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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def set_current_filename(self, filename, focus=True):

 """Set current filename and return the associated editor instance."""

        index = self.has_filename(filename)
        if index is not None:
            if focus:
                self.set_stack_index(index)
            editor = self.data[index].editor
            if focus:
                editor.setFocus()
            else:
                self.stack_history.remove_and_append(index)

            return editor