A Comprehensive Introduction to Python Programming and ... - MSDL

3.2 The Different Widgets 33 / 75
3.2.13 Scrollbar
The Scrollbar widget is a typical scrollbar, with arrows at both ends and a slider portion
in the middle. Unlike typical scrollbars, its color can be modified.
The Scrollbar widget provides the following options:
Option Description
command Used to update the associated widget. This is typically
the xview or yview method of the scrolled widget.
If the user drags the scrollbar slider, the command is
called as callback(MOVETO, offset) where offset 0.0
means that the slider is in its topmost (or leftmost) position,
and offset 1.0 means that it is in its bottommost
(or rightmost) position. If the user clicks the arrow buttons,
or clicks in the trough, the command is called as
callback(SCROLL, step, what). The second argument
is either ”-1” or ”1” depending on the direction, and
the third argument is UNITS to scroll lines (or other
units relevant for the scrolled widget), or PAGES to
scroll full pages.
orient Defines how to draw the scrollbar. Use one of HORI-
ZONTAL or VERTICAL. Default is VERTICAL.
The Scrollbar widget defines the following methods:
3.2.14 Text
3.2.15 Canvas
Method Description
The canvas widget provides the basic graphics facilities for Tkinter, and so more advanced
functions. Drawing on the canvas is done by creating various items on it. It is
important to note at this stage that items are not widgets, even though they are similar
in many ways. Each item on a canvas is enclosed by a bounding box, which is defined
using 2 points: the top-left corner and the bottom-right corner of the box. Tkinter uses
two coordinate systems simultaneously: the canvas coordinate system and the window
coordinate system. Both systems express positions relative to the top-left corner, with
X-coordinates increasing to the right and Y-coordinates increasing downwards. However,
the origin for the two systems is different. The window system expresses the
coordinates by placing the origin at the top-left corner of the visible portion of the canvas,
while the canvas system places its origin at the top-corner of the canvas widget,
even it is not visible. The difference is important when handling mouse events bound
to the canvas since the event object receives its coordinates in the window system.
Luckily, the canvas widget provides the necessary methods to convert coordinates to
the canvas system, through calls to the canvasx( ) and canvasy( ) methods.
The Tkinter canvas supports the following standard items (more can also be added):