This method sets the initial grab and focus on the calling widget so that, for example, if a GtkText were to call it, any keyboard events would be picked up immediately by the text widget.

grab_focus() can only be called when the widget in question has the GTK_CAN_FOCUS flag set. Some will have this set at source, others will not, and will generate an error message to that effect when the method is called. Use set_flags() to override the source settings.