What is z3c.form all about?
HTML forms are the cornerstone of modern web applications. When you interact with Plone, you use forms all the time - to search the content store, to edit content items, to fill in your personal details. You will notice that most of these forms use the same layout and conventions, and that they all rely on common patterns such as server-side validation and different buttons resulting in different actions.
Over the years, several approaches have evolved to deal with forms. A few of the most important ones are:
- Creating a simple view with an HTML form that submits to itself (or another view), where the request is validated and processed in custom Python code. This is very flexible and requires little learning, but can also be fairly cumbersome, and it is harder to maintain a common look and feel and behaviour across all forms. See the Views and viewlets for some hints on one way to build such views.
- Using the CMFFormController library. This relies on special page objects known as “controller page templates” that submit to “controller python scripts”. The form controller takes care of the flow between forms and actions, and can invoke validator scripts. This only superficially addresses the creation of standard form layouts and widgets, however. It is largely deprecated, although Plone still uses it internally in places.
- Using zope.formlib. This is a library which ships with Zope. It is based on the principle that a schema interface defines a number of form fields, constraints and so on. Special views are then used to render these using a standard set of widgets. Formlib takes care of page flow, validation and the invocation of actions - methods that correspond to buttons on the form. Formlib is used for Plone’s control panels and portlets. However, it can be cumbersome to use, especially when it comes to creating custom widgets or more dynamic forms.
- Using `z3c.form`_. This is a newer library, inspired by formlib, but more flexible and modern.
This manual will show you how to use z3c.form in a Plone context. It will use tools and patterns that are consistent with those used for Dexterity development, as shown in the Dexterity developer manual, but the information contained herein is not Dexterity specific. Note that Dexterity’s standard add and edit forms are all based on z3c.form.
As a library, z3c.form has spawned a number of add-on modules, ranging from new field types and widgets, to extensions that add functionality to the forms built using the framework. We will refer to a number of packages in this tutorial. The most important packages are:
- z3c.form itself, the basic form library. This defines the standard form view base classes, as well the default widgets. The z3c.form documentation applies to the forms created here, but some of the packages below simplify or enhance the integration experience.
- plone.z3cform makes z3c.form usable in Zope 2. It also adds a number of features useful in Zope 2 applications, notably a mechanism to extend or modify the fields in forms on the fly.
- plone.app.z3cform configures z3c.form to use Plone-looking templates by default, and adds few services, such as a widget to use Plone’s visual editor and “inline” on-the-fly validation of forms. This package must be installed for z3c.form-based forms to work in Plone.
- plone.autoform improves z3c.form’s ability to create a form from a schema interface. By using the base classes in this package, schemata can be more self-describing, for example specifying a custom widget, or specifying relative field ordering. We will use plone.autoform in this tutorial to simplify form setup.
- plone.directives.form provides tools for registering forms using convention-over-configuration instead of ZCML. We will use plone.directives.form to configure our forms in this manual.
A note about versions¶
This manual is targeted at Plone 4.1 and above (Zope 2.13).