Why Does Tkinter's Default Bindtags Cause a 'Lag by One' Issue in Keypress Events?

Bindtags in Tkinter: Unveiling the 'Lag by One' Issue

Consider the following Tkinter code where default bindtags are employed:

entry1.bind("<KeyPress>", OnKeyPress)

The event value will not be accessible within the OnKeyPress function, resulting in a "lag by one" effect. This occurs because the default bindtags prioritize the widget tag, which evaluates the widget contents before the class binding, which inserts the character.

Explanation:

When an event is triggered, Tkinter determines the relevant bind tags for the widget with focus. By default, the initial tag is the widget itself. If a binding exists for this tag, Tkinter executes it, potentially outputting the widget contents. However, if no binding is defined, Tkinter proceeds to the next tag.

In this scenario, the subsequent tag is the widget class. The class binding inserts the character into the widget. Therefore, the widget contents are updated after executing both the widget and class bindings. This results in the "lag by one" issue.

Resolution:

To resolve this issue, modify the order of bindtags to ensure the class binding is checked before the widget binding. For example:

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

This modification ensures that the class binding processes the event first, inserting the character before the widget binding attempts to retrieve its contents. Consequently, the event value is accessible within OnKeyPress.

Summary:

Using default bindtags in Tkinter can lead to a "lag by one" effect where the event value is not immediately available within the event handler. By altering the bindtag order to prioritize the class binding, this issue can be resolved.

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