How Do Tkinter's Bind Tags Affect Event Value Access in Bindings?

Understanding Bind Tags in Tkinter

In the provided code, the question arose regarding the behavior of bind tags in Tkinter, particularly the discrepancy between using default and modified bind tags.

Default Bindtags

By default, widgets have four bind tags:

• The widget itself (e.g., ".entry1")
• The widget class (e.g., "Entry")
• "." (all widgets)
• "all" (all events)

When an event occurs, Tkinter iterates through these bind tags in order. If a binding is found for the event on a particular tag, it is executed.

In the given code, the first widget ("entry1") uses the default bind tags. This means that the class binding for "KeyPress" will execute after the widget binding for "KeyPress". As a result, the event value (e.g., the character key pressed) is not yet present in the widget when the class binding executes.

Modified Bindtags

To address this issue, the second widget ("entry2") modifies the order of bind tags. By placing the class binding before the widget binding, the class binding executes before the widget binding. This allows the class binding to copy the event value into the widget, making it available to the widget binding.

Third Widget ("entry3")

The third widget ("entry3") adds a new bind tag, "post-class-bindings." This tag is used to bind events that should execute after the class bindings. By using this tag, the widget binding is ensured to execute after the class binding, ensuring that the event value is available.

Conclusion

Using default bind tags can result in a slight delay in accessing the event value within bindings. By modifying the order of bind tags or using additional tags like "post-class-bindings," this delay can be eliminated, ensuring that event values are accessible during handler execution.

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