Specifies either the widget in the subtree structure which is to receive the keyboard event, or None. Note that it is not an error to specify None when no input focus was previously set.
Specifies the widget for which the keyboard focus is to be set.
When there is no modal cascade, keyboard events can occur within a widget W in one of three ways:
W has the X input focus.
W has the keyboard focus of one of its ancestors, and the event occurs within the ancestor or one of the ancestor's descendants.
No ancestor of W has a descendant within the keyboard focus, and the pointer is within W.
When there is a modal cascade, a widget W receives keyboard events if an ancestor of W is in the active subset of the modal cascade and one or more of the previous conditions is True.
When subtree or one of its descendants acquires the X input focus or the pointer moves into the subtree such that keyboard events would now be delivered to subtree, a FocusIn event is generated for the descendant if FocusNotify events have been selected by the descendant. Similarly, when W loses the X input focus or the keyboard focus for one of its ancestors, a FocusOut event is generated for descendant if FocusNotify events have been selected by the descendant.
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