Forms
=====
.. module:: wtforms.form

Forms provide the highest level API in WTForms. They contain your field
definitions, delegate validation, take input, aggregate errors, and in
general function as the glue holding everything together.

The Form class
--------------

.. class:: Form

    Declarative Form base class.

    **Construction**

    .. automethod:: __init__

        Initialize a Form. This is usually done in the context of a
        view/controller in your application. When a Form is constructed, the
        fields populate their input based on the formdata, obj, and kwargs.

        **Note**
        Backing-store objects and kwargs are both expected to be provided
        with the values being already-coerced datatypes. WTForms does not
        check the types of incoming object-data or coerce them like it will
        for `formdata` as it is expected this data is defaults or data from
        a backing store which this form represents. See the section on
        :ref:`using Forms <using-forms>` for more information.


    **Properties**

    .. attribute:: data

        A dict containing the data for each field.

        Note that this is generated each time you access the property, so care
        should be taken when using it, as it can potentially be very expensive
        if you repeatedly access it. Typically used if you need to iterate all
        data in the form. If you just need to access the data for known fields,
        you should use `form.<field>.data`, not this proxy property.

    .. attribute:: errors

        A dict containing a list of errors for each field. Empty if the form
        hasn't been validated, or there were no errors.

        Note that this is a lazy property, and will only be generated when you
        first access it. If you call :meth:`validate` after accessing it, the
        cached result will be invalidated and regenerated on next access.

    .. attribute:: meta

        This is an object which contains various configuration options and also
        ability to customize the behavior of the form. See the :doc:`meta` doc
        for more information on what can be customized with the class Meta options.

    **Methods**

    .. automethod:: validate

    .. automethod:: populate_obj

        One common usage of this is an edit profile view::

            def edit_profile(request):
                user = User.objects.get(pk=request.session['userid'])
                form = EditProfileForm(request.POST, obj=user)

                if request.POST and form.validate():
                    form.populate_obj(user)
                    user.save()
                    return redirect('/home')
                return render_to_response('edit_profile.html', form=form)

        In the above example, because the form isn't directly tied to the user
        object, you don't have to worry about any dirty data getting onto there
        until you're ready to move it over.

    .. automethod:: __iter__

        .. code-block:: django

            {% for field in form %}
                <tr>
                    <th>{{ field.label }}</th>
                    <td>{{ field }}</td>
                </tr>
            {% endfor %}

    .. automethod:: __contains__

    .. automethod:: _get_translations

Defining Forms
--------------

To define a form, one makes a subclass of :class:`Form` and defines the fields
declaratively as class attributes::

    class MyForm(Form):
        first_name = StringField(u'First Name', validators=[validators.input_required()])
        last_name  = StringField(u'Last Name', validators=[validators.optional()])

Field names can be any valid python identifier, with the following restrictions:

* Field names are case-sensitive.
* Field names may not begin with "_" (underscore)
* Field names may not begin with "validate"

Form Inheritance
~~~~~~~~~~~~~~~~

Forms may subclass other forms as needed.  The new form will contain all fields
of the parent form, as well as any new fields defined on the subclass.  A field
name re-used on a subclass causes the new definition to obscure the original.

.. code-block:: python

    class PastebinEdit(Form):
        language = SelectField(u'Programming Language', choices=PASTEBIN_LANGUAGES)
        code     = TextAreaField()

    class PastebinEntry(PastebinEdit):
        name = StringField(u'User Name')


.. _inline-validators:

In-line Validators
~~~~~~~~~~~~~~~~~~

In order to provide custom validation for a single field without needing to
write a one-time-use validator, validation can be defined inline by defining a
method with the convention `validate_fieldname`::

    class SignupForm(Form):
        age = IntegerField(u'Age')

        def validate_age(form, field):
            if field.data < 13:
                raise ValidationError("We're sorry, you must be 13 or older to register")


.. _using-forms:

Using Forms
-----------

A form is most often constructed in the controller code for handling an action,
with the form data wrapper from the framework passed to its constructor, and
optionally an ORM object. A typical view begins something like:

.. code-block:: python

    def edit_article(request):
        article = Article.get(...)
        form = MyForm(request.POST, article)

A typical CRUD view has a user editing an object that needs various fields
updated. The Form would have fields describing the fields to be updated and
the validation rules, where the attribute names of the fields match those of
the attribute names on the object. The second parameter to the Form, the `obj`
parameter, is used to populate form defaults on the initial view.

.. note::
    While we did pass an object as the data source, this object data is only
    used if there is no POST data. If there is any POST data at all, then the
    object data is ignored. This is done for security and consistency reasons.

    This pattern is mostly a convenience since most application controllers
    don't separate GET and POST requests into separate view methods.

The constructed form can then validate any input data and generate errors if
invalid. Typically, the validation pattern in the view looks like::

    if request.POST and form.validate():
        form.populate_obj(article)
        article.save()
        return redirect('/articles')

Note that we have it so :meth:`~Form.validate` is only called if there is
POST data. The reason we gate the validation check this way is that when
there is no POST data (such as in a typical CRUD form) we don't want to
cause validation errors.

Inside the gated block, we call :meth:`~Form.populate_obj` to copy the data
onto fields on the 'article' object. We also then redirect after a successful
completion. The reason we redirect after the post is a best-practice associated
with the `Post/Redirect/Get <https://en.wikipedia.org/wiki/Post/Redirect/Get>`_
design pattern.

If there is no POST data, or the data fails to validate, then the view "falls
through" to the rendering portion. The Form object can be passed into the
template and its attributes can be used to render the fields and also for
displaying errors::

        return render('edit.html', form=form, article=article)

So there we have a full simple "edit object" page setup which
illustrates a best-practice way of using WTForms. This is by no means the only
way to use WTForms, but just an illustration of how the various features work.

Here is the full code for the view we just made::

    def edit_article(request):
        article = Article.get(...)
        form = MyForm(request.POST, article)

        if request.POST and form.validate():
            form.populate_obj(article)
            article.save()
            return redirect('/articles')

        return render('edit.html', form=form, article=article)



Low-Level API
-------------

.. warning::

    This section is provided for completeness; and is aimed at authors of
    complementary libraries and those looking for very special behaviors.
    Don't use `BaseForm` unless you know exactly *why* you are using it.

For those looking to customize how WTForms works, for libraries or special
applications, it might be worth using the :class:`BaseForm` class. `BaseForm` is
the parent class of :class:`Form`, and most of the implementation
logic from Form is actually handled by BaseForm.

The major difference on the surface between `BaseForm` and `Form` is that
fields are not defined declaratively on a subclass of `BaseForm`. Instead, you
must pass a dict of fields to the constructor. Likewise, you cannot add fields
by inheritance. In addition, `BaseForm` does not provide: sorting fields by
definition order, or inline `validate_foo` validators.  Because of this, for
the overwhelming majority of uses we recommend you use Form instead of BaseForm
in your code.

What `BaseForm` provides is a container for a collection of fields, which
it will bind at instantiation, and hold in an internal dict. Dict-style access
on a BaseForm instance will allow you to access (and modify) the enclosed
fields.

.. autoclass:: BaseForm()

    **Construction**

    .. automethod:: __init__(fields, prefix='', meta=<DefaultMeta>)

        .. code-block:: python

            form = BaseForm({
                'name': StringField(),
                'customer.age': IntegerField("Customer's Age")
            })

        Because BaseForm does not require field names to be valid identifiers,
        they can be most any python string. We recommend keeping it
        simple to avoid incompatibility with browsers and various form input
        frameworks where possible.

    **Properties**

    .. attribute:: data

        see :attr:`Form.data`

    .. attribute:: errors

        see :attr:`Form.errors`

    **Methods**

    .. automethod:: process

        Since BaseForm does not take its data at instantiation, you must call
        this to provide form data to the enclosed fields. Accessing the field's
        data before calling process is not recommended.

    .. automethod:: validate

    .. automethod:: __iter__

        Unlike :class:`Form`, fields are not iterated in definition order, but
        rather in whatever order the dict decides to yield them.

    .. automethod:: __contains__

    .. automethod:: __getitem__

    .. automethod:: __setitem__

        .. code-block:: python

            form['openid.name'] = StringField()

        Fields can be added and replaced in this way, but this must be done
        **before** :meth:`process` is called, or the fields will not have the
        opportunity to receive input data. Similarly, changing fields after
        :meth:`validate` will have undesired effects.

    .. automethod:: __delitem__

        The same caveats apply as with :meth:`__setitem__`.
