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.

    If present, the key ``None`` contains the content of
    :attr:`~wtforms.forms.Form.form_errors`.

.. attribute:: form_errors

    A list of form-level errors. Those are errors that does not concern a
    particuliar field, but the whole form consistency. Those errors are
    often set when overriding :meth:`~wtforms.forms.Form.validate`.

.. 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__

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('First Name', validators=[validators.input_required()])
    last_name  = StringField('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('Programming Language', choices=PASTEBIN_LANGUAGES)
        code     = TextAreaField()

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


.. _inline-validators:

In-line Validators and Filters

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('Age')

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

The same principle applies for filters with the convention filter_fieldname::

class SignupForm(Form):
    name = StringField('name')

    def filter_name(form, field):
        return field.strip()

Note that filters are applied after processing the default and incoming data, but before validation.

.. _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__`.