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YAFOWIL targets rendering form widgets and extracting/validating the data send by the browser per widget.

YAFOWIL widgets are just configuration. It provides a factory which can produce widget instances from blueprints.

There is a library of existing blueprints ready to be extended on demand.

YAFOWIL provides blueprints for all HTML standard inputs, lots of helper blueprints for buidling complex widgets and a bunch of add-ons (usally in namespace yafowil.widget.*).


Tired of inventing widgets again and again after switching the Python framework YAFOWIL is intentionally written framework-independent. By just feeding it with configuration it can be used and extended in most of existing python web frameworks. Zope, Pyramid, Django, Flask, CherryPy and similar are candidates.

Another common problem with form libs is a non-unique look and feel of the available widget collection. YAFOWIL tries to provide some useful addon widgets which tries to take care of a unified user experience.


YAFOWIL aims to have no dependencies to any web framework. It utilizes the node package. YAFOWIL does not know about data-storage, but offers a hook to add processing callback handler functions.


YAFOWIL currently integrates with the following packages:

For details read the chapter integrations.


For the impatient, code says more than 1000 words: A simple example form works like so:

import yafowil.loader
from yafowil.base import factory
from yafowil.controller import Controller

Produce a form.:

form = factory('form', name='myform', props={
    'action': 'http://www.domain.tld/someform',

form['someinput'] = factory('label:text', props={
    'label': 'Your Text',

def formaction(widget, data):

def formnext(request):
    return 'http://www.domain.tld/result'

form['submit'] = factory('submit', props={
    'handler': formaction,
    'next': formnext,
    'action': True,

Render empty form by calling the form object:

rendered = form()

This results in:

<form action="http://www.domain.tld/someform"
    <label for="input-myform-someinput">Your Text</label>
    <input id="input-myform-someinput"
    <input id="input-myform-submit"
           value="submit" />

Get form data from of request (request is expected dict-like):

request = {'myform.someinput': 'Hello World',
           'action.myform.submit': 'submit'}
controller = Controller(form, request)
<RuntimeData myform, value=None, extracted=None at ...>
  <RuntimeData myform.someinput, value=None, extracted='Hello World',
  attrs={'input_field_type': 'text'} at ...>
  <RuntimeData myform.submit, value=None, extracted=<UNSET> at ...>

Creating a widget

A widget is an instance of a blueprint created by the factory. Factory is a singleton and operates also as a registry for blueprints.

By calling the factory a widget is created, here a naked text input field from the blueprint text:

widget = factory('text')

Blueprints can be chained by colon separated names or given as list:

widget = factory('field:label:text')

This causes the created widget to chain the registered renderers, extractors, and other parts of the blueprints field, label and text in order.

Blueprint chains can be organised using as macros to reduce the complexity of factory calls (details below). I.e.:

widget = factory('#field:text')

expands the macro #field to field:label:error and appends :text so the result is field:label:error:text.

Widgets are Organized as a Tree

Any HTML form can be visualized as a tree: The <form> is the root, <input> elements are its children. Also a <fieldset> grouping a bunch of <input> is a compound with children. In YAFOWIL the form is organized as a tree which is like a ordered python dict with each value a dict again. Widget instances are dict-like objects.

Thus, a widget is either a compound node (containing children) or a leaf node in this tree.

Building widget trees is as simple as using python dicts:

form = factory('form', 'UNIQUENAME', props={
    'action': 'someurl',
form['somefield'] = factory('field:label:text', props={
    'label': 'Some Field',
form['somefieldset'] = factory('fieldset', props={
    'legend': 'A Fieldset',
form['somefieldset']['innerfield'] = factory('field:label:text', props={
    'label': 'Inner Field',

Rendering Mode

The way a widget is rendered is controlled by the mode. Every widget may given a mode keyword argument to the factory as a string or a callable accepting two parameters widget and ``data``returning a string.

These modes are supported:

Default classic mode, editing of form is possible. Rendering follows the registered edit_renderers.
No form elements are rendered, just the data as defined by registerd display_renders.
Renders just an empty string.


In YAFOWIL validation and extraction happens at the same time. Extraction means to get a meaningful value out of the request. Validation means to check constraints, i.e if a number is positive or an e-mail-adress is valid.


Invariants are implemented as extractors on compounds. Usally they are put as a custom blueprint (see below) with one extractor on some parent of the elements to validate.

Here is a short example (extension of the hello world example) for a custom invariant extractor which checks if one or the other field is filled, but never both or none (XOR):

from yafowil.base import ExtractionError
# ... see helloworld example whats missing here

def myinvariant_extractor(widget, data):
    if not (bool(data['hello']) != bool(data['world']):
        error = ExtractionError(
            'provide hello or world, not both or none')
    return data.extracted

def application(environ, start_response):
    # ... see helloworld example whats missing here
    form = factory(
            'action': url,
            'myinvariant': dict(extractors=[myinvariant_extractor]),
    form['hello'] = factory('field:label:error:text', props={
        'label': 'Enter some text here',
        'value': '',
    form['world'] = factory('field:label:error:text', props={
        'label': 'OR Enter some text here',
        'value': '',
    # ... see helloworld example whats missing here

Providing blueprints

General behaviours (rendering, extracting, etc...) can be registered as blueprint in the factory:


and then used as regular blueprint when calling the factory:

widget = factory('field:label:myblueprint:text', props={
    'label': 'Inner Field',

Adding custom behaviour

It’s possible to inject custom behaviour by marking a part of the blueprint chain with the asterisk * character. Behaviours are one or a combination of a

extracts, validates and/or converts form-data from the request.
build the markup for editing.
build the markup for display only.
Generic hook called once at factory time of the widget. Here i.e. subwidgets can be created.
Generic hook to prepare runtime-data. Runs once per runtime-data instance before extractors or renderers are running.
def myvalidator(widget, data):
   # validate the data, raise ExtractionError if somethings wrong
   if data.extracted != 'something:'
       raise ExtractionError("only 'something' is allowed as input.")
   return data.extracted

widget = factory(
        'label': 'Inner Field',
        'myvalidation': dict(extractor=[myvalidator]),

Delivering resources

YAFOWIL addon widgets are shipped with related Javascript and Stylesheet resources. These resources are registered to the factory with additional information like delivery order and resources group.

To help the integrator delivering these resources through the used web framework, the helper object yafowil.resources.YafowilResources is supposed to be used.

The function configure_resource_directory should be overwritten on deriving class which is responsible to make the given physical resource directory somehow available to the web.

The object can be instanciated with js_skip and css_skip keyword arguments, which contain iterable resource group names to skip when calculating resources. This is useful if basic or dependency resources are already shipped in another way.

The following example shows how to integrate YAFOWIL resources in a pyramid application.

from pyramid.static import static_view
from yafowil.resources import YafowilResources
import mypackage.views

class Resources(YafowilResources):

   def __init__(self, js_skip=[], css_skip=[], config=None):
       self.config = config
       super(Resources, self).__init__(js_skip=js_skip, css_skip=css_skip)

   def configure_resource_directory(self, plugin_name, resourc_edir):
       # instanciate static view
       resources_view = static_view(resourc_edir, use_subpath=True)
       # attach resources view to package
       view_name = '%s_resources' % plugin_name.replace('.', '_')
       setattr(mypackage.views, view_name, resources_view)
       # register view via config
       view_path = 'mypackage.views.%s' % view_name
       resource_base = '++resource++%s' % plugin_name
       self.config.add_view(view_path, name=resource_base)
       return resource_base

def includeme(config):
    # resources object gets instanciated only once
    resources = Resources(config=config)

    # sorted JS resources URL's. Supposed to be rendered to HTML

    # sorted CSS resources URL's. Supposed to be rendered to HTML