nltk.draw.util.CanvasWidget

A collection of graphical elements and bindings used to display a complex object on a Tkinter Canvas. A canvas widget is responsible for managing the Canvas tags and callback bindings necessary to display and interact with the object. Canvas widgets are often organized into hierarchies, where parent canvas widgets control aspects of their child widgets.

Each canvas widget is bound to a single Canvas. This Canvas is specified as the first argument to the CanvasWidget's constructor.

Attributes. Each canvas widget can support a variety of "attributes", which control how the canvas widget is displayed. Some typical examples attributes are color, font, and radius. Each attribute has a default value. This default value can be overridden in the constructor, using keyword arguments of the form attribute=value:

>>> from nltk.draw.util import TextWidget
>>> cn = TextWidget(c, 'test', color='red')

Attribute values can also be changed after a canvas widget has been constructed, using the __setitem__ operator:

>>> cn['font'] = 'times'

The current value of an attribute value can be queried using the __getitem__ operator:

>>> cn['color']
red

For a list of the attributes supported by a type of canvas widget, see its class documentation.

Interaction. The attribute 'draggable' controls whether the user can drag a canvas widget around the canvas. By default, canvas widgets are not draggable.

CanvasWidget provides callback support for two types of user interaction: clicking and dragging. The method bind_click registers a callback function that is called whenever the canvas widget is clicked. The method bind_drag registers a callback function that is called after the canvas widget is dragged. If the user clicks or drags a canvas widget with no registered callback function, then the interaction event will propagate to its parent. For each canvas widget, only one callback function may be registered for an interaction event. Callback functions can be deregistered with the unbind_click and unbind_drag methods.

Subclassing. CanvasWidget is an abstract class. Subclasses are required to implement the following methods:

• __init__: Builds a new canvas widget. It must perform the following three tasks (in order):

  • Create any new graphical elements.
  • Call _add_child_widget on each child widget.
  • Call the CanvasWidget constructor.

• _tags: Returns a list of the canvas tags for all graphical elements managed by this canvas widget, not including graphical elements managed by its child widgets.

• _manage: Arranges the child widgets of this canvas widget. This is typically only called when the canvas widget is created.

• _update: Update this canvas widget in response to a change in a single child.

For a CanvasWidget with no child widgets, the default definitions for _manage and _update may be used.

If a subclass defines any attributes, then it should implement __getitem__ and __setitem__. If either of these methods is called with an unknown attribute, then they should propagate the request to CanvasWidget.

Most subclasses implement a number of additional methods that modify the CanvasWidget in some way. These methods must call parent.update(self) after making any changes to the canvas widget's graphical elements. The canvas widget must also call parent.update(self) after changing any attribute value that affects the shape or position of the canvas widget's graphical elements.