Why Does Bindtag Order in Tkinter Affect Event Value Visibility?

Understanding Bindtags in Tkinter

When working with Tkinter, bindtags play a crucial role in event handling. As you mentioned in your question, the bindtag order can impact the visibility of event values. Let's explore why this behavior occurs.

In Tkinter, when an event is triggered, the system checks for bindings attached to various bind tags associated with the widget that received the event. By default, these bind tags include the widget itself, its class, and some global tags.

First Case: Default Bindtags

When using default bindtags for your entry widgets, the order is: ('.entry1', 'Entry', '.', 'all'). This means:

1. The event is first checked against the bind tag associated with the widget ('.entry1').
2. If no binding is found, it checks the bind tag for the widget's class ('Entry').
3. If still no binding matches, it proceeds to the general bind tag ('.') and the global bind tag ('all').

Issue: In this case, the event value is not visible in the function definition because it is retrieved using event.widget.get(). By the time the function is called, the class binding has already run, and the event value (in our case, the character "x") has been inserted into the widget.

Second Case: Modified Bindtags

In the second case, the order of bindtags for the third entry widget is altered to: ('.entry1','Entry','post-class-bindings', '.', 'all').

• The 'post-class-bindings' bindtag is added before the global tags.
• This means the event is first checked against the widget bindtag ('.entry1').
• If no binding is found, it checks the class bindtag ('Entry').
• If still no binding matches, it proceeds to 'post-class-bindings', which is where the modified behavior occurs.
• Crucially: The value of the event is retrieved at this stage, before the class binding has run and inserted the character into the widget.
• Hence, the value is still available.

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