===========
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.core.exceptions.ValidationError`` exception or returns the clean
value:

.. code-block:: pycon

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

.. code-block:: pycon

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

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

.. code-block:: pycon

    >>> f = forms.CharField(required=False)
    >>> f.clean("foo")
    'foo'
    >>> f.clean("")
    ''
    >>> f.clean(None)
    ''
    >>> f.clean(0)
    '0'
    >>> f.clean(True)
    'True'
    >>> f.clean(False)
    '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 return
:attr:`~CharField.empty_value` which defaults to an empty string. For other
``Field`` classes, it might be ``None``. (This varies from field to field.)

Widgets of required form fields have the ``required`` HTML attribute. Set the
:attr:`Form.use_required_attribute` attribute to ``False`` to disable it. The
``required`` attribute isn't included on forms of formsets because the browser
validation may not be correct when adding and deleting formsets.

``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:

.. code-block:: pycon

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

``label_suffix``
----------------

.. attribute:: Field.label_suffix

The ``label_suffix`` argument lets you override the form's
:attr:`~django.forms.Form.label_suffix` on a per-field basis:

.. code-block:: pycon

    >>> class ContactForm(forms.Form):
    ...     age = forms.IntegerField()
    ...     nationality = forms.CharField()
    ...     captcha_answer = forms.IntegerField(label="2 + 2", label_suffix=" =")
    ...
    >>> f = ContactForm(label_suffix="?")
    >>> print(f.as_p())
    <p><label for="id_age">Age?</label> <input id="id_age" name="age" type="number" required></p>
    <p><label for="id_nationality">Nationality?</label> <input id="id_nationality" name="nationality" type="text" required></p>
    <p><label for="id_captcha_answer">2 + 2 =</label> <input id="id_captcha_answer" name="captcha_answer" type="number" required></p>

``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:

.. code-block:: pycon

    >>> from django import forms
    >>> 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" required></td></tr>
    <tr><th>Url:</th><td><input type="url" name="url" value="http://" required></td></tr>
    <tr><th>Comment:</th><td><input type="text" name="comment" required></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:

.. code-block:: pycon

    >>> 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" required></td></tr>
    <tr><th>Url:</th><td><ul class="errorlist"><li>Enter a valid URL.</li></ul><input type="url" name="url" value="http://" required></td></tr>
    <tr><th>Comment:</th><td><ul class="errorlist"><li>This field is required.</li></ul><input type="text" name="comment" required></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:

.. code-block:: pycon

    >>> 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': ['This field is required.'], 'name': ['This field is required.']}

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

.. code-block:: pycon

    >>> 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" required><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()``).

Like the model field's :attr:`~django.db.models.Field.help_text`, this value
isn't HTML-escaped in automatically-generated forms.

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:

.. code-block:: pycon

    >>> from django import forms
    >>> 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" required><br><span class="helptext">100 characters max.</span></td></tr>
    <tr><th>Message:</th><td><input type="text" name="message" required></td></tr>
    <tr><th>Sender:</th><td><input type="email" name="sender" required><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" required> <span class="helptext">100 characters max.</span></li>
    <li>Message: <input type="text" name="message" required></li>
    <li>Sender: <input type="email" name="sender" required> 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" required> <span class="helptext">100 characters max.</span></p>
    <p>Message: <input type="text" name="message" required></p>
    <p>Sender: <input type="email" name="sender" required> 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:

.. code-block:: pycon

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

And here is a custom error message:

.. code-block:: pycon

    >>> name = forms.CharField(error_messages={"required": "Please enter your name"})
    >>> name.clean("")
    Traceback (most recent call last):
      ...
    ValidationError: ['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 :doc:`format localization </topics/i18n/formatting>` documentation for
more information.

``disabled``
------------

.. attribute:: Field.disabled

The ``disabled`` boolean argument, when set to ``True``, disables a form field
using the ``disabled`` HTML attribute so that it won't be editable by users.
Even if a user tampers with the field's value submitted to the server, it will
be ignored in favor of the value from the form's initial data.

Checking if the field data has changed
======================================

``has_changed()``
-----------------

.. method:: Field.has_changed()

The ``has_changed()`` method is used to determine if the field value has changed
from the initial value. Returns ``True`` or ``False``.

See the :class:`Form.has_changed()` 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: Whatever you've given as :attr:`empty_value`.
    * Normalizes to: A string.
    * Uses :class:`~django.core.validators.MaxLengthValidator` and
      :class:`~django.core.validators.MinLengthValidator` if ``max_length`` and
      ``min_length`` are provided. Otherwise, all inputs are valid.
    * Error message keys: ``required``, ``max_length``, ``min_length``

    Has the following 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.

    .. attribute:: strip

       If ``True`` (default), the value will be stripped of leading and
       trailing whitespace.

    .. attribute:: empty_value

       The value to use to represent "empty". Defaults to an empty string.

``ChoiceField``
---------------

.. class:: ChoiceField(**kwargs)

    * Default widget: :class:`Select`
    * Empty value: ``''`` (an empty string)
    * Normalizes to: A string.
    * 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 argument:

    .. attribute:: choices

        Either an :term:`iterable` of 2-tuples to use as choices for this
        field, :ref:`enumeration <field-choices-enum-types>` choices, or a
        callable that returns such an iterable. 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. If the argument is a callable, it is evaluated each
        time the field's form is initialized, in addition to during rendering.
        Defaults to an empty list.

    .. admonition:: Choice type

        This field normalizes choices to strings, so if choices are required in
        other data types, such as integers or booleans, consider using
        :class:`TypedChoiceField` instead.

``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

        An iterable 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
    taken from :setting:`DATE_INPUT_FORMATS` if :setting:`USE_L10N` is
    ``False``, or from the active locale format ``DATE_INPUT_FORMATS`` key if
    localization is enabled. See also :doc:`format localization
    </topics/i18n/formatting>`.

``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

        An iterable of formats used to attempt to convert a string to a valid
        ``datetime.datetime`` object, in addition to ISO 8601 formats.

    The field always accepts strings in ISO 8601 formatted dates or similar
    recognized by :func:`~django.utils.dateparse.parse_datetime`. Some examples
    are:

    * ``'2006-10-25 14:30:59'``
    * ``'2006-10-25T14:30:59'``
    * ``'2006-10-25 14:30'``
    * ``'2006-10-25T14:30'``
    * ``'2006-10-25T14:30Z'``
    * ``'2006-10-25T14:30+02:00'``
    * ``'2006-10-25'``

    If no ``input_formats`` argument is provided, the default input formats are
    taken from :setting:`DATETIME_INPUT_FORMATS` and
    :setting:`DATE_INPUT_FORMATS` if :setting:`USE_L10N` is ``False``, or from
    the active locale format ``DATETIME_INPUT_FORMATS`` and
    ``DATE_INPUT_FORMATS`` keys if localization is enabled. See also
    :doc:`format localization </topics/i18n/formatting>`.

``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. Uses
      :class:`~django.core.validators.MaxValueValidator` and
      :class:`~django.core.validators.MinValueValidator` if ``max_value`` and
      ``min_value`` are provided. Uses
      :class:`~django.core.validators.StepValueValidator` if ``step_size`` is
      provided. Leading and trailing whitespace is ignored.
    * Error message keys: ``required``, ``invalid``, ``max_value``,
      ``min_value``, ``max_digits``, ``max_decimal_places``,
      ``max_whole_digits``, ``step_size``.

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

    Takes five 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.

    .. attribute:: step_size

        Limit valid inputs to an integral multiple of ``step_size``.

    .. versionchanged:: 4.1

        The ``step_size`` argument was added.

``DurationField``
-----------------

.. class:: DurationField(**kwargs)

    * Default widget: :class:`TextInput`
    * Empty value: ``None``
    * Normalizes to: A Python :class:`~python:datetime.timedelta`.
    * Validates that the given value is a string which can be converted into a
      ``timedelta``. The value must be between :attr:`datetime.timedelta.min`
      and :attr:`datetime.timedelta.max`.
    * Error message keys: ``required``, ``invalid``, ``overflow``.

    Accepts any format understood by
    :func:`~django.utils.dateparse.parse_duration`.

``EmailField``
--------------

.. class:: EmailField(**kwargs)

    * Default widget: :class:`EmailInput`
    * Empty value: Whatever you've given as ``empty_value``.
    * Normalizes to: A string.
    * Uses :class:`~django.core.validators.EmailValidator` to validate that
      the given value is a valid email address, using a moderately complex
      regular expression.
    * Error message keys: ``required``, ``invalid``

    Has the optional arguments ``max_length``, ``min_length``, and
    ``empty_value`` which work just as they do for :class:`CharField`. The
    ``max_length`` argument defaults to 320 (see :rfc:`3696#section-3`).

    .. versionchanged:: 3.2.20

        The default value for ``max_length`` was changed to 320 characters.

``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 the 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: ``''`` (an empty string)
    * Normalizes to: A string.
    * 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 five
    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

        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

        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 a float. Uses
      :class:`~django.core.validators.MaxValueValidator` and
      :class:`~django.core.validators.MinValueValidator` if ``max_value`` and
      ``min_value`` are provided. Uses
      :class:`~django.core.validators.StepValueValidator` if ``step_size`` is
      provided. Leading and trailing whitespace is allowed, as in Python's
      ``float()`` function.
    * Error message keys: ``required``, ``invalid``, ``max_value``,
      ``min_value``, ``step_size``.

    Takes three optional arguments:

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

        These control the range of values permitted in the field.

    .. attribute:: step_size

        .. versionadded:: 4.1

        Limit valid inputs to an integral multiple of ``step_size``.

``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 string. 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'``.

``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. Also uses
      :class:`~django.core.validators.FileExtensionValidator` to validate that
      the file extension is supported by Pillow.
    * Error message keys: ``required``, ``invalid``, ``missing``, ``empty``,
      ``invalid_image``

    Using an ``ImageField`` requires that `Pillow`_ is installed with support
    for the image formats you use. If you encounter a ``corrupt image`` error
    when you upload an image, it usually means that Pillow doesn't understand
    its format. To fix this, install the appropriate library and reinstall
    Pillow.

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

    After the field has been cleaned and validated, the ``UploadedFile``
    object will have an additional ``image`` attribute containing the Pillow
    `Image`_ instance used to check if the file was a valid image. Pillow
    closes the underlying file descriptor after verifying an image, so while
    non-image data attributes, such as ``format``, ``height``, and ``width``,
    are available, methods that access the underlying image data, such as
    ``getdata()`` or ``getpixel()``, cannot be used without reopening the file.
    For example:

    .. code-block:: pycon

        >>> from PIL import Image
        >>> from django import forms
        >>> from django.core.files.uploadedfile import SimpleUploadedFile
        >>> class ImageForm(forms.Form):
        ...     img = forms.ImageField()
        ...
        >>> file_data = {"img": SimpleUploadedFile("test.png", b"file data")}
        >>> form = ImageForm({}, file_data)
        # Pillow closes the underlying file descriptor.
        >>> form.is_valid()
        True
        >>> image_field = form.cleaned_data["img"]
        >>> image_field.image
        <PIL.PngImagePlugin.PngImageFile image mode=RGBA size=191x287 at 0x7F5985045C18>
        >>> image_field.image.width
        191
        >>> image_field.image.height
        287
        >>> image_field.image.format
        'PNG'
        >>> image_field.image.getdata()
        # Raises AttributeError: 'NoneType' object has no attribute 'seek'.
        >>> image = Image.open(image_field)
        >>> image.getdata()
        <ImagingCore object at 0x7f5984f874b0>

    Additionally, ``UploadedFile.content_type`` will be updated with the
    image's content type if Pillow can determine it, otherwise it will be set
    to ``None``.

.. _Pillow: https://pillow.readthedocs.io/en/latest/
.. _Image: https://pillow.readthedocs.io/en/latest/reference/Image.html

``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.
    * Validates that the given value is an integer. Uses
      :class:`~django.core.validators.MaxValueValidator` and
      :class:`~django.core.validators.MinValueValidator` if ``max_value`` and
      ``min_value`` are provided. Uses
      :class:`~django.core.validators.StepValueValidator` if ``step_size`` is
      provided. Leading and trailing whitespace is allowed, as in Python's
      ``int()`` function.
    * Error message keys: ``required``, ``invalid``, ``max_value``,
      ``min_value``, ``step_size``

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

    Takes three optional arguments for validation:

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

        These control the range of values permitted in the field.

    .. attribute:: step_size

        .. versionadded:: 4.1

        Limit valid inputs to an integral multiple of ``step_size``.

``JSONField``
-------------

.. class:: JSONField(encoder=None, decoder=None, **kwargs)

    A field which accepts JSON encoded data for a
    :class:`~django.db.models.JSONField`.

    * Default widget: :class:`Textarea`
    * Empty value: ``None``
    * Normalizes to: A Python representation of the JSON value (usually as a
      ``dict``, ``list``, or ``None``), depending on :attr:`JSONField.decoder`.
    * Validates that the given value is a valid JSON.
    * Error message keys: ``required``, ``invalid``

    Takes two optional arguments:

    .. attribute:: encoder

        A :py:class:`json.JSONEncoder` subclass to serialize data types not
        supported by the standard JSON serializer (e.g. ``datetime.datetime``
        or :class:`~python:uuid.UUID`). For example, you can use the
        :class:`~django.core.serializers.json.DjangoJSONEncoder` class.

        Defaults to ``json.JSONEncoder``.

    .. attribute:: decoder

        A :py:class:`json.JSONDecoder` subclass to deserialize the input. Your
        deserialization may need to account for the fact that you can't be
        certain of the input type. For example, you run the risk of returning a
        ``datetime`` that was actually a string that just happened to be in the
        same format chosen for ``datetime``\s.

        The ``decoder`` can be used to validate the input. If
        :py:class:`json.JSONDecodeError` is raised during the deserialization,
        a ``ValidationError`` will be raised.

        Defaults to ``json.JSONDecoder``.

    .. note::

        If you use a :class:`ModelForm <django.forms.ModelForm>`, the
        ``encoder`` and ``decoder`` from :class:`~django.db.models.JSONField`
        will be used.

    .. admonition:: User friendly forms

        ``JSONField`` is not particularly user friendly in most cases. However,
        it is a useful way to format data from a client-side widget for
        submission to the server.

``MultipleChoiceField``
-----------------------

.. class:: MultipleChoiceField(**kwargs)

    * Default widget: :class:`SelectMultiple`
    * Empty value: ``[]`` (an empty list)
    * Normalizes to: A list of strings.
    * 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 :class:`ChoiceField`.

``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``).

    ``NullBooleanField`` may be used with widgets such as
    :class:`~django.forms.Select` or :class:`~django.forms.RadioSelect`
    by providing the widget ``choices``::

        NullBooleanField(
            widget=Select(
                choices=[
                    ("", "Unknown"),
                    (True, "Yes"),
                    (False, "No"),
                ]
            )
        )

``RegexField``
--------------

.. class:: RegexField(**kwargs)

    * Default widget: :class:`TextInput`
    * Empty value: Whatever you've given as ``empty_value``.
    * Normalizes to: A string.
    * Uses :class:`~django.core.validators.RegexValidator` to validate that
      the given value matches 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``, ``min_length``, ``strip``, and ``empty_value``
    which work just as they do for :class:`CharField`.

    .. attribute:: strip

        Defaults to ``False``. If enabled, stripping will be applied before the
        regex validation.

``SlugField``
-------------

.. class:: SlugField(**kwargs)

   * Default widget: :class:`TextInput`
   * Empty value: Whatever you've given as :attr:`empty_value`.
   * Normalizes to: A string.
   * Uses :class:`~django.core.validators.validate_slug` or
     :class:`~django.core.validators.validate_unicode_slug` to validate 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.

   Takes two optional parameters:

   .. attribute:: allow_unicode

       A boolean instructing the field to accept Unicode letters in addition
       to ASCII letters. Defaults to ``False``.

   .. attribute:: empty_value

       The value to use to represent "empty". Defaults to an empty string.

``TimeField``
-------------

.. class:: TimeField(**kwargs)

    * Default widget: :class:`TimeInput`
    * 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

        An iterable 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
    taken from :setting:`TIME_INPUT_FORMATS` if :setting:`USE_L10N` is
    ``False``, or from the active locale format ``TIME_INPUT_FORMATS`` key if
    localization is enabled. See also :doc:`format localization
    </topics/i18n/formatting>`.

``TypedChoiceField``
--------------------

.. class:: TypedChoiceField(**kwargs)

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

    * Default widget: :class:`Select`
    * Empty value: Whatever you've given as :attr:`empty_value`.
    * Normalizes to: A value of the type provided by the :attr:`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. Note that coercion happens after input
        validation, so it is possible to coerce to a value not present in
        ``choices``.

    .. 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.

``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
    :class:`TypedChoiceField`.

``URLField``
------------

.. class:: URLField(**kwargs)

    * Default widget: :class:`URLInput`
    * Empty value: Whatever you've given as ``empty_value``.
    * Normalizes to: A string.
    * Uses :class:`~django.core.validators.URLValidator` to validate that the
      given value is a valid URL.
    * Error message keys: ``required``, ``invalid``

    Has the optional arguments ``max_length``, ``min_length``, and
    ``empty_value`` which work just as they do for :class:`CharField`.

``UUIDField``
-------------

.. class:: UUIDField(**kwargs)

    * Default widget: :class:`TextInput`
    * Empty value: ``None``
    * Normalizes to: A :class:`~python:uuid.UUID` object.
    * Error message keys: ``required``, ``invalid``

    This field will accept any string format accepted as the ``hex`` argument
    to the :class:`~python:uuid.UUID` constructor.

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

``ComboField``
--------------

.. class:: ComboField(**kwargs)

    * Default widget: :class:`TextInput`
    * Empty value: ``''`` (an empty string)
    * Normalizes to: A string.
    * Validates 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).

            >>> from django.forms import ComboField
            >>> f = ComboField(fields=[CharField(max_length=20), EmailField()])
            >>> f.clean('test@example.com')
            'test@example.com'
            >>> f.clean('longemailaddress@example.com')
            Traceback (most recent call last):
            ...
            ValidationError: ['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 the given value against each of the fields specified
      as an argument to the ``MultiValueField``.
    * Error message keys: ``required``, ``invalid``, ``incomplete``

    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`.

    Also takes some optional arguments:

    .. attribute:: require_all_fields

        Defaults to ``True``, in which case a ``required`` validation error
        will be raised if no value is supplied for any field.

        When set to ``False``, the :attr:`Field.required` attribute can be set
        to ``False`` for individual fields to make them optional. If no value
        is supplied for a required field, an ``incomplete`` validation error
        will be raised.

        A default ``incomplete`` error message can be defined on the
        :class:`MultiValueField` subclass, or different messages can be defined
        on each individual field. For example::

            from django.core.validators import RegexValidator


            class PhoneField(MultiValueField):
                def __init__(self, **kwargs):
                    # Define one message for all fields.
                    error_messages = {
                        "incomplete": "Enter a country calling code and a phone number.",
                    }
                    # Or define a different message for each field.
                    fields = (
                        CharField(
                            error_messages={"incomplete": "Enter a country calling code."},
                            validators=[
                                RegexValidator(r"^[0-9]+$", "Enter a valid country calling code."),
                            ],
                        ),
                        CharField(
                            error_messages={"incomplete": "Enter a phone number."},
                            validators=[RegexValidator(r"^[0-9]+$", "Enter a valid phone number.")],
                        ),
                        CharField(
                            validators=[RegexValidator(r"^[0-9]+$", "Enter a valid extension.")],
                            required=False,
                        ),
                    )
                    super().__init__(
                        error_messages=error_messages,
                        fields=fields,
                        require_all_fields=False,
                        **kwargs
                    )

    .. 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 :class:`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 :class:`TimeField` are used.

.. _fields-which-handle-relationships:

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.

For more complex uses, you can specify ``queryset=None`` when declaring the
form field and then populate the ``queryset`` in the form's ``__init__()``
method::

    class FooMultipleChoiceForm(forms.Form):
        foo_select = forms.ModelMultipleChoiceField(queryset=None)

        def __init__(self, *args, **kwargs):
            super().__init__(*args, **kwargs)
            self.fields["foo_select"].queryset = ...

Both ``ModelChoiceField`` and ``ModelMultipleChoiceField`` have an ``iterator``
attribute which specifies the class used to iterate over the queryset when
generating choices. See :ref:`iterating-relationship-choices` for details.

``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``

    The ``invalid_choice`` error message may contain ``%(value)s``, which will
    be replaced with the selected 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
        are derived and which is used to validate the user's selection. It's
        evaluated when the form is rendered.

    ``ModelChoiceField`` also takes several optional arguments:

    .. 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 no empty choice is created (regardless of the value of
        ``empty_label``) if a ``ModelChoiceField`` is required and has a
        default initial value, or a ``widget`` is set to
        :class:`~django.forms.RadioSelect` and the
        :attr:`~ModelChoiceField.blank` argument is ``False``.

    .. attribute:: to_field_name

        This optional argument is used to specify the field to use as the value
        of the choices in the field's widget. Be sure it's a unique field for
        the model, otherwise the selected value could match more than one
        object. By default it is set to ``None``, in which case the primary key
        of each object will be used. For example::

            # No custom to_field_name
            field1 = forms.ModelChoiceField(queryset=...)

        would yield:

        .. code-block:: html

            <select id="id_field1" name="field1">
            <option value="obj1.pk">Object1</option>
            <option value="obj2.pk">Object2</option>
            ...
            </select>

        and::

            # to_field_name provided
            field2 = forms.ModelChoiceField(queryset=..., to_field_name="name")

        would yield:

        .. code-block:: html

            <select id="id_field2" name="field2">
            <option value="obj1.name">Object1</option>
            <option value="obj2.name">Object2</option>
            ...
            </select>

    .. attribute:: blank

        When using the :class:`~django.forms.RadioSelect` widget, this optional
        boolean argument determines whether an empty choice is created. By
        default, ``blank`` is ``False``, in which case no empty choice is
        created.

    ``ModelChoiceField`` also has the attribute:

    .. attribute:: iterator

        The iterator class used to generate field choices from ``queryset``. By
        default, :class:`ModelChoiceIterator`.

    The ``__str__()`` 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::

        from django.forms import ModelChoiceField


        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``, ``invalid_list``, ``invalid_choice``,
      ``invalid_pk_value``

    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.

    A single argument is required:

    .. attribute:: queryset

        Same as :class:`ModelChoiceField.queryset`.

    Takes one optional argument:

    .. attribute:: to_field_name

        Same as :class:`ModelChoiceField.to_field_name`.

    ``ModelMultipleChoiceField`` also has the attribute:

    .. attribute:: iterator

        Same as :class:`ModelChoiceField.iterator`.

.. _iterating-relationship-choices:

Iterating relationship choices
------------------------------

By default, :class:`ModelChoiceField` and :class:`ModelMultipleChoiceField` use
:class:`ModelChoiceIterator` to generate their field ``choices``.

When iterated, ``ModelChoiceIterator`` yields 2-tuple choices containing
:class:`ModelChoiceIteratorValue` instances as the first ``value`` element in
each choice. ``ModelChoiceIteratorValue`` wraps the choice value while
maintaining a reference to the source model instance that can be used in custom
widget implementations, for example, to add `data-* attributes`_ to
``<option>`` elements.

.. _`data-* attributes`: https://developer.mozilla.org/en-US/docs/Web/HTML/Global_attributes/data-*

For example, consider the following models::

    from django.db import models


    class Topping(models.Model):
        name = models.CharField(max_length=100)
        price = models.DecimalField(decimal_places=2, max_digits=6)

        def __str__(self):
            return self.name


    class Pizza(models.Model):
        topping = models.ForeignKey(Topping, on_delete=models.CASCADE)

You can use a :class:`~django.forms.Select` widget subclass to include
the value of ``Topping.price`` as the HTML attribute ``data-price`` for each
``<option>`` element::

    from django import forms


    class ToppingSelect(forms.Select):
        def create_option(
            self, name, value, label, selected, index, subindex=None, attrs=None
        ):
            option = super().create_option(
                name, value, label, selected, index, subindex, attrs
            )
            if value:
                option["attrs"]["data-price"] = value.instance.price
            return option


    class PizzaForm(forms.ModelForm):
        class Meta:
            model = Pizza
            fields = ["topping"]
            widgets = {"topping": ToppingSelect}

This will render the ``Pizza.topping`` select as:

.. code-block:: html

    <select id="id_topping" name="topping" required>
    <option value="" selected>---------</option>
    <option value="1" data-price="1.50">mushrooms</option>
    <option value="2" data-price="1.25">onions</option>
    <option value="3" data-price="1.75">peppers</option>
    <option value="4" data-price="2.00">pineapple</option>
    </select>

For more advanced usage you may subclass ``ModelChoiceIterator`` in order to
customize the yielded 2-tuple choices.

``ModelChoiceIterator``
~~~~~~~~~~~~~~~~~~~~~~~

.. class:: ModelChoiceIterator(field)

    The default class assigned to the ``iterator`` attribute of
    :class:`ModelChoiceField` and :class:`ModelMultipleChoiceField`. An
    iterable that yields 2-tuple choices from the queryset.

    A single argument is required:

    .. attribute:: field

        The instance of ``ModelChoiceField`` or ``ModelMultipleChoiceField`` to
        iterate and yield choices.

    ``ModelChoiceIterator`` has the following method:

    .. method:: __iter__()

        Yields 2-tuple choices, in the ``(value, label)`` format used by
        :attr:`ChoiceField.choices`. The first ``value`` element is a
        :class:`ModelChoiceIteratorValue` instance.

``ModelChoiceIteratorValue``
~~~~~~~~~~~~~~~~~~~~~~~~~~~~

.. class:: ModelChoiceIteratorValue(value, instance)

    Two arguments are required:

    .. attribute:: value

        The value of the choice. This value is used to render the ``value``
        attribute of an HTML ``<option>`` element.

    .. attribute:: instance

        The model instance from the queryset. The instance can be accessed in
        custom ``ChoiceWidget.create_option()`` implementations to adjust the
        rendered HTML.

    ``ModelChoiceIteratorValue`` has the following method:

    .. method:: __str__()

        Return ``value`` as a string to be rendered in HTML.

Creating custom fields
======================

If the built-in ``Field`` classes don't meet your needs, you can create custom
``Field`` classes. To do this, 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``).

You can also customize how a field will be accessed by overriding
:meth:`~django.forms.Field.get_bound_field()`.
