django/docs/ref/forms/api.txt

=============
The Forms API
=============

.. module:: django.forms

.. admonition:: About this document

    This document covers the gritty details of Django's forms API. You should
    read the :doc:`introduction to working with forms </topics/forms/index>`
    first.

.. _ref-forms-api-bound-unbound:

Bound and unbound forms
=======================

A :class:`Form` instance is either **bound** to a set of data, or **unbound**.

* If it's **bound** to a set of data, it's capable of validating that data
  and rendering the form as HTML with the data displayed in the HTML.

* If it's **unbound**, it cannot do validation (because there's no data to
  validate!), but it can still render the blank form as HTML.

.. class:: Form

To create an unbound :class:`Form` instance, simply instantiate the class::

    >>> f = ContactForm()

To bind data to a form, pass the data as a dictionary as the first parameter to
your :class:`Form` class constructor::

    >>> data = {'subject': 'hello',
    ...         'message': 'Hi there',
    ...         'sender': 'foo@example.com',
    ...         'cc_myself': True}
    >>> f = ContactForm(data)

In this dictionary, the keys are the field names, which correspond to the
attributes in your :class:`Form` class. The values are the data you're trying to
validate. These will usually be strings, but there's no requirement that they be
strings; the type of data you pass depends on the :class:`Field`, as we'll see
in a moment.

.. attribute:: Form.is_bound

If you need to distinguish between bound and unbound form instances at runtime,
check the value of the form's :attr:`~Form.is_bound` attribute::

    >>> f = ContactForm()
    >>> f.is_bound
    False
    >>> f = ContactForm({'subject': 'hello'})
    >>> f.is_bound
    True

Note that passing an empty dictionary creates a *bound* form with empty data::

    >>> f = ContactForm({})
    >>> f.is_bound
    True

If you have a bound :class:`Form` instance and want to change the data somehow,
or if you want to bind an unbound :class:`Form` instance to some data, create
another :class:`Form` instance. There is no way to change data in a
:class:`Form` instance. Once a :class:`Form` instance has been created, you
should consider its data immutable, whether it has data or not.

Using forms to validate data
============================

.. method:: Form.clean()

Implement a ``clean()`` method on your ``Form`` when you must add custom
validation for fields that are interdependent. See
:ref:`validating-fields-with-clean` for example usage.

.. method:: Form.is_valid()

The primary task of a :class:`Form` object is to validate data. With a bound
:class:`Form` instance, call the :meth:`~Form.is_valid` method to run validation
and return a boolean designating whether the data was valid::

    >>> data = {'subject': 'hello',
    ...         'message': 'Hi there',
    ...         'sender': 'foo@example.com',
    ...         'cc_myself': True}
    >>> f = ContactForm(data)
    >>> f.is_valid()
    True

Let's try with some invalid data. In this case, ``subject`` is blank (an error,
because all fields are required by default) and ``sender`` is not a valid
email address::

    >>> data = {'subject': '',
    ...         'message': 'Hi there',
    ...         'sender': 'invalid email address',
    ...         'cc_myself': True}
    >>> f = ContactForm(data)
    >>> f.is_valid()
    False

.. attribute:: Form.errors

Access the :attr:`~Form.errors` attribute to get a dictionary of error
messages::

    >>> f.errors
    {'sender': ['Enter a valid email address.'], 'subject': ['This field is required.']}

In this dictionary, the keys are the field names, and the values are lists of
strings representing the error messages. The error messages are stored
in lists because a field can have multiple error messages.

You can access :attr:`~Form.errors` without having to call
:meth:`~Form.is_valid` first. The form's data will be validated the first time
either you call :meth:`~Form.is_valid` or access :attr:`~Form.errors`.

The validation routines will only get called once, regardless of how many times
you access :attr:`~Form.errors` or call :meth:`~Form.is_valid`. This means that
if validation has side effects, those side effects will only be triggered once.

.. method:: Form.errors.as_data()

Returns a ``dict`` that maps fields to their original ``ValidationError``
instances.

    >>> f.errors.as_data()
    {'sender': [ValidationError(['Enter a valid email address.'])],
    'subject': [ValidationError(['This field is required.'])]}

Use this method anytime you need to identify an error by its ``code``. This
enables things like rewriting the error's message or writing custom logic in a
view when a given error is present. It can also be used to serialize the errors
in a custom format (e.g. XML); for instance, :meth:`~Form.errors.as_json()`
relies on ``as_data()``.

The need for the ``as_data()`` method is due to backwards compatibility.
Previously ``ValidationError`` instances were lost as soon as their
**rendered** error messages were added to the ``Form.errors`` dictionary.
Ideally ``Form.errors`` would have stored ``ValidationError`` instances
and methods with an ``as_`` prefix could render them, but it had to be done
the other way around in order not to break code that expects rendered error
messages in ``Form.errors``.

.. method:: Form.errors.as_json(escape_html=False)

Returns the errors serialized as JSON.

    >>> f.errors.as_json()
    {"sender": [{"message": "Enter a valid email address.", "code": "invalid"}],
    "subject": [{"message": "This field is required.", "code": "required"}]}

By default, ``as_json()`` does not escape its output. If you are using it for
something like AJAX requests to a form view where the client interprets the
response and inserts errors into the page, you'll want to be sure to escape the
results on the client-side to avoid the possibility of a cross-site scripting
attack. It's trivial to do so using a JavaScript library like jQuery - simply
use ``$(el).text(errorText)`` rather than ``.html()``.

If for some reason you don't want to use client-side escaping, you can also
set ``escape_html=True`` and error messages will be escaped so you can use them
directly in HTML.

.. method:: Form.errors.get_json_data(escape_html=False)

.. versionadded:: 2.0

Returns the errors as a dictionary suitable for serializing to JSON.
:meth:`Form.errors.as_json()` returns serialized JSON, while this returns the
error data before it's serialized.

The ``escape_html`` parameter behaves as described in
:meth:`Form.errors.as_json()`.

.. method:: Form.add_error(field, error)

This method allows adding errors to specific fields from within the
``Form.clean()`` method, or from outside the form altogether; for instance
from a view.

The ``field`` argument is the name of the field to which the errors
should be added. If its value is ``None`` the error will be treated as
a non-field error as returned by :meth:`Form.non_field_errors()
<django.forms.Form.non_field_errors>`.

The ``error`` argument can be a simple string, or preferably an instance of
``ValidationError``. See :ref:`raising-validation-error` for best practices
when defining form errors.

Note that ``Form.add_error()`` automatically removes the relevant field from
``cleaned_data``.

.. method:: Form.has_error(field, code=None)

This method returns a boolean designating whether a field has an error with
a specific error ``code``. If ``code`` is ``None``, it will return ``True``
if the field contains any errors at all.

To check for non-field errors use
:data:`~django.core.exceptions.NON_FIELD_ERRORS` as the ``field`` parameter.

.. method:: Form.non_field_errors()

This method returns the list of errors from :attr:`Form.errors
<django.forms.Form.errors>`  that aren't associated with a particular field.
This includes ``ValidationError``\s that are raised in :meth:`Form.clean()
<django.forms.Form.clean>` and errors added using :meth:`Form.add_error(None,
"...") <django.forms.Form.add_error>`.

Behavior of unbound forms
-------------------------

It's meaningless to validate a form with no data, but, for the record, here's
what happens with unbound forms::

    >>> f = ContactForm()
    >>> f.is_valid()
    False
    >>> f.errors
    {}

Dynamic initial values
======================

.. attribute:: Form.initial

Use :attr:`~Form.initial` to declare the initial value of form fields at
runtime. For example, you might want to fill in a ``username`` field with the
username of the current session.

To accomplish this, use the :attr:`~Form.initial` argument to a :class:`Form`.
This argument, if given, should be a dictionary mapping field names to initial
values. Only include the fields for which you're specifying an initial value;
it's not necessary to include every field in your form. For example::

    >>> f = ContactForm(initial={'subject': 'Hi there!'})

These values are only displayed for unbound forms, and they're not used as
fallback values if a particular value isn't provided.

If a :class:`~django.forms.Field` defines :attr:`~Field.initial` *and* you
include :attr:`~Form.initial` when instantiating the ``Form``, then the latter
``initial`` will have precedence. In this example, ``initial`` is provided both
at the field level and at the form instance level, and the latter gets
precedence::

    >>> from django import forms
    >>> class CommentForm(forms.Form):
    ...     name = forms.CharField(initial='class')
    ...     url = forms.URLField()
    ...     comment = forms.CharField()
    >>> f = CommentForm(initial={'name': 'instance'}, auto_id=False)
    >>> print(f)
    <tr><th>Name:</th><td><input type="text" name="name" value="instance" required /></td></tr>
    <tr><th>Url:</th><td><input type="url" name="url" required /></td></tr>
    <tr><th>Comment:</th><td><input type="text" name="comment" required /></td></tr>

.. method:: Form.get_initial_for_field(field, field_name)

Use :meth:`~Form.get_initial_for_field()` to retrieve initial data for a form
field. It retrieves data from :attr:`Form.initial` and :attr:`Field.initial`,
in that order, and evaluates any callable initial values.

Checking which form data has changed
====================================

.. method:: Form.has_changed()

Use the ``has_changed()`` method on your ``Form`` when you need to check if the
form data has been changed from the initial data.

    >>> data = {'subject': 'hello',
    ...         'message': 'Hi there',
    ...         'sender': 'foo@example.com',
    ...         'cc_myself': True}
    >>> f = ContactForm(data, initial=data)
    >>> f.has_changed()
    False

When the form is submitted, we reconstruct it and provide the original data
so that the comparison can be done:

    >>> f = ContactForm(request.POST, initial=data)
    >>> f.has_changed()

``has_changed()`` will be ``True`` if the data from ``request.POST`` differs
from what was provided in :attr:`~Form.initial` or ``False`` otherwise. The
result is computed by calling :meth:`Field.has_changed` for each field in the
form.

.. attribute:: Form.changed_data

The ``changed_data`` attribute returns a list of the names of the fields whose
values in the form's bound data (usually ``request.POST``) differ from what was
provided in :attr:`~Form.initial`. It returns an empty list if no data differs.

    >>> f = ContactForm(request.POST, initial=data)
    >>> if f.has_changed():
    ...     print("The following fields changed: %s" % ", ".join(f.changed_data))

Accessing the fields from the form
==================================

.. attribute:: Form.fields

You can access the fields of :class:`Form` instance from its ``fields``
attribute::

    >>> for row in f.fields.values(): print(row)
    ...
    <django.forms.fields.CharField object at 0x7ffaac632510>
    <django.forms.fields.URLField object at 0x7ffaac632f90>
    <django.forms.fields.CharField object at 0x7ffaac3aa050>
    >>> f.fields['name']
    <django.forms.fields.CharField object at 0x7ffaac6324d0>

You can alter the field of :class:`Form` instance to change the way it is
presented in the form::

    >>> f.as_table().split('\n')[0]
    '<tr><th>Name:</th><td><input name="name" type="text" value="instance" required /></td></tr>'
    >>> f.fields['name'].label = "Username"
    >>> f.as_table().split('\n')[0]
    '<tr><th>Username:</th><td><input name="name" type="text" value="instance" required /></td></tr>'

Beware not to alter the ``base_fields`` attribute because this modification
will influence all subsequent ``ContactForm`` instances within the same Python
process::

    >>> f.base_fields['name'].label = "Username"
    >>> another_f = CommentForm(auto_id=False)
    >>> another_f.as_table().split('\n')[0]
    '<tr><th>Username:</th><td><input name="name" type="text" value="class" required /></td></tr>'

Accessing "clean" data
======================

.. attribute:: Form.cleaned_data

Each field in a :class:`Form` class is responsible not only for validating
data, but also for "cleaning" it -- normalizing it to a consistent format. This
is a nice feature, because it allows data for a particular field to be input in
a variety of ways, always resulting in consistent output.

For example, :class:`~django.forms.DateField` normalizes input into a
Python ``datetime.date`` object. Regardless of whether you pass it a string in
the format ``'1994-07-15'``, a ``datetime.date`` object, or a number of other
formats, ``DateField`` will always normalize it to a ``datetime.date`` object
as long as it's valid.

Once you've created a :class:`~Form` instance with a set of data and validated
it, you can access the clean data via its ``cleaned_data`` attribute::

    >>> data = {'subject': 'hello',
    ...         'message': 'Hi there',
    ...         'sender': 'foo@example.com',
    ...         'cc_myself': True}
    >>> f = ContactForm(data)
    >>> f.is_valid()
    True
    >>> f.cleaned_data
    {'cc_myself': True, 'message': 'Hi there', 'sender': 'foo@example.com', 'subject': 'hello'}

Note that any text-based field -- such as ``CharField`` or ``EmailField`` --
always cleans the input into a string. We'll cover the encoding implications
later in this document.

If your data does *not* validate, the ``cleaned_data`` dictionary contains
only the valid fields::

    >>> data = {'subject': '',
    ...         'message': 'Hi there',
    ...         'sender': 'invalid email address',
    ...         'cc_myself': True}
    >>> f = ContactForm(data)
    >>> f.is_valid()
    False
    >>> f.cleaned_data
    {'cc_myself': True, 'message': 'Hi there'}

``cleaned_data`` will always *only* contain a key for fields defined in the
``Form``, even if you pass extra data when you define the ``Form``. In this
example, we pass a bunch of extra fields to the ``ContactForm`` constructor,
but ``cleaned_data`` contains only the form's fields::

    >>> data = {'subject': 'hello',
    ...         'message': 'Hi there',
    ...         'sender': 'foo@example.com',
    ...         'cc_myself': True,
    ...         'extra_field_1': 'foo',
    ...         'extra_field_2': 'bar',
    ...         'extra_field_3': 'baz'}
    >>> f = ContactForm(data)
    >>> f.is_valid()
    True
    >>> f.cleaned_data # Doesn't contain extra_field_1, etc.
    {'cc_myself': True, 'message': 'Hi there', 'sender': 'foo@example.com', 'subject': 'hello'}

When the ``Form`` is valid, ``cleaned_data`` will include a key and value for
*all* its fields, even if the data didn't include a value for some optional
fields. In this example, the data dictionary doesn't include a value for the
``nick_name`` field, but ``cleaned_data`` includes it, with an empty value::

    >>> from django import forms
    >>> class OptionalPersonForm(forms.Form):
    ...     first_name = forms.CharField()
    ...     last_name = forms.CharField()
    ...     nick_name = forms.CharField(required=False)
    >>> data = {'first_name': 'John', 'last_name': 'Lennon'}
    >>> f = OptionalPersonForm(data)
    >>> f.is_valid()
    True
    >>> f.cleaned_data
    {'nick_name': '', 'first_name': 'John', 'last_name': 'Lennon'}

In this above example, the ``cleaned_data`` value for ``nick_name`` is set to an
empty string, because ``nick_name`` is ``CharField``, and ``CharField``\s treat
empty values as an empty string. Each field type knows what its "blank" value
is -- e.g., for ``DateField``, it's ``None`` instead of the empty string. For
full details on each field's behavior in this case, see the "Empty value" note
for each field in the "Built-in ``Field`` classes" section below.

You can write code to perform validation for particular form fields (based on
their name) or for the form as a whole (considering combinations of various
fields). More information about this is in :doc:`/ref/forms/validation`.

.. _ref-forms-api-outputting-html:

Outputting forms as HTML
========================

The second task of a ``Form`` object is to render itself as HTML. To do so,
simply ``print`` it::

    >>> f = ContactForm()
    >>> print(f)
    <tr><th><label for="id_subject">Subject:</label></th><td><input id="id_subject" type="text" name="subject" maxlength="100" required /></td></tr>
    <tr><th><label for="id_message">Message:</label></th><td><input type="text" name="message" id="id_message" required /></td></tr>
    <tr><th><label for="id_sender">Sender:</label></th><td><input type="email" name="sender" id="id_sender" required /></td></tr>
    <tr><th><label for="id_cc_myself">Cc myself:</label></th><td><input type="checkbox" name="cc_myself" id="id_cc_myself" /></td></tr>

If the form is bound to data, the HTML output will include that data
appropriately. For example, if a field is represented by an
``<input type="text">``, the data will be in the ``value`` attribute. If a
field is represented by an ``<input type="checkbox">``, then that HTML will
include ``checked`` if appropriate::

    >>> data = {'subject': 'hello',
    ...         'message': 'Hi there',
    ...         'sender': 'foo@example.com',
    ...         'cc_myself': True}
    >>> f = ContactForm(data)
    >>> print(f)
    <tr><th><label for="id_subject">Subject:</label></th><td><input id="id_subject" type="text" name="subject" maxlength="100" value="hello" required /></td></tr>
    <tr><th><label for="id_message">Message:</label></th><td><input type="text" name="message" id="id_message" value="Hi there" required /></td></tr>
    <tr><th><label for="id_sender">Sender:</label></th><td><input type="email" name="sender" id="id_sender" value="foo@example.com" required /></td></tr>
    <tr><th><label for="id_cc_myself">Cc myself:</label></th><td><input type="checkbox" name="cc_myself" id="id_cc_myself" checked /></td></tr>

This default output is a two-column HTML table, with a ``<tr>`` for each field.
Notice the following:

* For flexibility, the output does *not* include the ``<table>`` and
  ``</table>`` tags, nor does it include the ``<form>`` and ``</form>``
  tags or an ``<input type="submit">`` tag. It's your job to do that.

* Each field type has a default HTML representation. ``CharField`` is
  represented by an ``<input type="text">`` and ``EmailField`` by an
  ``<input type="email">``.
  ``BooleanField`` is represented by an ``<input type="checkbox">``. Note
  these are merely sensible defaults; you can specify which HTML to use for
  a given field by using widgets, which we'll explain shortly.

* The HTML ``name`` for each tag is taken directly from its attribute name
  in the ``ContactForm`` class.

* The text label for each field -- e.g. ``'Subject:'``, ``'Message:'`` and
  ``'Cc myself:'`` is generated from the field name by converting all
  underscores to spaces and upper-casing the first letter. Again, note
  these are merely sensible defaults; you can also specify labels manually.

* Each text label is surrounded in an HTML ``<label>`` tag, which points
  to the appropriate form field via its ``id``. Its ``id``, in turn, is
  generated by prepending ``'id_'`` to the field name. The ``id``
  attributes and ``<label>`` tags are included in the output by default, to
  follow best practices, but you can change that behavior.

* The output uses HTML5 syntax, targeting ``<!DOCTYPE html>``. For example,
  it uses boolean attributes such as ``checked`` rather than the XHTML style
  of ``checked='checked'``.

Although ``<table>`` output is the default output style when you ``print`` a
form, other output styles are available. Each style is available as a method on
a form object, and each rendering method returns a string.

``as_p()``
----------

.. method:: Form.as_p()

``as_p()`` renders the form as a series of ``<p>`` tags, with each ``<p>``
containing one field::

    >>> f = ContactForm()
    >>> f.as_p()
    '<p><label for="id_subject">Subject:</label> <input id="id_subject" type="text" name="subject" maxlength="100" required /></p>\n<p><label for="id_message">Message:</label> <input type="text" name="message" id="id_message" required /></p>\n<p><label for="id_sender">Sender:</label> <input type="text" name="sender" id="id_sender" required /></p>\n<p><label for="id_cc_myself">Cc myself:</label> <input type="checkbox" name="cc_myself" id="id_cc_myself" /></p>'
    >>> print(f.as_p())
    <p><label for="id_subject">Subject:</label> <input id="id_subject" type="text" name="subject" maxlength="100" required /></p>
    <p><label for="id_message">Message:</label> <input type="text" name="message" id="id_message" required /></p>
    <p><label for="id_sender">Sender:</label> <input type="email" name="sender" id="id_sender" required /></p>
    <p><label for="id_cc_myself">Cc myself:</label> <input type="checkbox" name="cc_myself" id="id_cc_myself" /></p>

``as_ul()``
-----------

.. method:: Form.as_ul()

``as_ul()`` renders the form as a series of ``<li>`` tags, with each
``<li>`` containing one field. It does *not* include the ``<ul>`` or
``</ul>``, so that you can specify any HTML attributes on the ``<ul>`` for
flexibility::

    >>> f = ContactForm()
    >>> f.as_ul()
    '<li><label for="id_subject">Subject:</label> <input id="id_subject" type="text" name="subject" maxlength="100" required /></li>\n<li><label for="id_message">Message:</label> <input type="text" name="message" id="id_message" required /></li>\n<li><label for="id_sender">Sender:</label> <input type="email" name="sender" id="id_sender" required /></li>\n<li><label for="id_cc_myself">Cc myself:</label> <input type="checkbox" name="cc_myself" id="id_cc_myself" /></li>'
    >>> print(f.as_ul())
    <li><label for="id_subject">Subject:</label> <input id="id_subject" type="text" name="subject" maxlength="100" required /></li>
    <li><label for="id_message">Message:</label> <input type="text" name="message" id="id_message" required /></li>
    <li><label for="id_sender">Sender:</label> <input type="email" name="sender" id="id_sender" required /></li>
    <li><label for="id_cc_myself">Cc myself:</label> <input type="checkbox" name="cc_myself" id="id_cc_myself" /></li>

``as_table()``
--------------

.. method:: Form.as_table()

Finally, ``as_table()`` outputs the form as an HTML ``<table>``. This is
exactly the same as ``print``. In fact, when you ``print`` a form object,
it calls its ``as_table()`` method behind the scenes::

    >>> f = ContactForm()
    >>> f.as_table()
    '<tr><th><label for="id_subject">Subject:</label></th><td><input id="id_subject" type="text" name="subject" maxlength="100" required /></td></tr>\n<tr><th><label for="id_message">Message:</label></th><td><input type="text" name="message" id="id_message" required /></td></tr>\n<tr><th><label for="id_sender">Sender:</label></th><td><input type="email" name="sender" id="id_sender" required /></td></tr>\n<tr><th><label for="id_cc_myself">Cc myself:</label></th><td><input type="checkbox" name="cc_myself" id="id_cc_myself" /></td></tr>'
    >>> print(f)
    <tr><th><label for="id_subject">Subject:</label></th><td><input id="id_subject" type="text" name="subject" maxlength="100" required /></td></tr>
    <tr><th><label for="id_message">Message:</label></th><td><input type="text" name="message" id="id_message" required /></td></tr>
    <tr><th><label for="id_sender">Sender:</label></th><td><input type="email" name="sender" id="id_sender" required /></td></tr>
    <tr><th><label for="id_cc_myself">Cc myself:</label></th><td><input type="checkbox" name="cc_myself" id="id_cc_myself" /></td></tr>

.. _ref-forms-api-styling-form-rows:

Styling required or erroneous form rows
---------------------------------------

.. attribute:: Form.error_css_class
.. attribute:: Form.required_css_class

It's pretty common to style form rows and fields that are required or have
errors. For example, you might want to present required form rows in bold and
highlight errors in red.

The :class:`Form` class has a couple of hooks you can use to add ``class``
attributes to required rows or to rows with errors: simply set the
:attr:`Form.error_css_class` and/or :attr:`Form.required_css_class`
attributes::

    from django import forms

    class ContactForm(forms.Form):
        error_css_class = 'error'
        required_css_class = 'required'

        # ... and the rest of your fields here

Once you've done that, rows will be given ``"error"`` and/or ``"required"``
classes, as needed. The HTML will look something like::

    >>> f = ContactForm(data)
    >>> print(f.as_table())
    <tr class="required"><th><label class="required" for="id_subject">Subject:</label>    ...
    <tr class="required"><th><label class="required" for="id_message">Message:</label>    ...
    <tr class="required error"><th><label class="required" for="id_sender">Sender:</label>      ...
    <tr><th><label for="id_cc_myself">Cc myself:<label> ...
    >>> f['subject'].label_tag()
    <label class="required" for="id_subject">Subject:</label>
    >>> f['subject'].label_tag(attrs={'class': 'foo'})
    <label for="id_subject" class="foo required">Subject:</label>

.. _ref-forms-api-configuring-label:

Configuring form elements' HTML ``id`` attributes and ``<label>`` tags
----------------------------------------------------------------------

.. attribute:: Form.auto_id

By default, the form rendering methods include:

* HTML ``id`` attributes on the form elements.

* The corresponding ``<label>`` tags around the labels. An HTML ``<label>`` tag
  designates which label text is associated with which form element. This small
  enhancement makes forms more usable and more accessible to assistive devices.
  It's always a good idea to use ``<label>`` tags.

The ``id`` attribute values are generated by prepending ``id_`` to the form
field names.  This behavior is configurable, though, if you want to change the
``id`` convention or remove HTML ``id`` attributes and ``<label>`` tags
entirely.

Use the ``auto_id`` argument to the ``Form`` constructor to control the ``id``
and label behavior. This argument must be ``True``, ``False`` or a string.

If ``auto_id`` is ``False``, then the form output will not include ``<label>``
tags nor ``id`` attributes::

    >>> f = ContactForm(auto_id=False)
    >>> print(f.as_table())
    <tr><th>Subject:</th><td><input type="text" name="subject" maxlength="100" required /></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 /></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 /></li>
    <li>Message: <input type="text" name="message" required /></li>
    <li>Sender: <input type="email" name="sender" required /></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 /></p>
    <p>Message: <input type="text" name="message" required /></p>
    <p>Sender: <input type="email" name="sender" required /></p>
    <p>Cc myself: <input type="checkbox" name="cc_myself" /></p>

If ``auto_id`` is set to ``True``, then the form output *will* include
``<label>`` tags and will simply use the field name as its ``id`` for each form
field::

    >>> f = ContactForm(auto_id=True)
    >>> print(f.as_table())
    <tr><th><label for="subject">Subject:</label></th><td><input id="subject" type="text" name="subject" maxlength="100" required /></td></tr>
    <tr><th><label for="message">Message:</label></th><td><input type="text" name="message" id="message" required /></td></tr>
    <tr><th><label for="sender">Sender:</label></th><td><input type="email" name="sender" id="sender" required /></td></tr>
    <tr><th><label for="cc_myself">Cc myself:</label></th><td><input type="checkbox" name="cc_myself" id="cc_myself" /></td></tr>
    >>> print(f.as_ul())
    <li><label for="subject">Subject:</label> <input id="subject" type="text" name="subject" maxlength="100" required /></li>
    <li><label for="message">Message:</label> <input type="text" name="message" id="message" required /></li>
    <li><label for="sender">Sender:</label> <input type="email" name="sender" id="sender" required /></li>
    <li><label for="cc_myself">Cc myself:</label> <input type="checkbox" name="cc_myself" id="cc_myself" /></li>
    >>> print(f.as_p())
    <p><label for="subject">Subject:</label> <input id="subject" type="text" name="subject" maxlength="100" required /></p>
    <p><label for="message">Message:</label> <input type="text" name="message" id="message" required /></p>
    <p><label for="sender">Sender:</label> <input type="email" name="sender" id="sender" required /></p>
    <p><label for="cc_myself">Cc myself:</label> <input type="checkbox" name="cc_myself" id="cc_myself" /></p>

If ``auto_id`` is set to a string containing the format character ``'%s'``,
then the form output will include ``<label>`` tags, and will generate ``id``
attributes based on the format string. For example, for a format string
``'field_%s'``, a field named ``subject`` will get the ``id`` value
``'field_subject'``. Continuing our example::

    >>> f = ContactForm(auto_id='id_for_%s')
    >>> print(f.as_table())
    <tr><th><label for="id_for_subject">Subject:</label></th><td><input id="id_for_subject" type="text" name="subject" maxlength="100" required /></td></tr>
    <tr><th><label for="id_for_message">Message:</label></th><td><input type="text" name="message" id="id_for_message" required /></td></tr>
    <tr><th><label for="id_for_sender">Sender:</label></th><td><input type="email" name="sender" id="id_for_sender" required /></td></tr>
    <tr><th><label for="id_for_cc_myself">Cc myself:</label></th><td><input type="checkbox" name="cc_myself" id="id_for_cc_myself" /></td></tr>
    >>> print(f.as_ul())
    <li><label for="id_for_subject">Subject:</label> <input id="id_for_subject" type="text" name="subject" maxlength="100" required /></li>
    <li><label for="id_for_message">Message:</label> <input type="text" name="message" id="id_for_message" required /></li>
    <li><label for="id_for_sender">Sender:</label> <input type="email" name="sender" id="id_for_sender" required /></li>
    <li><label for="id_for_cc_myself">Cc myself:</label> <input type="checkbox" name="cc_myself" id="id_for_cc_myself" /></li>
    >>> print(f.as_p())
    <p><label for="id_for_subject">Subject:</label> <input id="id_for_subject" type="text" name="subject" maxlength="100" required /></p>
    <p><label for="id_for_message">Message:</label> <input type="text" name="message" id="id_for_message" required /></p>
    <p><label for="id_for_sender">Sender:</label> <input type="email" name="sender" id="id_for_sender" required /></p>
    <p><label for="id_for_cc_myself">Cc myself:</label> <input type="checkbox" name="cc_myself" id="id_for_cc_myself" /></p>

If ``auto_id`` is set to any other true value -- such as a string that doesn't
include ``%s`` -- then the library will act as if ``auto_id`` is ``True``.

By default, ``auto_id`` is set to the string ``'id_%s'``.

.. attribute:: Form.label_suffix

A translatable string (defaults to a colon (``:``) in English) that will be
appended after any label name when a form is rendered.

It's possible to customize that character, or omit it entirely, using the
``label_suffix`` parameter::

    >>> f = ContactForm(auto_id='id_for_%s', label_suffix='')
    >>> print(f.as_ul())
    <li><label for="id_for_subject">Subject</label> <input id="id_for_subject" type="text" name="subject" maxlength="100" required /></li>
    <li><label for="id_for_message">Message</label> <input type="text" name="message" id="id_for_message" required /></li>
    <li><label for="id_for_sender">Sender</label> <input type="email" name="sender" id="id_for_sender" required /></li>
    <li><label for="id_for_cc_myself">Cc myself</label> <input type="checkbox" name="cc_myself" id="id_for_cc_myself" /></li>
    >>> f = ContactForm(auto_id='id_for_%s', label_suffix=' ->')
    >>> print(f.as_ul())
    <li><label for="id_for_subject">Subject -></label> <input id="id_for_subject" type="text" name="subject" maxlength="100" required /></li>
    <li><label for="id_for_message">Message -></label> <input type="text" name="message" id="id_for_message" required /></li>
    <li><label for="id_for_sender">Sender -></label> <input type="email" name="sender" id="id_for_sender" required /></li>
    <li><label for="id_for_cc_myself">Cc myself -></label> <input type="checkbox" name="cc_myself" id="id_for_cc_myself" /></li>

Note that the label suffix is added only if the last character of the
label isn't a punctuation character (in English, those are ``.``, ``!``, ``?``
or ``:``).

Fields can also define their own :attr:`~django.forms.Field.label_suffix`.
This will take precedence over :attr:`Form.label_suffix
<django.forms.Form.label_suffix>`. The suffix can also be overridden at runtime
using the ``label_suffix`` parameter to
:meth:`~django.forms.BoundField.label_tag`.

.. attribute:: Form.use_required_attribute

When set to ``True`` (the default), required form fields will have the
``required`` HTML attribute.

:doc:`Formsets </topics/forms/formsets>` instantiate forms with
``use_required_attribute=False`` to avoid incorrect browser validation when
adding and deleting forms from a formset.

Configuring the rendering of a form's widgets
---------------------------------------------

.. attribute:: Form.default_renderer

Specifies the :doc:`renderer <renderers>` to use for the form. Defaults to
``None`` which means to use the default renderer specified by the
:setting:`FORM_RENDERER` setting.

You can set this as a class attribute when declaring your form or use the
``renderer`` argument to ``Form.__init__()``. For example::

    from django import forms

    class MyForm(forms.Form):
        default_renderer = MyRenderer()

or::

    form = MyForm(renderer=MyRenderer())

Notes on field ordering
-----------------------

In the ``as_p()``, ``as_ul()`` and ``as_table()`` shortcuts, the fields are
displayed in the order in which you define them in your form class. For
example, in the ``ContactForm`` example, the fields are defined in the order
``subject``, ``message``, ``sender``, ``cc_myself``. To reorder the HTML
output, just change the order in which those fields are listed in the class.

There are several other ways to customize the order:

.. attribute:: Form.field_order

By default ``Form.field_order=None``, which retains the order in which you
define the fields in your form class. If ``field_order`` is a list of field
names, the fields are ordered as specified by the list and remaining fields are
appended according to the default order. Unknown field names in the list are
ignored. This makes it possible to disable a field in a subclass by setting it
to ``None`` without having to redefine ordering.

You can also use the ``Form.field_order`` argument to a :class:`Form` to
override the field order. If a :class:`~django.forms.Form` defines
:attr:`~Form.field_order` *and* you include ``field_order`` when instantiating
the ``Form``, then the latter ``field_order`` will have precedence.

.. method:: Form.order_fields(field_order)

You may rearrange the fields any time using ``order_fields()`` with a list of
field names as in :attr:`~django.forms.Form.field_order`.

How errors are displayed
------------------------

If you render a bound ``Form`` object, the act of rendering will automatically
run the form's validation if it hasn't already happened, and the HTML output
will include the validation errors as a ``<ul class="errorlist">`` near the
field. The particular positioning of the error messages depends on the output
method you're using::

    >>> data = {'subject': '',
    ...         'message': 'Hi there',
    ...         'sender': 'invalid email address',
    ...         'cc_myself': True}
    >>> f = ContactForm(data, auto_id=False)
    >>> print(f.as_table())
    <tr><th>Subject:</th><td><ul class="errorlist"><li>This field is required.</li></ul><input type="text" name="subject" maxlength="100" required /></td></tr>
    <tr><th>Message:</th><td><input type="text" name="message" value="Hi there" required /></td></tr>
    <tr><th>Sender:</th><td><ul class="errorlist"><li>Enter a valid email address.</li></ul><input type="email" name="sender" value="invalid email address" required /></td></tr>
    <tr><th>Cc myself:</th><td><input checked type="checkbox" name="cc_myself" /></td></tr>
    >>> print(f.as_ul())
    <li><ul class="errorlist"><li>This field is required.</li></ul>Subject: <input type="text" name="subject" maxlength="100" required /></li>
    <li>Message: <input type="text" name="message" value="Hi there" required /></li>
    <li><ul class="errorlist"><li>Enter a valid email address.</li></ul>Sender: <input type="email" name="sender" value="invalid email address" required /></li>
    <li>Cc myself: <input checked type="checkbox" name="cc_myself" /></li>
    >>> print(f.as_p())
    <p><ul class="errorlist"><li>This field is required.</li></ul></p>
    <p>Subject: <input type="text" name="subject" maxlength="100" required /></p>
    <p>Message: <input type="text" name="message" value="Hi there" required /></p>
    <p><ul class="errorlist"><li>Enter a valid email address.</li></ul></p>
    <p>Sender: <input type="email" name="sender" value="invalid email address" required /></p>
    <p>Cc myself: <input checked type="checkbox" name="cc_myself" /></p>

.. _ref-forms-error-list-format:

Customizing the error list format
---------------------------------

By default, forms use ``django.forms.utils.ErrorList`` to format validation
errors. If you'd like to use an alternate class for displaying errors, you can
pass that in at construction time::

    >>> from django.forms.utils import ErrorList
    >>> class DivErrorList(ErrorList):
    ...     def __str__(self):
    ...         return self.as_divs()
    ...     def as_divs(self):
    ...         if not self: return ''
    ...         return '<div class="errorlist">%s</div>' % ''.join(['<div class="error">%s</div>' % e for e in self])
    >>> f = ContactForm(data, auto_id=False, error_class=DivErrorList)
    >>> f.as_p()
    <div class="errorlist"><div class="error">This field is required.</div></div>
    <p>Subject: <input type="text" name="subject" maxlength="100" required /></p>
    <p>Message: <input type="text" name="message" value="Hi there" required /></p>
    <div class="errorlist"><div class="error">Enter a valid email address.</div></div>
    <p>Sender: <input type="email" name="sender" value="invalid email address" required /></p>
    <p>Cc myself: <input checked type="checkbox" name="cc_myself" /></p>

More granular output
====================

The ``as_p()``, ``as_ul()``, and ``as_table()`` methods are simply shortcuts --
they're not the only way a form object can be displayed.

.. class:: BoundField

   Used to display HTML or access attributes for a single field of a
   :class:`Form` instance.

   The ``__str__()`` method of this object displays the HTML for this field.

To retrieve a single ``BoundField``, use dictionary lookup syntax on your form
using the field's name as the key::

    >>> form = ContactForm()
    >>> print(form['subject'])
    <input id="id_subject" type="text" name="subject" maxlength="100" required />

To retrieve all ``BoundField`` objects, iterate the form::

    >>> form = ContactForm()
    >>> for boundfield in form: print(boundfield)
    <input id="id_subject" type="text" name="subject" maxlength="100" required />
    <input type="text" name="message" id="id_message" required />
    <input type="email" name="sender" id="id_sender" required />
    <input type="checkbox" name="cc_myself" id="id_cc_myself" />

The field-specific output honors the form object's ``auto_id`` setting::

    >>> f = ContactForm(auto_id=False)
    >>> print(f['message'])
    <input type="text" name="message" required />
    >>> f = ContactForm(auto_id='id_%s')
    >>> print(f['message'])
    <input type="text" name="message" id="id_message" required />

Attributes of ``BoundField``
----------------------------

.. attribute:: BoundField.auto_id

    The HTML ID attribute for this ``BoundField``. Returns an empty string
    if :attr:`Form.auto_id` is ``False``.

.. attribute:: BoundField.data

    This property returns the data for this :class:`~django.forms.BoundField`
    extracted by the widget's :meth:`~django.forms.Widget.value_from_datadict`
    method, or ``None`` if it wasn't given::

        >>> unbound_form = ContactForm()
        >>> print(unbound_form['subject'].data)
        None
        >>> bound_form = ContactForm(data={'subject': 'My Subject'})
        >>> print(bound_form['subject'].data)
        My Subject

.. attribute:: BoundField.errors

    A :ref:`list-like object <ref-forms-error-list-format>` that is displayed
    as an HTML ``<ul class="errorlist">`` when printed::

        >>> data = {'subject': 'hi', 'message': '', 'sender': '', 'cc_myself': ''}
        >>> f = ContactForm(data, auto_id=False)
        >>> print(f['message'])
        <input type="text" name="message" required />
        >>> f['message'].errors
        ['This field is required.']
        >>> print(f['message'].errors)
        <ul class="errorlist"><li>This field is required.</li></ul>
        >>> f['subject'].errors
        []
        >>> print(f['subject'].errors)

        >>> str(f['subject'].errors)
        ''

.. attribute:: BoundField.field

    The form :class:`~django.forms.Field` instance from the form class that
    this :class:`~django.forms.BoundField` wraps.

.. attribute:: BoundField.form

    The :class:`~django.forms.Form` instance this :class:`~django.forms.BoundField`
    is bound to.

.. attribute:: BoundField.help_text

    The :attr:`~django.forms.Field.help_text` of the field.

.. attribute:: BoundField.html_name

    The name that will be used in the widget's HTML ``name`` attribute. It takes
    the form :attr:`~django.forms.Form.prefix` into account.

.. attribute:: BoundField.id_for_label

    Use this property to render the ID of this field. For example, if you are
    manually constructing a ``<label>`` in your template (despite the fact that
    :meth:`~BoundField.label_tag` will do this for you):

    .. code-block:: html+django

        <label for="{{ form.my_field.id_for_label }}">...</label>{{ my_field }}

    By default, this will be the field's name prefixed by ``id_``
    ("``id_my_field``" for the example above). You may modify the ID by setting
    :attr:`~django.forms.Widget.attrs` on the field's widget. For example,
    declaring a field like this::

        my_field = forms.CharField(widget=forms.TextInput(attrs={'id': 'myFIELD'}))

    and using the template above, would render something like:

    .. code-block:: html

        <label for="myFIELD">...</label><input id="myFIELD" type="text" name="my_field" required />

.. attribute:: BoundField.is_hidden

    Returns ``True`` if this :class:`~django.forms.BoundField`'s widget is
    hidden.

.. attribute:: BoundField.label

    The :attr:`~django.forms.Field.label` of the field. This is used in
    :meth:`~BoundField.label_tag`.

.. attribute:: BoundField.name

    The name of this field in the form::

        >>> f = ContactForm()
        >>> print(f['subject'].name)
        subject
        >>> print(f['message'].name)
        message

Methods of ``BoundField``
-------------------------

.. method:: BoundField.as_hidden(attrs=None, **kwargs)

    Returns a string of HTML for representing this as an ``<input type="hidden">``.

    ``**kwargs`` are passed to :meth:`~django.forms.BoundField.as_widget`.

    This method is primarily used internally. You should use a widget instead.

.. method:: BoundField.as_widget(widget=None, attrs=None, only_initial=False)

    Renders the field by rendering the passed widget, adding any HTML
    attributes passed as ``attrs``.  If no widget is specified, then the
    field's default widget will be used.

    ``only_initial`` is used by Django internals and should not be set
    explicitly.

.. method:: BoundField.css_classes()

    When you use Django's rendering shortcuts, CSS classes are used to
    indicate required form fields or fields that contain errors. If you're
    manually rendering a form, you can access these CSS classes using the
    ``css_classes`` method::

        >>> f = ContactForm(data={'message': ''})
        >>> f['message'].css_classes()
        'required'

    If you want to provide some additional classes in addition to the
    error and required classes that may be required, you can provide
    those classes as an argument::

        >>> f = ContactForm(data={'message': ''})
        >>> f['message'].css_classes('foo bar')
        'foo bar required'

.. method:: BoundField.label_tag(contents=None, attrs=None, label_suffix=None)

    To separately render the label tag of a form field, you can call its
    ``label_tag()`` method::

        >>> f = ContactForm(data={'message': ''})
        >>> print(f['message'].label_tag())
        <label for="id_message">Message:</label>

    You can provide the ``contents`` parameter which will replace the
    auto-generated label tag. An ``attrs`` dictionary may contain additional
    attributes for the ``<label>`` tag.

    The HTML that's generated includes the form's
    :attr:`~django.forms.Form.label_suffix` (a colon, by default) or, if set, the
    current field's :attr:`~django.forms.Field.label_suffix`. The optional
    ``label_suffix`` parameter allows you to override any previously set
    suffix. For example, you can use an empty string to hide the label on selected
    fields. If you need to do this in a template, you could write a custom
    filter to allow passing parameters to ``label_tag``.

.. method:: BoundField.value()

    Use this method to render the raw value of this field as it would be rendered
    by a ``Widget``::

        >>> initial = {'subject': 'welcome'}
        >>> unbound_form = ContactForm(initial=initial)
        >>> bound_form = ContactForm(data={'subject': 'hi'}, initial=initial)
        >>> print(unbound_form['subject'].value())
        welcome
        >>> print(bound_form['subject'].value())
        hi

Customizing ``BoundField``
==========================

If you need to access some additional information about a form field in a
template and using a subclass of :class:`~django.forms.Field` isn't
sufficient, consider also customizing :class:`~django.forms.BoundField`.

A custom form field can override ``get_bound_field()``:

.. method:: Field.get_bound_field(form, field_name)

    Takes an instance of :class:`~django.forms.Form` and the name of the field.
    The return value will be used when accessing the field in a template. Most
    likely it will be an instance of a subclass of
    :class:`~django.forms.BoundField`.

If you have a ``GPSCoordinatesField``, for example, and want to be able to
access additional information about the coordinates in a template, this could
be implemented as follows::

    class GPSCoordinatesBoundField(BoundField):
        @property
        def country(self):
            """
            Return the country the coordinates lie in or None if it can't be
            determined.
            """
            value = self.value()
            if value:
                return get_country_from_coordinates(value)
            else:
                return None

    class GPSCoordinatesField(Field):
        def get_bound_field(self, form, field_name):
            return GPSCoordinatesBoundField(form, self, field_name)

Now you can access the country in a template with
``{{ form.coordinates.country }}``.

.. _binding-uploaded-files:

Binding uploaded files to a form
================================

Dealing with forms that have ``FileField`` and ``ImageField`` fields
is a little more complicated than a normal form.

Firstly, in order to upload files, you'll need to make sure that your
``<form>`` element correctly defines the ``enctype`` as
``"multipart/form-data"``::

  <form enctype="multipart/form-data" method="post" action="/foo/">

Secondly, when you use the form, you need to bind the file data. File
data is handled separately to normal form data, so when your form
contains a ``FileField`` and ``ImageField``, you will need to specify
a second argument when you bind your form. So if we extend our
ContactForm to include an ``ImageField`` called ``mugshot``, we
need to bind the file data containing the mugshot image::

    # Bound form with an image field
    >>> from django.core.files.uploadedfile import SimpleUploadedFile
    >>> data = {'subject': 'hello',
    ...         'message': 'Hi there',
    ...         'sender': 'foo@example.com',
    ...         'cc_myself': True}
    >>> file_data = {'mugshot': SimpleUploadedFile('face.jpg', <file data>)}
    >>> f = ContactFormWithMugshot(data, file_data)

In practice, you will usually specify ``request.FILES`` as the source
of file data (just like you use ``request.POST`` as the source of
form data)::

    # Bound form with an image field, data from the request
    >>> f = ContactFormWithMugshot(request.POST, request.FILES)

Constructing an unbound form is the same as always -- just omit both
form data *and* file data::

    # Unbound form with an image field
    >>> f = ContactFormWithMugshot()

Testing for multipart forms
---------------------------

.. method:: Form.is_multipart()

If you're writing reusable views or templates, you may not know ahead of time
whether your form is a multipart form or not. The ``is_multipart()`` method
tells you whether the form requires multipart encoding for submission::

    >>> f = ContactFormWithMugshot()
    >>> f.is_multipart()
    True

Here's an example of how you might use this in a template::

    {% if form.is_multipart %}
        <form enctype="multipart/form-data" method="post" action="/foo/">
    {% else %}
        <form method="post" action="/foo/">
    {% endif %}
    {{ form }}
    </form>

Subclassing forms
=================

If you have multiple ``Form`` classes that share fields, you can use
subclassing to remove redundancy.

When you subclass a custom ``Form`` class, the resulting subclass will
include all fields of the parent class(es), followed by the fields you define
in the subclass.

In this example, ``ContactFormWithPriority`` contains all the fields from
``ContactForm``, plus an additional field, ``priority``. The ``ContactForm``
fields are ordered first::

    >>> class ContactFormWithPriority(ContactForm):
    ...     priority = forms.CharField()
    >>> f = ContactFormWithPriority(auto_id=False)
    >>> print(f.as_ul())
    <li>Subject: <input type="text" name="subject" maxlength="100" required /></li>
    <li>Message: <input type="text" name="message" required /></li>
    <li>Sender: <input type="email" name="sender" required /></li>
    <li>Cc myself: <input type="checkbox" name="cc_myself" /></li>
    <li>Priority: <input type="text" name="priority" required /></li>

It's possible to subclass multiple forms, treating forms as mixins. In this
example, ``BeatleForm`` subclasses both ``PersonForm`` and ``InstrumentForm``
(in that order), and its field list includes the fields from the parent
classes::

    >>> from django import forms
    >>> class PersonForm(forms.Form):
    ...     first_name = forms.CharField()
    ...     last_name = forms.CharField()
    >>> class InstrumentForm(forms.Form):
    ...     instrument = forms.CharField()
    >>> class BeatleForm(InstrumentForm, PersonForm):
    ...     haircut_type = forms.CharField()
    >>> b = BeatleForm(auto_id=False)
    >>> print(b.as_ul())
    <li>First name: <input type="text" name="first_name" required /></li>
    <li>Last name: <input type="text" name="last_name" required /></li>
    <li>Instrument: <input type="text" name="instrument" required /></li>
    <li>Haircut type: <input type="text" name="haircut_type" required /></li>

It's possible to declaratively remove a ``Field`` inherited from a parent class
by setting the name of the field to ``None`` on the subclass. For example::

    >>> from django import forms

    >>> class ParentForm(forms.Form):
    ...     name = forms.CharField()
    ...     age = forms.IntegerField()

    >>> class ChildForm(ParentForm):
    ...     name = None

    >>> list(ChildForm().fields)
    ['age']

.. _form-prefix:

Prefixes for forms
==================

.. attribute:: Form.prefix

You can put several Django forms inside one ``<form>`` tag. To give each
``Form`` its own namespace, use the ``prefix`` keyword argument::

    >>> mother = PersonForm(prefix="mother")
    >>> father = PersonForm(prefix="father")
    >>> print(mother.as_ul())
    <li><label for="id_mother-first_name">First name:</label> <input type="text" name="mother-first_name" id="id_mother-first_name" required /></li>
    <li><label for="id_mother-last_name">Last name:</label> <input type="text" name="mother-last_name" id="id_mother-last_name" required /></li>
    >>> print(father.as_ul())
    <li><label for="id_father-first_name">First name:</label> <input type="text" name="father-first_name" id="id_father-first_name" required /></li>
    <li><label for="id_father-last_name">Last name:</label> <input type="text" name="father-last_name" id="id_father-last_name" required /></li>

The prefix can also be specified on the form class::

    >>> class PersonForm(forms.Form):
    ...     ...
    ...     prefix = 'person'