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POPlease explain reasoning behind code snippet from argparse module
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  1. POPlease explain reasoning behind code snippet from argparse module
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    copied!<p>I was looking at the source code for the built-in <a href="http://docs.python.org/2/library/argparse.html#action" rel="nofollow noreferrer">argparse._AppendAction</a>, which implements the <code>"append"</code> action, and puzzled over the <a href="http://hg.python.org/cpython/file/1f66fc397c8d/Lib/argparse.py#l946" rel="nofollow noreferrer">way it is implemented</a>:</p> <pre><code> 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) </code></pre> <p>To break it down:</p> <ul> <li><code>_ensure_value</code> is like <code>dict.setdefault</code> for attributes. That is, if <code>namespace</code> has an attribute with the name <code>self.dest</code> then it is returned, if not it is set to <code>[]</code> and returned.</li> <li><code>_copy.copy(x)</code> returns just a shallow copy. When <code>x</code> is a list it is exactly like <code>list(x)</code> (but slower).</li> <li>Then the item is appended to the <em>copy</em> of the list gotten from <code>namespace</code>.</li> <li>Finally the <code>self.dest</code> attribute of <code>namespace</code> is overwritten with the copy, which should cause the old list to be garbage collected.</li> </ul> <p>Why do it in such a roundabout and inefficient way, throwing away a whole list for each append? Why doesn't this suffice?</p> <pre><code> def __call__(self, parser, namespace, values, option_string=None): items = _ensure_value(namespace, self.dest, []) items.append(values) </code></pre>
 

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