Python in a Nutshell

The Canvas Widget

Class Canvas is a powerful, flexible widget used for many purposes, including plotting and, in particular, building custom widgets. Building custom widgets is an advanced topic, and I do not cover it further in this book. This section covers only a subset of Canvas functionality used for the simplest kind of plotting.

Coordinates within a Canvas instance c are in pixels, with the origin at the upper left corner of c and positive coordinates growing rightward and downward. There are advanced methods that let you change c’s coordinate system, but I do not cover them in this book.

What you draw on a Canvas instance c are canvas items, which can be lines, polygons, Tkinter images, arcs, ovals, texts, and others. Each item has an item handle by which you can refer to the item. You can also assign symbolic names called tags to sets of canvas items (the sets of items with different tags can overlap). ALL is a predefined tag that applies to all items; CURRENT is a predefined tag that applies to the item under the mouse pointer.

Tags on a Canvas instance are different from tags on a Text instance. The canvas tags are nothing more than sets of items with no independent existence. When you perform any operation, passing a Canvas tag as the item identifier, the operation occurs on those items that are in the tag’s current set. It makes no difference if items are later removed from or added to that tag’s set.

You create a canvas item by calling on c a method with a name of ...