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I was looking at the source code for the built-in argparse._AppendAction , which implements the "append" action, and puzzled over the way it is implemented : def __call__(self, parser, namespace, values, option_string=None): items = _copy.copy(_ensure_value(namespace, self.dest, [])) items.append(values) setattr(namespace, self.dest, items) To break it down: _ensure_value is like dict.setdefault for attributes. That is, if namespace has an attribute with the name self.dest then it is returned, if not it is set to [] and returned. _copy.copy(x) returns just a shallow copy. When x is a list it is exactly like list(x) (but slower). Then the item is appended to the copy of the list gotten from namespace . Finally the self.dest attribute of namespace is overwritten with the copy, which should cause the old list to be garbage collected. Why do it in such a roundabout and inefficient way, throwing away a whole list for each append? Why doesn't this suffice? def __call__(self, parser, namespace, values, option_string=None): items = _ensure_value(namespace, self.dest, []) items.append(values)

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