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(u'foo@example.com')
    u'foo@example.com'
    >>> f.clean('invalid e-mail address')
    Traceback (most recent call last):
    ...
    ValidationError: [u'Enter a valid e-mail address.']

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="text" 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="text" 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="text" 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 e-mail 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 />100 characters max.</td></tr>
    <tr><th>Message:</th><td><input type="text" name="message" /></td></tr>
    <tr><th>Sender:</th><td><input type="text" name="sender" /><br />A valid e-mail 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" /> 100 characters max.</li>
    <li>Message: <input type="text" name="message" /></li>
    <li>Sender: <input type="text" name="sender" /> A valid e-mail 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" /> 100 characters max.</p>
    <p>Message: <input type="text" name="message" /></p>
    <p>Sender: <input type="text" name="sender" /> A valid e-mail address, please.</p>
    <p>Cc myself: <input type="checkbox" name="cc_myself" /></p>

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

.. versionadded:: 1.0

.. 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``
~~~~~~~~~~~~~~

.. versionadded:: 1.2

.. 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``
~~~~~~~~~~~~

.. versionadded:: 1.2

.. 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 ``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: ``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``

.. versionchanged:: 1.0
   The empty value for a ``CheckboxInput`` (and hence the standard
   ``BooleanField``) has changed to return ``False`` instead of ``None`` in
   the Django 1.0.

.. 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: ``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:: CharField.max_length
.. attribute:: CharField.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: ``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``

Takes one extra required argument:

.. attribute:: ChoiceField.choices

    An iterable (e.g., a list or tuple) of 2-tuples to use as choices for this
    field.

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

.. class:: TypedChoiceField(**kwargs)

Just like a :class:`ChoiceField`, except :class:`TypedChoiceField` takes an
extra ``coerce`` argument.

    * Default widget: ``Select``
    * Empty value: Whatever you've given as ``empty_value``
    * Normalizes to: the value returned by the ``coerce`` argument.
    * Validates that the given value exists in the list of choices.
    * Error message keys: ``required``, ``invalid_choice``

Takes extra arguments:

.. attribute:: TypedChoiceField.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:: TypedChoiceField.empty_value

    The value to use to represent "empty." Defaults to the empty string;
    ``None`` is another common choice here.

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

.. class:: DateField(**kwargs)

    * Default widget: ``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:: DateField.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', '%m/%d/%Y', '%m/%d/%y', # '2006-10-25', '10/25/2006', '10/25/06'
    '%b %d %Y', '%b %d, %Y',            # 'Oct 25 2006', 'Oct 25, 2006'
    '%d %b %Y', '%d %b, %Y',            # '25 Oct 2006', '25 Oct, 2006'
    '%B %d %Y', '%B %d, %Y',            # 'October 25 2006', 'October 25, 2006'
    '%d %B %Y', '%d %B, %Y',            # '25 October 2006', '25 October, 2006'

.. versionchanged:: 1.1
   The ``DateField`` previously used a ``TextInput`` widget by default. It now
   uses a ``DateInput`` widget.

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

.. class:: DateTimeField(**kwargs)

    * Default widget: ``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:: DateTimeField.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'

.. versionchanged:: 1.0
   The ``DateTimeField`` used to use a ``TextInput`` widget by default. This has now changed.

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

.. versionadded:: 1.0

.. class:: DecimalField(**kwargs)

    * Default widget: ``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``

Takes four optional arguments:

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

    These attributes define the limits for the fields value.

.. attribute:: DecimalField.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:: DecimalField.decimal_places

    The maximum number of decimal places permitted.

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

.. class:: EmailField(**kwargs)

    * Default widget: ``TextInput``
    * Empty value: ``''`` (an empty string)
    * Normalizes to: A Unicode object.
    * Validates that the given value is a valid e-mail 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.

.. versionchanged:: 1.2
   The EmailField previously did not recognize e-mail addresses as valid that
   contained an IDN (Internationalized Domain Name; a domain containing
   unicode characters) domain part. This has now been corrected.

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

.. versionadded:: 1.0

.. class:: FileField(**kwargs)

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

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

.. versionadded:: 1.0

.. class:: FilePathField(**kwargs)

    * Default widget: ``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:: FilePathField.path

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

.. attribute:: FilePathField.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:: FilePathField.match

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

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

    * Default widget: ``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``
~~~~~~~~~~~~~~

.. versionadded:: 1.0

.. class:: ImageField(**kwargs)

    * Default widget: ``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 PIL.
    * Error message keys: ``required``, ``invalid``, ``missing``, ``empty``,
      ``invalid_image``

Using an ImageField requires that the `Python Imaging Library`_ is installed.

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

.. _Python Imaging Library: http://www.pythonware.com/products/pil/

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

.. class:: IntegerField(**kwargs)

    * Default widget: ``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``

Takes two optional arguments for validation:

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

    These control the range of values permitted in the field.

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

.. class:: IPAddressField(**kwargs)

    * Default widget: ``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``

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

.. class:: MultipleChoiceField(**kwargs)

    * Default widget: ``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``

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

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

.. class:: NullBooleanField(**kwargs)

    * Default widget: ``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: ``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:: RegexField.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: ``TextInput``
   * Empty value: ``''`` (an empty string)
   * Normalizes to: A Unicode object.
   * Validates that the given value contains only letters, numbers 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: ``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:: TimeField.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: ``TextInput``
    * Empty value: ``''`` (an empty string)
    * Normalizes to: A Unicode object.
    * Validates that the given value is a valid URL.
    * Error message keys: ``required``, ``invalid``, ``invalid_link``

Takes the following optional arguments:

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

    Same as ``CharField.max_length`` and ``CharField.min_length``.

.. attribute:: URLField.verify_exists

    If ``True``, the validator will attempt to load the given URL, raising
    ``ValidationError`` if the page gives a 404. Defaults to ``False``.

.. attribute:: URLField.validator_user_agent

    String used as the user-agent used when checking for a URL's existence.
    Defaults to the value of the ``URL_VALIDATOR_USER_AGENT`` setting.

.. versionchanged:: 1.2
   The URLField previously did not recognize URLs as valid that contained an IDN
   (Internationalized Domain Name; a domain name containing unicode characters)
   domain name. This has now been corrected.


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

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

.. class:: ComboField(**kwargs)

    * Default widget: ``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:: ComboField.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(**kwargs)

    * Default widget: ``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``

    This abstract field (must be subclassed) aggregates the logic of multiple
    fields. Subclasses should not have to implement clean(). Instead, they must
    implement compress(), which takes a list of valid values and returns a
    "compressed" version of those values -- a single value.  For example,
    :class:`SplitDateTimeField` is a subclass which combines a time field and
    a date field into a datetime object.

Takes one extra required argument:

.. attribute:: MultiValueField.fields

    A list of fields which are cleaned into a single field. Each value in
    ``clean`` 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 "compressed" into a single value.

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

.. class:: SplitDateTimeField(**kwargs)

    * Default widget: ``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``

Takes two optional arguments:

.. attribute:: SplitDateTimeField.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:: SplitDateTimeField.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.

.. versionchanged:: 1.1
   The ``SplitDateTimeField`` previously used two ``TextInput`` widgets by
   default. The ``input_date_formats`` and ``input_time_formats`` arguments
   are also new.

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)

Allows the selection of a single model object, suitable for
representing a foreign key.  A single argument is required:

.. attribute:: ModelChoiceField.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:: ModelChoiceField.empty_label

    By default the ``<select>`` widget used by ``ModelChoiceField`` will have a
    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)

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:: ModelMultipleChoiceField.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``).