How Do Bindtag Orders Affect Event Processing in Tkinter?

Bindtags and Event Processing in Tkinter

In Tkinter, event processing is managed through bindtags, which are identifiers associated with widgets. When an event occurs, Tkinter determines the widget that intercepted it and checks its bindtags for matching bindings. However, the order of bindtags affects how events are processed.

In the example provided, three entries are created and bound to the same keypress event. Initially, their bindtags are set as follows:

entry1.bindtags(('.entry1', 'Entry', '.', 'all'))
entry2.bindtags(('Entry', '.entry1', '.', 'all'))
entry3.bindtags(('.entry1','Entry','post-class-bindings', '.', 'all'))

In the first two cases, the order is: the entry's own bindtag, Entry (the class of all entries), and a wildcard bindtag for all widgets. In the third case, an additional bindtag post-class-bindings is included.

When a keypress event occurs:

• entry1:

  • The .entry1 tag matches and fires its binding. However, at this point, the event's value (the entered character) has not been copied to the entry widget, so the status label will not yet display it.
  • The Entry tag fires next, causing the character to be inserted into the widget.

• entry2:

  • Similarly to entry1, the .entry1 tag fires first, but does not update the widget.
  • However, the Entry tag fires before the .entry1 tag in this case, so the character is inserted into the widget immediately, and the status label is updated correctly.

• entry3:

  • The same behavior as entry1 occurs: the .entry1 tag fires and does not update the widget.
  • The Entry tag is not included in its bindtags list.
  • The post-class-bindings tag is encountered, and its binding is executed before any other class bindings. In this case, the class binding for the entry is still executed and inserts the character into the widget, but because it fires after the post-class-bindings binding, the widget is not updated until after the event value is reported.

Therefore, the order of bindtags determines when the class binding for a widget is triggered. In the first two cases, the class binding fires after the widget's event value has been updated, resulting in a lag. In the third case, the class binding is executed before any other class bindings, resolving the lag issue.

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