce`` and ``empty_value``, as for ``TypedChoiceField``.

``NullBooleanField``
~~~~~~~~~~~~~~~~~~~~

.. class:: NullBooleanField(**kwargs)

    * Default widget: :class:`NullBooleanSelect`
    * Empty value: ``None``
    * Normalizes to: A Python ``True``, ``False`` or ``None`` value.
    * Validates nothing (i.e., it never raises a ``ValidationError``).

``RegexField``
~~~~~~~~~~~~~~

.. class:: RegexField(**kwargs)

    * Default widget: :class:`TextInput`
    * Empty value: ``''`` (an empty string)
    * Normalizes to: A Unicode object.
    * Validates that the given value matches against a certain regular
      expression.
    * Error message keys: ``required``, ``invalid``

    Takes one required argument:

    .. attribute:: regex

        A regular expression specified either as a string or a compiled regular
        expression object.

    Also takes ``max_length`` and ``min_length``, which work just as they do for
    ``CharField``.

    The optional argument ``error_message`` is also accepted for backwards
    compatibility. The preferred way to provide an error message is to use the
    ``error_messages`` argument, passing a dictionary with ``'invalid'`` as a key
    and the error message as the value.

``SlugField``
~~~~~~~~~~~~~

.. class:: SlugField(**kwargs)

   * Default widget: :class:`TextInput`
   * Empty value: ``''`` (an empty string)
   * Normalizes to: A Unicode object.
   * Validates that the given value contains only letters, numbers,
     underscores, and hyphens.
   * Error messages: ``required``, ``invalid``

   This field is intended for use in representing a model
   :class:`~django.db.models.SlugField` in forms.

``TimeField``
~~~~~~~~~~~~~

.. class:: TimeField(**kwargs)

    * Default widget: :class:`TextInput`
    * Empty value: ``None``
    * Normalizes to: A Python ``datetime.time`` object.
    * Validates that the given value is either a ``datetime.time`` or string
      formatted in a particular time format.
    * Error message keys: ``required``, ``invalid``

    Takes one optional argument:

    .. attribute:: input_formats

        A list of formats used to attempt to convert a string to a valid
        ``datetime.time`` object.

    If no ``input_formats`` argument is provided, the default input formats are::

        '%H:%M:%S',     # '14:30:59'
        '%H:%M',        # '14:30'

``URLField``
~~~~~~~~~~~~

.. class:: URLField(**kwargs)

    * Default widget: :class:`URLInput`
    * Empty value: ``''`` (an empty string)
    * Normalizes to: A Unicode object.
    * Validates that the given value is a valid URL.
    * Error message keys: ``required``, ``invalid``

    Takes the following optional arguments:

    .. attribute:: max_length
    .. attribute:: min_length

    These are the same as ``CharField.max_length`` and ``CharField.min_length``.


Slightly complex built-in ``Field`` classes
-------------------------------------------

``ComboField``
~~~~~~~~~~~~~~

.. class:: ComboField(**kwargs)

    * Default widget: :class:`TextInput`
    * Empty value: ``''`` (an empty string)
    * Normalizes to: A Unicode object.
    * Validates that the given value against each of the fields specified
      as an argument to the ``ComboField``.
    * Error message keys: ``required``, ``invalid``

    Takes one extra required argument:

    .. attribute:: fields

        The list of fields that should be used to validate the field's value (in
        the order in which they are provided).

            >>> f = ComboField(fields=[CharField(max_length=20), EmailField()])
            >>> f.clean('test@example.com')
            u'test@example.com'
            >>> f.clean('longemailaddress@example.com')
            Traceback (most recent call last):
            ...
            ValidationError: [u'Ensure this value has at most 20 characters (it has 28).']

``MultiValueField``
~~~~~~~~~~~~~~~~~~~

.. class:: MultiValueField(fields=(), **kwargs)

    * Default widget: :class:`TextInput`
    * Empty value: ``''`` (an empty string)
    * Normalizes to: the type returned by the ``compress`` method of the subclass.
    * Validates that the given value against each of the fields specified
      as an argument to the ``MultiValueField``.
    * Error message keys: ``required``, ``invalid``

    Aggregates the logic of multiple fields that together produce a single
    value.

    This field is abstract and must be subclassed. In contrast with the
    single-value fields, subclasses of :class:`MultiValueField` must not
    implement :meth:`~django.forms.Field.clean` but instead - implement
    :meth:`~MultiValueField.compress`.

    Takes one extra required argument:

    .. attribute:: fields

        A tuple of fields whose values are cleaned and subsequently combined
        into a single value.  Each value of the field is cleaned by the
        corresponding field in ``fields`` -- the first value is cleaned by the
        first field, the second value is cleaned by the second field, etc.
        Once all fields are cleaned, the list of clean values is combined into
        a single value by :meth:`~MultiValueField.compress`.

    .. attribute:: MultiValueField.widget

        Must be a subclass of :class:`django.forms.MultiWidget`.
        Default value is :class:`~django.forms.TextInput`, which
        probably is not very useful in this case.

    .. method:: compress(data_list)

        Takes a list of valid values and returns  a "compressed" version of
        those values -- in a single value. For example,
        :class:`SplitDateTimeField` is a subclass which combines a time field
        and a date field into a ``datetime`` object.

        This method must be implemented in the subclasses.

``SplitDateTimeField``
~~~~~~~~~~~~~~~~~~~~~~

.. class:: SplitDateTimeField(**kwargs)

    * Default widget: :class:`SplitDateTimeWidget`
    * Empty value: ``None``
    * Normalizes to: A Python ``datetime.datetime`` object.
    * Validates that the given value is a ``datetime.datetime`` or string
      formatted in a particular datetime format.
    * Error message keys: ``required``, ``invalid``, ``invalid_date``,
      ``invalid_time``

    Takes two optional arguments:

    .. attribute:: input_date_formats

        A list of formats used to attempt to convert a string to a valid
        ``datetime.date`` object.

    If no ``input_date_formats`` argument is provided, the default input formats
    for ``DateField`` are used.

    .. attribute:: input_time_formats

        A list of formats used to attempt to convert a string to a valid
        ``datetime.time`` object.

    If no ``input_time_formats`` argument is provided, the default input formats
    for ``TimeField`` are used.

Fields which handle relationships
---------------------------------

Two fields are available for representing relationships between
models: :class:`ModelChoiceField` and
:class:`ModelMultipleChoiceField`.  Both of these fields require a
single ``queryset`` parameter that is used to create the choices for
the field.  Upon form validation, these fields will place either one
model object (in the case of ``ModelChoiceField``) or multiple model
objects (in the case of ``ModelMultipleChoiceField``) into the
``cleaned_data`` dictionary of the form.

``ModelChoiceField``
~~~~~~~~~~~~~~~~~~~~

.. class:: ModelChoiceField(**kwargs)

    * Default widget: :class:`Select`
    * Empty value: ``None``
    * Normalizes to: A model instance.
    * Validates that the given id exists in the queryset.
    * Error message keys: ``required``, ``invalid_choice``

    Allows the selection of a single model object, suitable for representing a
    foreign key. Note that the default widget for ``ModelChoiceField`` becomes
    impractical when the number of entries increases. You should avoid using it
    for more than 100 items.

    A single argument is required:

    .. attribute:: queryset

        A ``QuerySet`` of model objects from which the choices for the
        field will be derived, and which will be used to validate the
        user's selection.

    ``ModelChoiceField`` also takes one optional argument:

    .. attribute:: empty_label

        By default the ``<select>`` widget used by ``ModelChoiceField`` will have an
        empty choice at the top of the list. You can change the text of this
        label (which is ``"---------"`` by default) with the ``empty_label``
        attribute, or you can disable the empty label entirely by setting
        ``empty_label`` to ``None``::

            # A custom empty label
            field1 = forms.ModelChoiceField(queryset=..., empty_label="(Nothing)")

            # No empty label
            field2 = forms.ModelChoiceField(queryset=..., empty_label=None)

        Note that if a ``ModelChoiceField`` is required and has a default
        initial value, no empty choice is created (regardless of the value
        of ``empty_label``).

    The ``__unicode__`` method of the model will be called to generate
    string representations of the objects for use in the field's choices;
    to provide customized representations, subclass ``ModelChoiceField``
    and override ``label_from_instance``. This method will receive a model
    object, and should return a string suitable for representing it. For
    example::

        class MyModelChoiceField(ModelChoiceField):
            def label_from_instance(self, obj):
                return "My Object #%i" % obj.id

``ModelMultipleChoiceField``
~~~~~~~~~~~~~~~~~~~~~~~~~~~~

.. class:: ModelMultipleChoiceField(**kwargs)

    * Default widget: :class:`SelectMultiple`
    * Empty value: An empty ``QuerySet`` (self.queryset.none())
    * Normalizes to: A ``QuerySet`` of model instances.
    * Validates that every id in the given list of values exists in the
      queryset.
    * Error message keys: ``required``, ``list``, ``invalid_choice``,
      ``invalid_pk_value``

    .. versionchanged:: 1.5

        The empty and normalized values were changed to be consistently
        ``QuerySets`` instead of ``[]`` and ``QuerySet`` respectively.

    .. versionchanged:: 1.6

        The ``invalid_choice`` message may contain ``%(value)s`` and the
        ``invalid_pk_value`` message may contain ``%(pk)s``, which will be
        substituted by the appropriate values.

    Allows the selection of one or more model objects, suitable for
    representing a many-to-many relation. As with :class:`ModelChoiceField`,
    you can use ``label_from_instance`` to customize the object
    representations, and ``queryset`` is a required parameter:

    .. attribute:: queryset

        A ``QuerySet`` of model objects from which the choices for the
        field will be derived, and which will be used to validate the
        user's selection.

Creating custom fields
----------------------

If the built-in ``Field`` classes don't meet your needs, you can easily create
custom ``Field`` classes. To do this, just create a subclass of
``django.forms.Field``. Its only requirements are that it implement a
``clean()`` method and that its ``__init__()`` method accept the core arguments
mentioned above (``required``, ``label``, ``initial``, ``widget``,
``help_text``).