python - What to consider before subclassing list?

Question

I was recently going over a coding problem I was having and someone looking at the code said that subclassing list was bad (my problem was unrelated to that class). He said that you shouldn't do it and that it came with a bunch of bad side effects. Is this true?

I'm asking if list is generally bad to subclass and if so, what are the reasons. Alternately, what should I consider before subclassing list in Python?

Answer 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.