tutorial.html

Urwid is a Python library for making text console applications. It has many features including fluid

<span style="color:black;background:silver">Bob</span><span style="color:silver;background:black"> </span><span style="color:black;background:silver">                   </span>
<span style="color:black;background:silver">                       </span>
<span style="color:black;background:silver">                       </span>
<span style="color:black;background:silver">                       </span>
<span style="color:black;background:silver">                       </span>
<span style="color:black;background:silver">                       </span>
<span style="color:black;background:silver">                       </span>
</pre></div>
<div class="shot"><pre><span style="color:black;background:teal">Press F8 to exit.      </span>
<span style="color:black;background:silver">Nice to meet you, Abe</span><span style="color:black;background:silver">  </span>
<span style="color:black;background:silver">                       </span>
<span style="color:black;background:silver">What is your name?</span><span style="color:black;background:silver">     </span>
<span style="color:black;background:silver">Bob                    </span>
<span style="color:black;background:silver">Nice to meet you, Bob</span><span style="color:black;background:silver">  </span>
<span style="color:black;background:silver">                       </span>
<span style="color:black;background:silver">What is your name?</span><span style="color:black;background:silver">     </span>
<span style="color:black;background:silver">Carl                   </span>
<span style="color:black;background:silver">Nice to meet you, Carl</span><span style="color:black;background:silver"> </span>
<span style="color:black;background:silver">                       </span>
<span style="color:black;background:silver">What is your name?</span><span style="color:black;background:silver">     </span>
<span style="color:silver;background:black"> </span><span style="color:black;background:silver">                      </span>
</pre></div>
<div class="shot"><pre><span style="color:black;background:teal">Press F8 to exit.      </span>
<span style="color:black;background:silver">Nice to meet you, Bob</span><span style="color:black;background:silver">  </span>
<span style="color:black;background:silver">                       </span>
<span style="color:black;background:silver">What is your name?</span><span style="color:black;background:silver">     </span>
<span style="color:black;background:silver">Carl                   </span>
<span style="color:black;background:silver">Nice to meet you, Carl</span><span style="color:black;background:silver"> </span>
<span style="color:black;background:silver">                       </span>
<span style="color:black;background:silver">What is your name?</span><span style="color:black;background:silver">     </span>
<span style="color:black;background:silver">Dave                   </span>
<span style="color:black;background:silver">Nice to meet you, Dave</span><span style="color:black;background:silver"> </span>
<span style="color:black;background:silver">                       </span>
<span style="color:black;background:silver">What is your name?</span><span style="color:black;background:silver">     </span>
<span style="color:silver;background:black"> </span><span style="color:black;background:silver">                      </span>
</pre></div>

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<br>

<h2>3. Zen of ListBox</h2>

<h3><a name="lbscr">3.1. ListBox Focus and Scrolling</a>
<span class="back">[<a href="#top">back to top</a>]</span></h3>

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">CONTENT = [ urwid.AttrWrap( w, None, 'reveal focus' ) for w in [
	urwid.Text(&quot;This is a text string that is fairly long&quot;),
	urwid.Divider(&quot;-&quot;),
	urwid.Text(&quot;Short one&quot;),
	urwid.Text(&quot;Another&quot;),
	urwid.Divider(&quot;-&quot;),
	urwid.Text(&quot;What could be after this?&quot;),
	urwid.Text(&quot;The end.&quot;),
] ]</pre>
<div class="shot"><pre><span style="color:white;background:black">Pressed:       </span>
<span style="color:black;background:teal">This is a text </span>
<span style="color:black;background:teal">string that is </span>
<span style="color:black;background:teal">fairly long    </span>
<span style="color:black;background:silver">---------------</span>
<span style="color:black;background:silver">Short one      </span>
<span style="color:black;background:silver">Another        </span>
</pre></div>
<div class="shot"><pre><span style="color:white;background:black">Pressed: down  </span>
<span style="color:black;background:silver">string that is </span>
<span style="color:black;background:silver">fairly long    </span>
<span style="color:black;background:silver">---------------</span>
<span style="color:black;background:silver">Short one      </span>
<span style="color:black;background:silver">Another        </span>
<span style="color:black;background:teal">---------------</span>
</pre></div>
<div class="shot"><pre><span style="color:white;background:black">Pressed: down  </span>
<span style="color:black;background:silver">fairly long    </span>
<span style="color:black;background:silver">---------------</span>
<span style="color:black;background:silver">Short one      </span>
<span style="color:black;background:silver">Another        </span>
<span style="color:black;background:silver">---------------</span>
<span style="color:black;background:teal">What could be  </span>
</pre></div>
<div class="shot"><pre><span style="color:white;background:black">Pressed: down  </span>
<span style="color:black;background:silver">---------------</span>
<span style="color:black;background:silver">Short one      </span>
<span style="color:black;background:silver">Another        </span>
<span style="color:black;background:silver">---------------</span>
<span style="color:black;background:teal">What could be  </span>
<span style="color:black;background:teal">after this?    </span>
</pre></div>
<div class="shot"><pre><span style="color:white;background:black">Pressed: up    </span>
<span style="color:black;background:teal">fairly long    </span>
<span style="color:black;background:silver">---------------</span>
<span style="color:black;background:silver">Short one      </span>
<span style="color:black;background:silver">Another        </span>
<span style="color:black;background:silver">---------------</span>
<span style="color:black;background:silver">What could be  </span>
</pre></div>
<div class="shot"><pre><span style="color:white;background:black">Pressed: up    </span>
<span style="color:black;background:teal">string that is </span>
<span style="color:black;background:teal">fairly long    </span>
<span style="color:black;background:silver">---------------</span>
<span style="color:black;background:silver">Short one      </span>
<span style="color:black;background:silver">Another        </span>
<span style="color:black;background:silver">---------------</span>
</pre></div>
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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"><pre><span style="color:white;background:black">Pressed:            </span>
<span style="color:black;background:teal">string that is      </span>
<span style="color:black;background:teal">fairly long         </span>
<span style="color:black;background:silver">--------------------</span>
<span style="color:black;background:silver">Short one           </span>
<span style="color:black;background:silver">Another             </span>
<span style="color:black;background:silver">--------------------</span>
<span style="color:black;background:silver">What could be after </span>
<span style="color:black;background:silver">this?               </span>
</pre></div>
<div class="shot"><pre><span style="color:white;background:black">Pressed:                 </span>
<span style="color:black;background:teal">This is a text string    </span>
<span style="color:black;background:teal">that is fairly long      </span>
<span style="color:black;background:silver">-------------------------</span>
<span style="color:black;background:silver">Short one                </span>
<span style="color:black;background:silver">Another                  </span>
<span style="color:black;background:silver">-------------------------</span>
</pre></div>
<div class="shot"><pre><span style="color:white;background:black">Pressed:   </span>
<span style="color:black;background:teal">This is a  </span>
<span style="color:black;background:teal">text string</span>
<span style="color:black;background:teal">that is    </span>
<span style="color:black;background:teal">fairly long</span>
<span style="color:black;background:silver">-----------</span>
<span style="color:black;background:silver">Short one  </span>
<span style="color:black;background:silver">Another    </span>
<span style="color:black;background:silver">-----------</span>
<span style="color:black;background:silver">What could </span>
<span style="color:black;background:silver">be after   </span>
<span style="color:black;background:silver">this?      </span>
<span style="color:black;background:silver">The end.   </span>
</pre></div>
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<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.

<br clear="left">
<br>

<h3><a name="lbdyn">3.2. Dynamic ListBox with List Walker</a>
<span class="back">[<a href="#top">back to top</a>]</span></h3>

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".

<br clear="left">
<br>

<h3><a name="lbfocus">3.3. Setting the Focus</a>
<span class="back">[<a href="#top">back to top</a>]</span></h3>

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.

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<br>

<h2>4. Combining Widgets</h2>

<h3><a name="pile">4.1. Piling Widgets</a>
<span class="back">[<a href="#top">back to top</a>]</span></h3>

<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>.

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<br>

<h3><a name="cols">4.2. Dividing into Columns</a>
<span class="back">[<a href="#top">back to top</a>]</span></h3>