File: tmpl_tutorial.html

package info (click to toggle)
urwid 0.9.9.1-1
links: PTS, VCS
area: main
in suites: squeeze
size: 1,372 kB
ctags: 2,406
sloc: python: 13,464; ansic: 607; makefile: 14
file content (743 lines) | stat: -rw-r--r-- 29,932 bytes
parent folder | download | duplicates (2)
<html>
<head>
<title>Urwid %version% Tutorial</title>
<style type="text/css">
    h1 { text-align: center; }
    h2 { margin: 40px 0 0 0; padding: 10px;  background: #6d96e8;}
    h3 { margin: 0 0 3px 0; padding: 12px 6px 6px 6px; background: #efef96;}
    .code { background: #dddddd; padding: 5px; margin: 7px 20px; }
    .l1 { margin: 12px 0 0 0; }
    .l2 { margin-left: 20px; }
    .shot { padding: 5px 20px 5px 0px; float: left; }
    .back { font-size:small; padding-left: 20px; }
</style>
<body>
<a name="top"></a>
<h1>Urwid %version% Tutorial</h1>

<div style="text-align: center;">
<a href="http://excess.org/urwid/">Urwid Home Page</a> /
<a href="http://excess.org/urwid/examples.html">Example Screenshots</a> /
<a href="http://excess.org/urwid/utf8examples.html">UTF-8 Screenshots</a> /
Tutorial /
<a href="reference.html">Reference</a>
</div>
<br>

<table width="100%"><tr><td width="50%" valign="top">
%toc_left%
</td><td width="50%" valign="top">
%toc_right%
</td></tr></table>

<p>
The programs in this tutorial are available in your Urwid distribution. 
Run the command "./docgen_tutorial.py -s" to output the example scripts.
</p>

{contents}


<div style="background:#ffff33; padding: 5px">
<h1>Urwid Tutorial Template File</h1>
This file is used by <b>docgen_tutorial.py</b> to generate the tutorial
documentation <b>tutorial.html</b>.
<p>
</div>
<div style="background:#ffffaa;">
<p>
Items in the list that follows are parsed by docgen_tutorial.py. 
Each item has a tag and a name, separated by a comma.  Whitespace
on either side of the comma is stripped.
</p><p>
Items without tags are new sections.
</p><p>
A --- separates the left and right columns in the table of contents.
</p>

<pre>
<b>Tag, Section or Item Name</b>
{section_data}
,            Hello World Example
min,         Minimal Urwid Application
input,       Handling Input
attr,        AttrMap Widgets and Text Attributes
highcolors,  High Color Modes
,            Conversation Example
edit,        Edit Widgets
frlb,        Events and ListBox Widgets
lbcont,      Modifying ListBox Content
,            Zen of ListBox
lbscr,       ListBox Focus and Scrolling
lbdyn,       Dynamic ListBox with List Walker
lbfocus,     Setting the Focus
---
,            Combining Widgets
pile,        Piling Widgets
cols,        Dividing into Columns
grid,        GridFlow Arrangement
overlay,     Overlay Widgets
,            Creating Custom Widgets
wmod,        Modifying Existing Widgets
wanat,       Anatomy of a Widget
wsel,        Creating Selectable Widgets
wcur,        Widgets Displaying the Cursor
{/section_data}
</pre>
</div>


 <p>
{toc_section}
<div class="l1">%snum%. %name%</div>
{/toc_section}
<p>
{toc_item}
<div class="l2"><a href="#%tag%">%snum%.%inum%. %name%</a></div>
{/toc_item}
<p>
{section_head}
<h2>%snum%. %name%</h2>
{/section_head}
<p>
{section_body}
<h3><a name="%tag%">%snum%.%inum%. %name%</a>
<span class="back">[<a href="#top">back to top</a>]</span></h3>
%content%
<br clear="left">
<br>
{/section_body}

<br clear="left"><hr>



{body[min]}
This program displays the string "Hello World" in the top left corner
of the screen and will run until interrupted with CTRL+C (^C).

<pre class="code">%example[0]%</pre>

<ul>
<li>The <a href="reference.html#Text">Text</a>
widget handles formatting blocks of text, wrapping to the next line when
necessary.  Widgets like this are called "flow widgets" because their
sizing can have a number of columns given, in this case the full screen
width, then they will flow to fill as many rows as necessary.
<li>The <a href="reference.html#Filler">Filler</a> is widget fills in blank
lines above or below flow widgets so that they can be displayed in a fixed
number of rows.  This Filler will align our Text to the top of the screen,
filling all the rows below with blank lines.  Widgets which are given both
the number of columns and number of rows they must be displayed in are called
"box widgets".  The "topmost" widget displayed on the screen must be 
a box widget.
<li>The <a href="reference.html#MainLoop">MainLoop</a> class handles displaying
our widgets as well as input from the user.  In this case our widgets can't
handle the input so we need to interrupt the program to exit with ^C.</ul>

<div align="center">%result[0]%</div>
{/body[min]}

<br clear="left"><hr>

{body[input]}
This program initially displays the string "Hello World", then it displays
each key pressed, exiting when the user presses Q.

<pre class="code">%example[0]%</pre>

<ul>
<li>The MainLoop class has an optional function parameter unhandled_input
This function will be called once for each keypress that is not handled
by the widgets being displayed.
<li>None of the widgets being displayed here handle input, so every key
the user presses will be passed to the show_or_exit function.
<li>The ExitMainLoop exception is used to 
exit cleanly from the MainLoop.run() function when the user presses Q.  All
other input is displayed by replacing the current Text widget's content.
</ul>

<div class="shot">%result[0]%</div>
<div class="shot">%result[1]%</div>
<div class="shot">%result[2]%</div>
<div class="shot">%result[3]%</div>
<div class="shot">%result[4]%</div>
{/body[input]}

<br clear="left"><hr>

{body[attr]}
This program displays the string "Hello World" in the center of the screen.
It uses different attributes for the text, the space on either side
of the text and the space above and below the text.  It waits for 
a keypress before exiting.

<pre class="code">%example[0]%</pre>

<ul>
<li>Attributes are defined as part of a palette.  Valid foreground,
background and setting values are documented in the
<a href="reference.html#AttrSpec">AttrSpec class</a>.
A palette is a list of tuples containing:
  <ol>
  <li>Name of the attribute, typically a string
  <li>Foreground color and settings for 16-color (normal) mode
  <li>Background color for normal mode
  <li>Settings for monochrome mode (optional)
  <li>Foreground color and settings for 88 and 256-color modes (optional, see next example)
  <li>Background color for 88 and 256-color modes (optional)
  </ol>
<li>The palette is passed to MainLoop to make it available to our program
<li>A <a href="reference.html#Text">Text</a> widget is created containing
the string " Hello World " with attribute "banner". The attributes of text
in a Text widget is set by using a (attribute, text) tuple instead of a
simple text string.  Text attributes will flow with the text, and
multiple attributes may be specified by combining tuples into a
list.
<li>An <a href="reference.html#AttrMap">AttrMap</a> widget is created to
wrap the text widget with attribute "streak". AttrMap widgets allow you
to map any attribute to any other attribute, but by default they will 
set the attribute of everything that does not already have an
attribute. In this case the text has an attribute, so only 
the areas around the text used for alignment will be have the new attribute.
<li>A second AttrMap widget is created to
wrap the filler widget with attribute "bg".
</ul>

When this program is run you can now clearly
see the separation of the text, the alignment around the text, and
the filler above and below the text.  This is how these widgets
react to being resized:

<div class="shot">%result[0]%</div>
<div class="shot">%result[1]%</div>
<div class="shot">%result[2]%</div>
<div class="shot">%result[3]%</div>
{/body[attr]}

<br clear="left"><hr>

{body[highcolors]}
This program displays the string "Hello World" in the center of the screen.
It uses a number of 256-color-mode background attributes to decorate the
text, and will work in any terminal that supports 256-color mode.
It will exit when Q is pressed.

<pre class="code">%example[0]%</pre>

<ul>
<li>This palette only defines values for the high color foreground
and backgrounds, because only the high colors will be used.
A real application should define values for all the modes in their
palette. Valid foreground,
background and setting values are documented in the
<a href="reference.html#AttrSpec">AttrSpec class</a>.
<li><a href="reference.html#Divider">Divider</a> widgets are used to
create blank lines, colored with AttrMap.
<li>A <a href="reference.html#Pile">Pile</a> widget arranges the
Divider widgets above and below our text.
<li>Behind the scenes our MainLoop class has created a 
raw_display.Screen object for drawing the screen.
The program is put into 256-color mode by using the screen
object's
<a href="reference.html#Screen-set_terminal_properties"
>set_terminal_properties</a> method.  This method works only when 
using the default raw_display Screen class in our MainLoop.
</ul>

<div align="center">%result[0]%</div>
{/body[highcolors]}

<br clear="left"><hr>

{body[edit]}
This program asks for your name then responds "Nice to meet you, (your name)."

<pre class="code">%example[0]%</pre>

<ul>
<li>An <a href="reference.html#Edit">Edit</a> widget is created with the
caption "What is your name?". A newline at the end of the caption makes
the user input start on the next row.
<li>Most keystrokes will be handled by the Edit widget, allowing
the user to enter their name.
<li>The unhandled_input function will replace the Edit widget inside
the Filler widget with a reply when the user presses ENTER.
<li>When the user presses ENTER again the unhandled_input function
will cause the program to exit.
</ul>

The Edit widget has many capabilities. It lets you make corrections and move
the cursor around with the HOME, END and arrow keys. It is based on the Text
widget so it supports the same wrapping and alignment modes.

<div class="shot">%result[0]%</div>
<div class="shot">%result[1]%</div>
<div class="shot">%result[2]%</div>
{/body[edit]}

<br clear="left"><hr>

{body[frlb]}
This program asks for your name and responds "Nice to meet you, (your name)"
<i>while</i> you type your name.  ENTER exits.

<pre class="code">%example[0]%</pre>

<ul>
<li>An Edit widget and Text reply widget are created, like in the
previous example.
<li>A <a href="reference.html#SimpleListWalker">SimpleListWalker</a>
is then created to manage the contents and focus of our ListBox.
The SimpleListWalker behaves just like a list of widgets, and can
be modified like a regular list.
<li>A <a href="reference.html#ListBox">ListBox</a> is created
and passed the SimpleListWalker.  The ListBox is a box widget and
allows scrolling through its contents.  This example is simple enough
that we could have used a Pile widget and Filler widget instead, but
we will see how the ListBox can be useful in later examples.
<li>The <a href="reference.html#connect_signal">connect_signal</a>
function is used to attach our on_ask_change function to our Edit
widget's "change" event.  Now any time the content of the Edit
widget changes on_ask_change will be called and passed the new 
content.
<li>Now on_ask_change updates the reply text as the user enters their
name.
</ul>

<div class="shot">%result[0]%</div>
<div class="shot">%result[1]%</div>
<div class="shot">%result[2]%</div>
<div class="shot">%result[3]%</div>
{/body[frlb]}

<br clear="left"><hr>

{body[lbcont]}
This program asks for your name and responds "Nice to meet you, (your name)."
It then asks again, and again. Old values may be changed and the responses
will be updated when you press ENTER.  ENTER on a blank line exits.
<br><br>
<pre class="code">%example[0]%</pre>

<ul>
<li>When the user presses ENTER:
  <ul>
  <li>The widget in focus and its current position is retrieved by calling the
  <a href="reference.html#ListBox-get_focus">get_focus</a> function.
  <li>If the widget in focus does not have an edit_text attribute, then it
  is not one of the Edit widgets we are interested in. 
  One of the Text widgets might receive focus
  if it covers the entire visible area of the ListBox widget and there is
  no Edit widget to take focus. While this is unlikely, it should be handled
  or the program will fail when trying to access it.
  <li>If there is no edit text we exit the program.
  <li>The widget after the widget in focus (if any exists) is replaced
  with a response.
  <li>If there is no widget after that widget, a new Edit widget is
  created.
  <li>The focus is then moved to the next Edit widget by calling
  <a href="reference.html#ListBox-set_focus">set_focus</a>.
  </ul>
<li>All other keys are passed to the top widget to handle. The ListBox widget
does most of the hard work:
  <ul>
  <li>UP and DOWN will change the focus and/or scroll the widgets in the list
  box.
  <li>PAGE UP and PAGE DOWN will try to move the focus one screen up or down.
  <li>The cursor's column is maintained as best as possible when moving
  from one Edit widget to another.
  </ul>
</ul>

<div class="shot">%result[0]%</div>
<div class="shot">%result[1]%</div>
<div class="shot">%result[2]%</div>
{/body[lbcont]}

<br clear="left"><hr>

{body[lbscr]}
The 
<a href="reference.html#ListBox">ListBox</a> 
is a box widget that contains flow widgets.  Its contents
are displayed stacked vertically, and the ListBox allows the user to scroll
through its content.  One of the flow widgets displayed
in the ListBox is the focus widget.  The ListBox passes key presses to the
focus widget to allow the user to interact with it. If the
focus widget does not handle a keypress then the ListBox may handle the
keypress by scrolling and/or selecting another widget to become the focus
widget.
<br><br>
The ListBox tries to do the most sensible thing when scrolling and
changing focus.  When the widgets displayed are all Text widgets or
other unselectable widgets then the ListBox will behave like a web browser
does when the user presses UP, DOWN, PAGE UP and PAGE DOWN: new text is
immediately scrolled in from the top or bottom.  The ListBox chooses one of the
visible widgets as its focus widget when scrolling. When scrolling up the
ListBox chooses the topmost widget as the focus, and when scrolling down 
the ListBox chooses the bottommost widget as the focus.
<br><br>
When all the widgets displayed are not selectable the user would typically
have no way to tell which widget is in focus, but if we wrap the widgets with
AttrWrap we can see what is happening while the focus changes:

<pre class="code">%example[0]%</pre>
<div class="shot">%result[0]%</div>
<div class="shot">%result[1]%</div>
<div class="shot">%result[2]%</div>
<div class="shot">%result[3]%</div>
<div class="shot">%result[4]%</div>
<div class="shot">%result[5]%</div>
<br clear="left">

The ListBox remembers the location of the widget in focus as either an
"offset" or an "inset". An offset is the number of rows between the top
of the ListBox and the beginning of the focus widget. An offset of zero
corresponds to a widget with its top aligned with the top of the ListBox.
An inset is the fraction of rows of the focus widget
that are "above" the top of the ListBox and not visible. 
The ListBox uses this method of remembering the focus widget location
so that when the ListBox is resized the text displayed will stay 
roughly aligned with the top of the ListBox.
<br><br>
<div class="shot">%result[6]%</div>
<div class="shot">%result[7]%</div>
<div class="shot">%result[8]%</div>
<br clear="left">
<br><br>
When there are selectable widgets in the ListBox the focus will move
between the selectable widgets, skipping the unselectable widgets.
The ListBox will try to scroll all the rows of a selectable widget into
view so that the user can see the new focus widget in its entirety.
This behavior can be used to bring more than a single widget into view
by using composite widgets to combine a selectable widget with other
widgets that should be displayed at the same time.
{/body[lbscr]}

<br clear="left"><hr>

{body[lbdyn]}
While the ListBox stores the location of its focus widget, it
does not directly store the actual focus widget or other contents of the
ListBox. The storage of a ListBox's content is delegated to a "List Walker" 
object. If a list of widgets is passed to the ListBox constructor then it
creates a 
<a href="reference.html#SimpleListWalker">SimpleListWalker</a> 
object to manage the list.
<br><br>
When the ListBox is 
<a href="reference.html#ListBox-render">rendering a canvas</a> or 
<a href="reference.html#ListBox-keypress">handling input</a> it will:
<ol>
<li>Call the 
<a href="reference.html#List_Walker_interface_definition-get_focus">get_focus</a> 
method of its list walker object. 
This method will return the focus widget and a position object.  
<li>Optionally call the 
<a href="reference.html#List_Walker_interface_definition-get_prev">get_prev</a> 
method of its List Walker object 
one or more times, initially passing 
the focus position and then passing the new position returned on each 
successive call.  This method will return the widget and position object
"above" the position passed.
<li>Optionally call the 
<a href="reference.html#List_Walker_interface_definition-get_next">get_next</a> 
method of its List Walker object 
one or more times, similarly, to collect widgets and position objects 
"below" the focus position.
<li>Optionally call the 
<a href="reference.html#List_Walker_interface_definition-set_focus">set_focus</a> 
method passing one of the position 
objects returned in the previous steps.
</ol>
This is the only way the ListBox accesses its contents, and it will not
store copies of any of the widgets or position objects beyond the current
rendering or input handling operation.
<br><br>
The SimpleListWalker stores a list of widgets, and uses integer indexes into
this list as its position objects.  It stores the focus
position as an integer, so if you insert a widget into the list above the
focus position then you need to remember to increment the focus position in
the SimpleListWalker object or the contents of the ListBox will shift.
<br><br>
A custom List Walker object may be passed to the ListBox constructor instead
of a plain list of widgets. List Walker objects must implement the 
<a href="reference.html#List_Walker_interface_definition">List Walker interface</a>.
<br><br>
The 
<a href="fib.py.html">fib.py</a> 
example program demonstrates a custom list walker that doesn't
store any widgets. It uses a tuple of two successive Fibonacci numbers
as its position objects and it generates Text widgets to display the numbers
on the fly. 
The result is a ListBox that can scroll through an unending list of widgets.
<br><br>
The 
<a href="edit.py.html">edit.py</a> 
example program demonstrates a custom list walker that
loads lines from a text file only as the user scrolls them into view.  
This allows even huge files to be opened almost instantly.
<br><br>
The 
<a href="browse.py.html">browse.py</a>
example program demonstrates a custom list walker that
uses a tuple of strings as position objects, one for the parent directory
and one for the file selected. The widgets are cached in a separate class
that is accessed using a dictionary indexed by parent directory names.
This allows the directories to be read only as required. The custom list
walker also allows directories to be hidden from view when they are
"collapsed".
{/body[lbdyn]}

<br clear="left"><hr>

{body[lbfocus]}
The easiest way to change the current ListBox focus is to call the
<a href="reference.html#ListBox-set_focus">set_focus</a> method.
This method doesn't require that you know the ListBox's
current dimensions (maxcol, maxrow).  It will wait until the next call to
either keypress or render to complete setting the offset and inset values 
using the dimensions passed to that method.
<br><br>
The position object passed to set_focus must be compatible with the List
Walker object that the ListBox is using. For SimpleListWalker the position
is the integer index of the widget within the list.
<br><br>
The coming_from parameter should be set if you know that the old position
is "above" or "below" the previous position.  When the ListBox completes
setting the offset and inset values it tries to find the old widget among
the visible widgets. If the old widget is still visible, if will try to
avoid causing the ListBox contents to scroll up or down from its previous
position. If the widget is not visible, then the ListBox will:
<ul>
<li>Display the new focus at the bottom of the ListBox if coming_from is
"above".
<li>Display the new focus at the top of the ListBox if coming_from is "below".
<li>Display the new focus in the middle of the ListBox if coming_from is None.
</ul>
If you know exactly where you want to display the new focus widget within
the ListBox you may call 
<a href="reference.html#ListBox-set_focus_valign">set_focus_valign</a>. 
This method lets you specify the
"top", "bottom", "middle", a relative position or the exact number of rows
from the top or bottom of the ListBox.
{/body[lbfocus]}

<br clear="left"><hr>

{body[pile]}
<a href="reference.html#Pile">Pile</a>
 widgets are used to combine multiple widgets by stacking them
vertically. A Pile can manage selectable widgets by keeping track of which
widget is in focus and it can handle moving the focus between widgets when
the user presses the UP and DOWN keys. A Pile will also work well when used 
within a ListBox.
<br><br>
A Pile is selectable only if its focus widget is selectable. If you create
a Pile containing one Text widget and one Edit widget the Pile will
choose the Edit widget as its default focus widget.
To change the pile's focus widget you can call
<a href="reference.html#Pile-set_focus">set_focus</a>.
{/body[pile]}

<br clear="left"><hr>

{body[cols]}
<a href="reference.html#Columns">Columns</a>
widgets may be used to arrange either flow widgets or box widgets
horizontally into columns. Columns widgets will manage selectable widgets
by keeping track of which column is in focus and it can handle moving the
focus between columns when the user presses the LEFT and RIGHT keys.
Columns widgets also work well when used within a ListBox.
<br><br>
Columns widgets are selectable only if the column in focus is selectable.
If a focus column is not specified the first selectable widget will be
chosen as the focus column. The 
<a href="reference.html#Columns-set_focus">set_focus</a>
method may be used to select the focus column.
{/body[cols]}

<br clear="left"><hr>

{body[grid]}
The 
<a href="reference.html#GridFlow">GridFlow</a>
widget is a flow widget designed for use with
<a href="reference.html#Button">Button</a>,
<a href="reference.html#CheckBox">CheckBox</a> and
<a href="reference.html#RadioButton">RadioButton</a> widgets.
It renders all the widgets it contains the same width and it arranges them
from left to right and top to bottom.
<br><br>
The GridFlow widget uses Pile, Columns, Padding and Divider widgets to build
a display widget that will handle the keyboard input and rendering. When the
GridFlow widget is resized it regenerates the display widget to accommodate
the new space.
{/body[grid]}

<br clear="left"><hr>

{body[overlay]}
The <a href="reference.html#Overlay">Overlay</a> widget is a box widget that
contains two other box widgets.  The bottom widget is rendered the full size
of the Overlay widget and the top widget is placed on top, obscuring an area
of the bottom widget. This widget can be used to create effects such as
overlapping "windows" or pop-up menus.
<br><br>
The Overlay widget always treats the top widget as the one in focus. All
keyboard input will be passed to the top widget. 
<br><br>
If you want to use a flow
flow widget for the top widget, first wrap the flow widget with a
<a href="reference.html#Filler">Filler</a> widget.
{/body[overlay]}

<br clear="left"><hr>

{body[wmod]}
The easiest way to create a custom widget is to modify an existing widget.
This can be done by either subclassing the original widget or by wrapping it.
Subclassing is appropriate when you need to interact at a very low level
with the original widget, such as if you are creating a custom edit widget
with different behavior than the usual Edit widgets.  If you are creating
a custom widget that doesn't need tight coupling with the original widget,
such as a widget that displays customer address information, then wrapping
is more appropriate.
<br><br>
The <a href="reference.html#WidgetWrap">WidgetWrap</a> class simplifies
wrapping existing widgets.  You can create a custom widget simply by 
creating a subclass of WidgetWrap and passing a widget into WidgetWrap's 
constructor. 
<br><br>
This is an example of a custom widget that uses WidgetWrap:

<pre class="code">%example[0]%</pre>

The above code creates a group of RadioButtons and provides a method to
query the state of the buttons.
<br><br>
Wrapped widgets may also override the standard widget methods. These methods
are described in following sections.
{/body[wmod]}

<br clear="left"><hr>

{body[wanat]}
Any object that follows the 
<a href="reference.html#Widget_interface_definition">Widget interface definition</a>
may be used as a widget.  Box widgets must implement 
<a href="reference.html#Widget_interface_definition-selectable">selectable</a>
and 
<a href="reference.html#Widget_interface_definition-render">render</a>
methods, and flow widgets must implement selectable, render and 
<a href="reference.html#Widget_interface_definition-rows">rows</a>
methods.

<pre class="code">%example[0]%</pre>

The above code implements two widget classes. Pudding is a flow widget and 
BoxPudding is a box widget. Pudding will render as much "Pudding" as will
fit in a single row, and BoxPudding will render as much "Pudding" as will
fit into the entire area given.
<br><br>
It is not strictly necessary to inherit from
<a href="reference.html#FlowWidget">BoxWidget</a> or 
<a href="reference.html#FlowWidget">FlowWidget</a>, but doing so does add
some documentation to your code.  
<br><br>
Note that the rows and render methods' focus parameter must have a default
value of False.  Also note that for flow widgets the number of rows returned 
by the rows method must match the number of rows rendered by the render
method.
<br><br>
In most cases it is easier to let other widgets handle the rendering and 
row calculations for you:

<pre class="code">%example[1]%</pre>

The NewPudding class behaves the same way as the Pudding class above, but in
NewPudding you can change the way the widget appears by modifying only the
display_widget method, whereas in the Pudding class you may have to modify both
the render and rows methods.
<br><br>
To improve the efficiency of your Urwid application you should be careful
of how long your rows methods take to execute.  The rows methods may be called
many times as part of input handling and rendering operations.  If you are
using a display widget that is time consuming to create you should consider
caching it to reduce its impact on performance.
<br><br>
It is possible to create a widget that will behave as either a flow widget
or box widget depending on what is required:

<pre class="code">%example[2]%</pre>

MultiPudding will work in place of either Pudding or BoxPudding above. The
number of elements in the size tuple determines whether the containing widget
is expecting a flow widget or a box widget.
{/body[wanat]}

<br clear="left"><hr>

{body[wsel]}
Selectable widgets such as Edit and Button widgets allow the user to
interact with the application.  A widget is selectable if its selectable
method returns True.  Selectable widgets must implement the
<a href="reference.html#Widget_interface_definition-keypress">keypress</a>
method to handle keyboard input.

<pre class="code">%example[0]%</pre>

The SelectablePudding widget will display its contents in uppercase when it
is in focus, and it allows the user to "eat" the pudding by pressing each of
the letters P, U, D, D, I, N and G on the keyboard.  When the user has "eaten"
all the pudding the widget will reset to its initial state.
<br><br>
Note that keys that are unhandled in the keypress method are returned so that
another widget may be able to handle them.  This is a good convention to follow
unless you have a very good reason not to.  In this case the UP and DOWN keys
are returned so that if this widget is in a ListBox the ListBox will behave as
the user expects and change the focus or scroll the ListBox.
{/body[wsel]}

<br clear="left"><hr>

{body[wcur]}
Widgets that display the cursor must implement the
<a href="reference.html#Widget_interface_definition-get_cursor_coords">get_cursor_coords</a> 
method. Similar to the rows method for flow widgets, this method lets other
widgets make layout decisions without rendering the entire widget.  The
ListBox widget in particular uses get_cursor_coords to make sure that the
cursor is visible within its focus widget.

<pre class="code">%example[0]%</pre>

CursorPudding will let the user move the cursor through the widget by 
pressing LEFT and RIGHT. The cursor must only be added to the canvas when
the widget is in focus. The get_cursor_coords method must always return
the same cursor coordinates that render does.

<br><br>A widget displaying a cursor may choose to implement
<a href="reference.html#Widget_interface_definition-get_pref_col">get_pref_col</a>.
This method returns the preferred column for the cursor, and is called when
the focus is moving up or down off this widget.
<br><br>
Another optional method is 
<a href="reference.html#Widget_interface_definition-move_cursor_to_coords">move_cursor_to_coords</a>.
This method allows other widgets to try to position the cursor within this
widget. The ListBox widget uses move_cursor_to_coords when changing focus and
when the user pressed PAGE UP or PAGE DOWN. This method must return True on
success and False on failure. If the cursor may be placed at any position
within the row specified (not only at the exact column specified) then this
method must move the cursor to that position and return True.


<pre class="code">%example[1]%</pre>
{/body[wcur]}

<br clear="left"><hr>

{/contents}

</body>
</html>