Quixote Widget Classes
======================

[This is reference documentation.  If you haven't yet read "Lesson 5:
widgets" of demo.txt, you should go and do so now.  This document also
assumes you have a good understanding of HTML forms and form elements.
If not, you could do worse than pick up a copy of *HTML: The Definitive
Guide* by Chuck Musciano & Bill Kennedy (O'Reilly).  I usually keep it
within arm's reach.]

Web forms are built out of form elements: string input, select lists,
checkboxes, submit buttons, and so forth.  Quixote provides a family of
classes for handling these form elements, or widgets, in the
quixote.form.widget module.  The class hierarchy is:

  Widget [A]
   |
   +-StringWidget
   |  |
   |  +-PasswordWidget
   |  |
   |  +-NumberWidget [*] [A]
   |     |
   |     +-FloatWidget [*]
   |     +-IntWidget [*]
   |     
   +-TextWidget
   | 
   +-CheckboxWidget
   | 
   +-SelectWidget [A]
   |  |
   |  +-SingleSelectWidget
   |  |  |
   |  |  +-RadiobuttonsWidget
   |  |  |
   |  |  +-OptionSelectWidget [*]
   |  |    
   |  +-MultipleSelectWidget
   |    
   +-SubmitButtonWidget
   | 
   +-HiddenWidget
   |
   +-ListWidget [*]

[*] Widget classes that do not correspond exactly with a particular
    HTML form element
[A] Abstract classes 


Widget: the base class
----------------------

Widget is the abstract base class for the widget hierarchy.  It provides
the following facilities
  * widget name ('name' attribute, set_name() method)
  * widget value ('value' attribute, set_value() and clear() methods)
  * __str__() and __repr__() methods
  * some facilities for writing composite widget classes

The Widget constructor signature is:
  Widget(name : string,
         value : any = None)

  name
    the name of the widget.  For non-compound widgets (ie. everything in
    the above class hierarchy), this will be used as the "name"
    attribute for the main HTML tag that defines the form element.
  value
    the current value of the form element.  The type of 'value' depends
    on the widget class.  Most widget classes support only a single
    type, eg. StringWidget always deals with strings and IntWidget with
    integers.  The SelectWidget classes are different; see the
    descriptions below for details.


Common widget methods
---------------------

The Widget base class also provides a couple of useful
methods:

  set_name(name : string)
    use this to change the widget name supplied to the constructor.
    Unless you know what you're doing, you should do this before
    rendering or parsing the widget.

  set_value(value : any)
    use this to set the widget value; this is the same as supplying
    a value to the constructor (and the same type rules apply, ie.
    the type of 'value' depends on the widget class).

  clear()
    clear the widget's current value.  Equivalent to
      widget.set_value(None)

The following two methods will be used on every widget object you
create; if you write your own widget classes, you will almost certainly
have to define both of these:

  render(request : HTTPRequest) -> string
    return a chunk of HTML that implements the form element
    corresponding to this widget.

  parse(request : HTTPRequest) -> any
    extract the form value for this widget from 'request.form', parse it
    according to the rules for this widget class, and return the
    resulting value.  The return value depends on the widget class, and
    will be of the same type as the value passed to the constructor
    and/or set_value().


StringWidget
------------

Used for short, single-line string input with no validation (ie. any
string will be accepted.)  Generates an <input type="text"> form
element.

[Constructor]

  StringWidget(name : string,
               value : string = None,
               size : int = None,
               maxlength : int = None)

  size
    used as the "size" attribute of the generated "<input>" tag;
    controls the physical size of the input field.
  maxlength
    used as the "maxlength" attribute; controls the maximum amount
    of input.

[Examples]

  StringWidget("foo", value="hello").render(request)
->
  <input name="foo" type="text" value="hello">

  StringWidget("foo", size=10, maxlength=20).render(request)
->
  <input name="foo" type="text" size="10" maxlength="20">


PasswordWidget
--------------

PasswordWidget is identical to StringWidget except for the type of the
HTML form element: "password" instead of "text".


TextWidget
----------

Used for multi-line text input.  The value is a single string with
newlines right where the browser supplied them.  ("\r\n", if present, is
converted to "\n".)  Generates a <textarea> form element.

[Constructor]

  TextWidget(name : string,
             value : string = None,
             cols : int = None,
             rows : int = None,
             wrap : string = "physical")

  cols
  rows
    number of columns/rows in the textarea
  wrap
    controls how the browser wraps text and includes newlines in the
    submitted form value; consult an HTML book for details.


CheckboxWidget
--------------

Used for single boolean (on/off) value.  The value you supply can be
anything, since Python has a boolean interpretation for all values; the
value returned by 'parse()' will always be 0 or 1 (but you shouldn't
rely on that!).  Generates an <input type="checkbox"> form element.

[Constructor]

  CheckboxWidget(name : string,
                 value : boolean = false)

[Examples]

  CheckboxWidget("foo", value=0).render(request)
->
  <input name="foo" type="checkbox" value="yes">
                 
  CheckboxWidget("foo", value="you bet").render(request)
->
  <input name="foo" type="checkbox" value="yes" checked>


RadiobuttonsWidget
------------------

Used for a *set* of related radiobuttons, ie. several
<input type="radio"> tags with the same name and different values.
The set of values are supplied to the constructor as 'allowed_values',
which may be a list of any Python objects (not just strings).  The
current value must be either None (the default) or one of the values in
'allowed_values'; if you supply a 'value' not in 'allowed_values', it
will be ignored.  parse() will return either None or one of the values
in 'allowed_values'.

[Constructor]

  RadiobuttonsWidget(name : string,
                     value : any = None,
                     allowed_values : [any] = None,
                     descriptions : [string] = map(str, allowed_values),
                     quote : boolean = true,
                     delim : string = "\n")

  allowed_values  
    specifies how many <input type="radio"> tags to generate and the
    values for each.  Eg. allowed_values=["foo", "bar"] will result in
    (roughly):
      <input type="radio" value="foo">
      <input type="radio" value="bar">
  descriptions
    the text that will actually be shown to the user in the web page
    that includes this widget.  Handy when the elements of
    'allowed_values' are too terse, or don't have a meaningful str(), or
    you want to add some additional cues for the user.  If not supplied,
    map(str, allowed_values) is used, with the exception that None in
    'allowed_values' becomes "" (the empty string) in 'descriptions'.
    If supplied, 'descriptions' must be the same length as
    'allowed_values'.
  quote
    if true (the default), the elements of 'descriptions' will be
    HTML-quoted (using quixote.html.html_quote()) when the widget is
    rendered.  This is essential if you might have characters like
    "&" or "<" in your descriptions.  However, you'll want to set
    'quote' to false if you are deliberately including HTML markup
    in your descriptions.
  delim
    the delimiter to separate the radiobuttons with when rendering
    the whole widget.  The default ensures that your HTML is readable
    (by putting each <input> tag on a separate line), and that there
    is horizontal whitespace between each radiobutton.

[Examples]

  RadiobuttonsWidget("foo",
      allowed_values=["red", "green", "blue", "pink"]).render(request)
->
  <input name="foo" type="radio" value="0">red
  <input name="foo" type="radio" value="1">green
  <input name="foo" type="radio" value="2">blue
  <input name="foo" type="radio" value="3">pink
  
(Note that the actual form values, ie. what the browser returns to the
server, are always stringified indices into the 'allowed_values' list.
This is irrelevant to you, since SingleSelectWidget takes care of
converting "1" to 1 and looking up allowed_values[1].)

  RadiobuttonsWidget("bar",
      allowed_values=[val1, val2, val3],
      descriptions=["thing <b>1</b>",
                    "thing <b>2</b>",
                    "thing <b>3</b>"],
      value=val3,
      delim="<br>\n",
      quote=0).render(request)
->
  <input name="bar" type="radio" value="0">thing <b>1</b><br>
  <input name="bar" type="radio" value="1">thing <b>2</b><br>
  <input name="bar" type="radio" value="2" checked>thing <b>3</b>



SingleSelectWidget
------------------

Used to select a single value from a list that's too long or ungainly
for a set of radiobuttons.  (Most browsers implement this as a scrolling
list; UNIX versions of Netscape 4.x and earlier used a pop-up menu.)
The value can be any Python object; parse() will return either None or
one of the values you supply to the constructor as 'allowed_values'.
Generates a <select>...</select> tag, with one <option> tag for each
element of 'allowed_values'.

[Constructor]

  SingleSelectWidget(name : string,
                     value : any = None,
                     allowed_values : [any] = None,
                     descriptions : [string] = map(str, allowed_values),
                     quote : boolean = true,
                     size : int = None)

  allowed_values
    determines the set of <option> tags that will go inside the
    <select> tag; these can be any Python values (not just strings).
    parse() will return either one of the allowed_values or None.
    If you supply a 'value' that is not in 'allowed_values', it
    will be ignored.
  descriptions
    (same as RadiobuttonsWidget above)
  quote
    (same as RadiobuttonsWidget above)
  size
    corresponds to the "size" attribute of the "<select>" tag: ask
    the browser to show a select list with 'size' items visible.
    Not always respected by the browser; consult an HTML book.

[Examples]

  SingleSelectWidget("foo",
                     allowed_values=["abc", 123, 5.5]).render(request)
->
  <select name="foo">
  <option value="0">abc
  <option value="1">123
  <option value="2">5.5
  </select>

  SingleSelectWidget("bar",
                     value=val2,  
                     allowed_values=[val1, val2, val3],
                     descriptions=["foo", "bar", "foo & bar"],
                     size=3).render(request)
->
  <select name="bar" size="3">
  <option value="0">foo
  <option selected value="1">bar
  <option value="2">foo &amp; bar
  </select>


MultipleSelectWidget
--------------------

Used to select multiple values from a list.  Everything is just like
SingleSelectWidget, except that 'value' can be a list of objects
selected from 'allowed_values' (in which case every object in 'value'
will initially be selected).  Generates a <select multiple>...</select>
tag, with one <option> tag for each element of 'allowed_values'.

[Constructor]

  MultipleSelectWidget(name : string,
                       value : any | [any] = None,
                       allowed_values : [any] = None,
                       descriptions : [string] = map(str, allowed_values),
                       quote : boolean = true,
                       size : int = None)


SubmitButtonWidget
------------------

Used for generating submit buttons.  Note that HTML submit buttons are
rather weird, and Quixote preserves this weirdness -- the Widget classes
are meant to be a fairly thin wrapper around HTML forms, after all.

In particular, the widget value for a submit button controls two things:
what the user sees in their browser (the text in the button) and what
the browser returns as the value for that form element.  You can't
control the two separately, as you can with radiobuttons or selection
widgets.

Also, SubmitButtonWidget is the only widget with an optional name.  In
many simple forms, all you care about is the fact that the form was
submitted -- which submit button the user used doesn't matter.

[Constructor]

  SubmitButtonWidget(name : string = None,
                     value : string = None)

  value
    the text that will be shown in the user's browser, *and* the
    value that will be returned for this form element (widget)
    if the user selects this submit button.

[Examples]

  SubmitButtonWidget(value="Submit Form").render(request)
->
  <input type="submit" value="Submit Form">


HiddenWidget
------------

Used to generate HTML hidden widgets, which can be useful for carrying
around non-sensitive application state.  (The Quixote form framework
uses hidden widgets for form tokens as a measure against cross-site
request forgery [CSRF] attacks.  So by "sensitive" I mean "information
which should not be revealed", rather than "security-related".  If you
wouldn't put it in a cookie or in email, don't put it in a hidden form
element.)

[Constructor]

  HiddenWidget(name : string,
               value : string)

[Examples]

  HiddenWidget("form_id", "2452345135").render(request)
->
  <input type="hidden" name="form_id" value="2452345135">


IntWidget
---------

The first derived widget class: this is a subclass of StringWidget
specifically for entering integer values.  As such, this is the first
widget class we've covered that can reject certain user input.  (The
selection widgets all have to validate their input in case of broken or
malicious clients, but they just drop bogus values.)  If the user enters
a string that Python's built-in int() can't convert to an integer,
IntWidget's parse() method raises FormValueError (also defined in the
quixote.form.widget module).  This exception is handled by Quixote's
form framework, but if you're using widget objects on their own, you'll
have to handle it yourself.

IntWidget.parse() always returns an integer or None.

[Constructor]

  IntWidget(name : string,
            value : int = None,
            size : int = None,
            maxlength : int = None)

  Constructor arguments are as for StringWidget, except that 'value'
  must be an integer (or None).  Note that 'size' and 'maxlength' have
  exactly the same meaning: they control the size of the input widget
  and the maximum number of characters of input.

[Examples]

  IntWidget("num", value=37, size=5).render(request)
->
  <input type="string" name="num" value="37" size="5">


FloatWidget
-----------

FloatWidget is identical to IntWidget, except:
  * 'value' must be a float
  * parse() returns a float or None
  * parse() raises FormValueError if the string entered by the
    user cannot be converted by Python's built-in float() function


OptionSelectWidget
------------------

OptionSelectWidget is simply a SingleSelectWidget that uses a bit of
Javascript to automatically submit the current form as soon as the user
selects a value.  This is useful for very simple one-element forms where
you don't want to bother with a submit button, or for very complex forms
where you need to revamp the user interface based on a user's selection.
Your form-processing code could then detect that style of form
submission, and regenerate a slightly different form for the user.  (Or
you could treat it as a full-blown form submission, if the only widget
of interest is the OptionSelectWidget.)

For example, if you're asking a user for their address, some of the
details will vary depending on which country they're in.  You might make
the country widget an OptionSelectWidget: if the user selects "Canada",
you'll ask them for a province and a postal code; if they select "United
tates", you ask for a state and a zip code; and so forth.  (I don't
really recommend a user interface that works this way: you'll spend way
too much time getting the details right ["How many states does Australia
have again?"], and you're bound to get something wrong -- there are over
200 countries in the world, after all.)

Be warned that since OptionSelectWidget relies on Javascript to work,
using it makes immediately makes your application less portable and more
fragile.  One thing to avoid: form elements with a name of "submit",
since that masks the Javascript function called by OptionSelectWidget.
