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    <p>There are several issues in your code:</p> <p>1.If you make any list operation the result will be a native list:</p> <pre><code>layers1 = Layers() layers2 = Layers() layers1 + layers2 -&gt; the result will be a native list </code></pre> <p>2.Why define input_string when you can override <a href="http://docs.python.org/reference/datamodel.html#object.__repr__" rel="noreferrer"><code>__repr__</code></a> or <a href="http://docs.python.org/reference/datamodel.html#object.__str__" rel="noreferrer"><code>__str__</code></a></p> <p>3.Why do you even have to derive from list in this case? You only need to derive from list if you want your class to behave exactly like a list. But in your case you seem to be looking for a container. All you need to do to get your class to behave similar to a list is to override some special python methods <a href="http://docs.python.org/reference/datamodel.html#emulating-container-types" rel="noreferrer">http://docs.python.org/reference/datamodel.html#emulating-container-types</a></p> <pre><code>class Layers(object): def __init__(self, container=None): if container is None: container = [] self.container = container def add(self,n,k,comment): self.container.append([n,k,comment]) def __str__(self): return str(self.container) def __repr__(self): return str(self.container) def __getitem__(self, key): return Layers(self.container[key]) def __len__(self): return len(self.container) &gt;&gt;&gt; l = Layers() &gt;&gt;&gt; l.add(1, 2, 'test') &gt;&gt;&gt; l.add(1, 2, 'test') &gt;&gt;&gt; l [[1, 2, 'test'], [1, 2, 'test']] &gt;&gt;&gt; l[0] [1, 2, 'test'] &gt;&gt;&gt; len(l) 2 </code></pre>
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    1. POHow to use classes derived from Python's list class
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    1. USAndrew T
    1. USNadia Alramli
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    1. CO@hopatcong: You can fix the first problem by implementing the __add__ method. See http://docs.python.org/reference/datamodel.html#emulating-numeric-types.
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      1. USNikhil Chelliah
    2. COJust an addition here, __repr__ is the proper replacement for input_string. __str__ is supposed to be for an informal/simple explanation - not a full representation of the object.
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      1. USmonokrome
    3. COAnd your code has an issue: mutable default value, see http://stackoverflow.com/q/1132941/13543
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      1. USAndrew T
 

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