repoze.formapi 0.4

Overview
========

The ``repoze.formapi`` provides a form library which integrates with
HTML forms instead of abstracting them away.

It provides a small framework to take you through the entire process
of rendering a form, provide default values, validate and execute form
actions.

Form fields are defined using Python base types which map out nested
data structures with end points that are either integers, strings,
floats or tuples of these. It's up to the application to bridge these
with more complex objects.


Using :mod:`repoze.formapi`
===========================

To create a form you subclass the ``repoze.formapi.Form`` class and
define the form field definitions in the ``fields`` attribute.

>>> class TapeForm(formapi.Form):
... """A form to edit a casette tape object."""
...
... fields = {
... 'artist': unicode,
... 'title': unicode,
... 'asin': str,
... 'year': int,
... 'playtime': float}

There are many forms for which default values don't apply (e.g. search
forms or :term:`add forms`); such forms can be instantiated with no
arguments:

>>> form = TapeForm()

The form data is available from the ``data`` attribute. Since we
didn't pass in a request, there's no data available.

>>> form.data['artist'] is None
True

In a web application, usually we'll anticipate data coming in through
the request object. Values are then reflected in the form data. We can
also validate the request against the form fields.

.. note:: There's no inherent concept of required fields.

We pass the request to the form as keyword argument.

>>> request = Request(
... params=(('title', u'Motorcity Detroit USA Live'),))
>>> form = TapeForm(request=request)

This will set the title of the record on the form data object. Since
it's a valid unicode string, we expect no validation errors.

>>> form.validate()
True
>>> form.data['title']
u'Motorcity Detroit USA Live'

Alternatively we can pass in the ``params`` attribute directly.

>>> form = TapeForm(params=request.params)
>>> form.data['title']
u'Motorcity Detroit USA Live'

We'll often want to initialize the form with default values. To this
effect we pass in a dictionary object.

>>> data = {
... 'artist': u'Bachman-Turner Overdrive',
... 'title': u'Four Wheel Drive',
... 'asin': 'B000001FL8',
... 'year': 1975,
... 'playtime': 33.53}
>>> form = TapeForm(data)

The values are available in the form data object.

>>> form.data['title']
u'Four Wheel Drive'

However, if we pass in the request from the former example, we'll see
that values from the request are used before the passed dictionary
object is queried.

>>> form = TapeForm(data, request=request)
>>> form.data['title']
u'Motorcity Detroit USA Live'

The provided ``data`` dictionary is unchanged at this point:

>>> data['title']
u'Four Wheel Drive'

We need to invoke the ``save`` method to commit the changes to the
provided dictionary.

>>> form.data.save()
>>> data['title']
u'Motorcity Detroit USA Live'


Additional validation
---------------------

It is possible to create validation methods for more complex
needs. These extra validators can be hooked up using the `validator`
decorator.

>>> class CDForm(formapi.Form):
... fields = {
... 'artist': unicode,
... 'title': unicode,
... 'asin': str,
... 'year': int,
... 'genre': str,
... 'playtime': float}
...
... @formapi.validator
... def check_genre(self):
... if self.data['genre'] != 'rock':
... yield 'Genre is invalid'

A validator can look at all the data that is available. This makes it
easy to create validators that need to check multiple fields.

>>> form = CDForm()
>>> form.validate()
False

The errors attribute contains our error message.

>>> form.errors[0]
'Genre is invalid'

Errors can also be assigned to a specific field. To do this a
validator can register itself for a specific field.

>>> class CDForm(formapi.Form):
...
... fields = {
... 'artist': unicode,
... 'title': unicode,
... 'asin': str,
... 'year': int,
... 'genre': str,
... 'playtime': float}
...
... @formapi.validator('genre')
... def check_genre(self):
... if self.data['genre'] != 'rock':
... yield 'Genre is invalid'

When this form is validated it will have the error available for the
specific field.

>>> form = CDForm()
>>> form.validate()
False
>>> form.errors['genre'][0]
'Genre is invalid'

Form context
------------

We can set the context of a form to some object.

>>> class Tape:
... title = u'Motorcity Detroit USA Live'
... asin = 'B000001FL8'
... year = 1975

>>> tape = Tape()

The form data will draw defaults from the context.

>>> form = TapeForm(context=tape)
>>> form.data['title']
u'Motorcity Detroit USA Live'

Parameters have priority.

>>> request = Request(params=(('asin', u''), ('year', u'')))
>>> form = TapeForm(context=tape, request=request)

This input does not validate.

>> form.validate()
False

The ``asin`` input validates and is coerced to a string (from
unicode).

>>> form.data['asin']
''

The ``year`` input does not validate and is left as-is.

>>> form.data['year']
u''

Form submission
---------------

If a form prefix has not been set, the request is applied by
default. However, most applications will want to set a form prefix and
require explicit form submission.

A form submits a "default action" if the prefix is submitted as a
parameter.

>>> request = Request(params=(
... ('tape_form', ''),
... ('title', u'Motorcity Detroit USA Live')))
>>> form = TapeForm(request=request, prefix='tape_form')
>>> form.data['title']
u'Motorcity Detroit USA Live'

As expected, if we submit a form with a different prefix, the request
is not applied.

>>> form = TapeForm(request=request, prefix='other_form')
>>> form.data['title'] is None
True

We can also define form actions on the form class itself.

>>> class TapeAddForm(TapeForm):
... """An add-form for a casette tape."""
...
... @formapi.action
... def handle_add(self, data):
... print "add"
...
... @formapi.action("add_and_edit")
... def handle_add_and_edit(self, data):
... print "add_and_edit"

The first action is a "default action"; if we submit the request we
set up before, this action will be read to be submitted.

>>> form = TapeAddForm(request=request, prefix='tape_form')
>>> form.actions
[<Action name="" submitted="True">,
<Action name="add_and_edit" submitted="False">]

The submitted action is available in the ``action`` parameter.

>>> form.action
<Action name="" submitted="True">

To call the form handler of the submitted action, we invoke the form's
call method.

>>> form()
add

To call the named form action, there must be a parameter in the
request which is a concatenation of the prefix and the form action
name. Accepted separation characters are '.' (dot), '_' (underscore)
and '-' (dash).

>>> request = Request(params=(
... ('tape_form-add_and_edit', ''),
... ('title', u'Motorcity Detroit USA Live'),))
>>> form = TapeAddForm(request=request, prefix='tape_form')
>>> form.actions
[<Action name="" submitted="False">,
<Action name="add_and_edit" submitted="True">]
>>> form()
add_and_edit

Data proxies
------------

We can bind a context object to a data object by using a proxy
object. This technique can be used to create edit or add-forms.

To illustrate this, let's define a content object. We'll hardcode
default values for simplicity.

>>> class Tape(object):
... artist = u'Bachman-Turner Overdrive'
... title = u'Four Wheel Drive'
... asin = 'B000001FL8'
... year = 1975
... playtime = 33.53

We can now create a data proxy for an instance of this class.

>>> tape = Tape()
>>> proxy = formapi.Proxy(tape)

With no further intervention, this data object acts as a proxy to read
and write attributes on the content object.

>>> proxy.title
u'Four Wheel Drive'
>>> proxy.title = u'Motorcity Detroit USA Live'
>>> tape.title
u'Motorcity Detroit USA Live'

If we want to have more control over this process, we can subclass and
define descriptors.

The following example defines custom behavior for the ``title``
attribute; the value is uppercased.

>>> class TapeProxy(formapi.Proxy):
... def get_title(self):
... return self.title
...
... def set_title(self, value):
... self.title = value.upper()
...
... title = property(get_title, set_title)
>>> proxy = TapeProxy(tape)

If we read and write to the ``title`` attribute of this proxy object,
the custom getter and setter functions are used.

>>> proxy.title = u'Motorcity Detroit USA Live'

As would be expected from a proxy, changes are actually made to the
underlying content object.

>>> tape.title
u'MOTORCITY DETROIT USA LIVE'

Saving form data
----------------

When instantiating a form, you can pass in a proxy object instead of
``data``. This binds the data object to the proxy, but it also allows
us to save the form data on the proxied object.

>>> form = TapeForm(proxy, request=request)
>>> form.data['title'] = u'Four Wheel Drive'

Assignment behaves logically.

>>> form.data['title']
u'Four Wheel Drive'

However, if we invoke the ``save`` action, changes take effect on the
proxied object.

>>> form.data.save()
>>> tape.title
u'FOUR WHEEL DRIVE'