What to consider before subclassing list?

Solution 1:

The abstract base classes provided in the collections module, particularly MutableSequence, can be useful when implementing list-like classes. These are available in Python 2.6 and later.

With ABCs you can implement the "core" functionality of your class and it will provide the methods which logically depend on what you've defined.

For example, implementing __getitem__ in a collections.Sequence-derived class will be enough to provide your class with __contains__, __iter__, and other methods.

You may still want to use a contained list object to do the heavy lifting.

Solution 2:

There are no benefits to subclassing list. None of the methods will use any methods you override, so you can have unexpected bugs. Further, it's very often confusing doing things like self.append instead of self.foos.append or especially self[4] rather than self.foos[4] to access your data. You can make something that works exactly like a list or (better) howevermuch like a list you really want while just subclassing object.

Solution 3:

I think the first question I'd ask myself is, "Is my new object really a list?". Does it walk like a list, talk like a list? Or is is something else?

If it is a list, then all the standard list methods should all make sense.

If the standard list methods don't make sense, then your object should contain a list, not be a list.

In old python (2.2?) sub-classing list was a bad idea for various technical reasons, but in a modern python it is fine.