In this chapter, we'll walk through basic usage of Deform to render a form, and capture and validate input.
The steps a developer must take to cause a form to be renderered and subsequently be ready to accept form submission input are:
- Define a schema
- Create a form object.
- Assign non-default widgets to fields in the form (optional).
- Render the form.
Once the form is rendered, a user will interact with the form in his browser, and some point, he will submit it. When the user submits the form, the data provided by the user will either validate properly, or the form will need to be rerendered with error markers which help to inform the user of which parts need to be filled in "properly" (as defined by the schema). We allow the user to continue filling in the form, submitting, and revalidating indefinitely.
Defining A Schema
The first step to using Deform is to create a :term:`schema` which represents the data structure you wish to be captured via a form rendering.
For example, let's imagine you want to create a form based roughly on a data structure you'll obtain by reading data from a relational database. An example of such a data structure might look something like this:
In other words, the database query we make returns a sequence of people; each person is represented by some data. We need to edit this data. There won't be many people in this list, so we don't need any sort of paging or batching to make our way through the list; we can display it all on one form page.
Deform designates a structure akin to the example above as an :term:`appstruct`. The term "appstruct" is shorthand for "application structure", because it's the kind of high-level structure that an application usually cares about: the data present in an appstruct is useful directly to an application itself.
An appstruct differs from other structures that Deform uses (such as :term:`pstruct` and :term:`cstruct` structures): pstructs and cstructs are typically only useful during intermediate parts of the rendering process.
Usually, given some appstruct, you can divine a :term:`schema` that would allow you to edit the data related to the appstruct. Let's define a schema which will attempt to serialize this particular appstruct to a form. Our application has these requirements of the resulting form:
- It must be possible to add and remove a person.
- It must be possible to change any person's name or age after they've been added.
Here's a schema that will help us meet those requirements:
The schemas used by Deform come from a package named :term:`Colander`. The canonical documentation for Colander exists at http://docs.pylonsproject.org/projects/colander/dev/ . To compose complex schemas, you'll need to read it to get comfy with the documentation of the default Colander data types. But for now, we can play it by ear.
For ease of reading, we've actually defined three schemas above, but
we coalesce them all into a single schema instance as
the last step. A
People schema is a collection of
schema nodes. As the result of our definitions, a
name, which must be a string.
age, which must be deserializable to an integer; after deserialization happens, a validator ensures that the integer is between 0 and 200 inclusive.
Schema Node Objects
This section repeats and contextualizes the :term:`Colander` documentation about schema nodes in order to prevent you from needing to switch away from this page to another while trying to learn about forms. But you can also get much the same information at http://docs.pylonsproject.org/projects/colander/dev/
A schema is composed of one or more schema node objects, each typically of the class :class:`colander.SchemaNode`, usually in a nested arrangement. Each schema node object has a required type, an optional preparer for adjusting data after deserialization, an optional validator for deserialized prepared data, an optional default, an optional missing, an optional title, an optional description, and a slightly less optional name. It also accepts arbitrary keyword arguments, which are attached directly as attributes to the node instance.
The preparer of a schema node is called after deserialization but before validation; it prepares a deserialized value for validation. Examples would be to prepend schemes that may be missing on url values or to filter html provided by a rich text editor. A preparer is not called during serialization, only during deserialization.
The validator of a schema node is called after deserialization and
preparation ; it makes sure the value matches a constraint. An example of
such a validator is provided in the schema above:
validator=colander.Range(0, 200). A validator is not called after
schema node serialization, only after node deserialization.
The default of a schema node indicates the value to be serialized if a value for the schema node is not found in the input data during serialization. It should be the deserialized representation.
The missing of a schema node indicates the value to be deserialized
if a value for the schema node is not found in the input data during
deserialization. It should be the deserialized representation. If a
schema node does not have a
missing value, a
:exc:`colander.Invalid` exception will be raised if the data structure
being deserialized does not contain a matching value.
The name of a schema node is used to relate schema nodes to each other. It is also used as the title if a title is not provided.
The title of a schema node is metadata about a schema node. It shows up in the legend above the form field(s) related to the schema node. By default, it is a capitalization of the name.
The description of a schema node is metadata about a schema node. It shows up as a tooltip when someone hovers over the form control(s) related to a :term:`field`. By default, it is empty.
The name of a schema node that is introduced as a class-level attribute of a :class:`colander.MappingSchema`, :class:`colander.TupleSchema` or a :class:`colander.SequenceSchema` is its class attribute name. For example:
The name of the schema node defined via
colander.SchemaNode(..) within the schema above is
The title of the same schema node is
In the examples above, if you've been paying attention, you'll have noticed that we're defining classes which subclass from :class:`colander.MappingSchema`, and :class:`colander.SequenceSchema`. It's turtles all the way down: the result of creating an instance of any of :class:`colander.MappingSchema`, :class:`colander.TupleSchema` or :class:`colander.SequenceSchema` object is also a :class:`colander.SchemaNode` object.
Creating Schemas Without Using a Class Statement (Imperatively)
for information about how to create schemas without using a
Creating a schema with or without
class statements is purely a
style decision; the outcome of creating a schema without
statements is the same as creating one with
Rendering a Form
Earlier we defined a schema:
Let's now use this schema to create, render and validate a form.
Creating a Form Object
To create a form object, we do this:
We used the
schema object (an instance of
:class:`colander.MappingSchema`) we created in the previous section as
the first positional parameter to the :class:`deform.Form` class; we
passed the value
('submit',) as the value of the
keyword argument. This will cause a single
submit input element
Submit to be injected at the bottom of the form rendering.
We chose to pass in the button names as a sequence of strings, but we
could have also passed a sequence of instances of the
:class:`deform.Button` class. Either is permissible.
Note that the first positional argument to :class:`deform.Form` must
be a schema node representing a mapping object (a structure which
maps a key to a value). We satisfied this constraint above by passing
schema object, which we obtained via the
:class:`colander.MappingSchema` constructor, as the
argument to the :class:`deform.Form` constructor
Although different kinds of schema nodes can be present in a schema
used by a Deform :class:`deform.Form` instance, a form instance cannot
deal with a schema node representing a sequence, a tuple schema, a
string, an integer, etc. as the value of its
only a schema node representing a mapping is permissible. This
typically means that the object passed as the
schema argument to a
:class:`deform.Form` constructor must be obtained as the result of
using the :class:`colander.MappingSchema` constructor (or the
equivalent imperative spelling).
Rendering the Form
Once we've created a Form object, we can render it without issue by calling the :meth:`deform.Field.render` method: the :class:`deform.Form` class is a subclass of the :class:`deform.Field` class, so this method is available to a :class:`deform.Form` instance.
If we wanted to render an "add" form (a form without initial data), we'd just omit the appstruct while calling :meth:`deform.Field.render`.
form = myform.render()
If we have some existing data already that we'd like to edit using the form (the form is an "edit form" as opposed to an "add form"). That data might look like this:
To inject it into the serialized form as the data to be edited, we'd pass it in to the :meth:`deform.Field.render` method to get a form rendering:
form = myform.render(appstruct)
If, finally, instead we wanted to render a "read-only" variant of an edit form
using the same appstruct, we'd pass the
readonly flag as
to the :meth:`deform.Field.render` method.
form = myform.render(appstruct, readonly=True)
This would cause a page to be rendered in a crude form without any form controls, so the user it's presented to cannot edit it.
Once any of the above statements runs, the
form variable is now a
Unicode object containing an HTML rendering of the edit form, useful
for serving out to a browser. The root tag of the rendering will be
<form> tag representing this form (or at least a
that contains this form tag), so the application using it will need to
wrap it in HTML
<body> tags as necessary. It will
need to be inserted as "structure" without any HTML escaping.
Serving up the Rendered Form
We now have an HTML rendering of a form as the variable named
form. But before we can serve it up successfully to a browser
user, we have to make sure that static resources used by Deform can be
resolved properly. Some Deform widgets (including at least one we've
implied in our sample schema) require access to static resources such
as images via HTTP.
For these widgets to work properly, we'll need to arrange that files
in the directory named
static within the :mod:`deform` package can
be resolved via a URL which lives at the same hostname and port number
as the page which serves up the form itself. For example, the URL
/static/images/close.png should be willing to return the
close.png image in the
static/images directory in the
:mod:`deform` package and
image/png content . How you arrange to do this is dependent on
your web framework. It's done in :mod:`pyramid` imperative
config = Configurator(...) ... config.add_static_view('static', 'deform:static') ...
Your web framework will use a different mechanism to offer up static files.
and images present in the
static directory of the :mod:`deform`
package are the following:
- CSS related to form element renderings.
Each of these libraries should be included in the
<head> tag of a
page which renders a Deform form, e.g.:
The :meth:`deform.field.get_widget_resources` method can be used to
tell you which
static directory-relative files are required by a
particular form rendering, so that you can inject only the ones
necessary into the page rendering.
deform.load() must be called by the HTML
page (usually in a script tag near the end of the page, ala
<script..>deform.load()</script>) which renders a Deform form in
binding. If this function is not called, built-in widgets which use
this within the body of the rendered page near its end:
As above, the head should also contain a
<meta> tag which names a
utf-8 charset in a
Content-Type http-equiv. This is a sane
setting for most systems.
Validating a Form Submission
Once the user seen the form and has chewed on its inputs a bit, he will eventually submit the form. When he submits it, the logic you use to deal with the form validation must do a few things:
- It must detect that a submit button was clicked.
- It must obtain the list of :term:`form controls` from the form POST data.
- It must call the :meth:`deform.Form.validate` method with the list of form controls.
- It must be willing to catch a :exc:`deform.ValidationFailure` exception and rerender the form if there were validation errors.
For example, using the :term:`WebOb` API for the above tasks, and the
form object we created earlier, such a dance might look like this:
The above set of statements is the sort of logic every web app that
uses Deform must do. If the validation stage does not fail, a
appstruct will exist with the data serialized from
the form to be used in your application. Otherwise the form will be
Note that by default, when any form submit button is clicked, the form
will send a post request to the same URL which rendered the form.
This can be changed by passing a different
action to the
Seeing it In Action
To see an "add form" in action that follows the schema in this chapter, visit http://deformdemo.repoze.org/sequence_of_mappings/.
To see a "readonly edit form" in action that follows the schema in this chapter, visit http://deformdemo.repoze.org/readonly_sequence_of_mappings/
The application at http://deformdemo.repoze.org is a :mod:`pyramid` application which demonstrates most of the features of Deform, including most of the widget and data types available for use within an application that uses Deform.