Check if modifiers pressed are only shifts

def _only_shifts(self, modifiers):
        """Check if modifiers pressed are only shifts"""
        if not modifiers or len(modifiers) > 2:
            return False
        if len(modifiers) == 2:
            return 'left shift' in modifiers and 'right shift' in modifiers
        if len(modifiers) == 1:
            return 'left shift' in modifiers or 'right shift' in modifiers