What to consider before subclassing list?

python list subclassing
Edward Williams picture Edward Williams · Oct 15, 2010 · Viewed 9.2k times · Source

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

intuited picture intuited · Oct 15, 2010

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.

Related questions

creating a defaultlist in python

I'm trying to create a list equivalent for the very useful collections.defaultdict. The following design works nicely: class defaultlist(list): def __init__(self, fx): self._fx = fx def __setitem__(self, index, value): while len(self) <= index: self.append(…

Read More
Finding the index of an item in a list

Given a list ["foo", "bar", "baz"] and an item in the list "bar", how do I get its index (1) in Python?

Read More
How do I get the number of elements in a list?

Consider the following: items = [] items.append("apple") items.append("orange") items.append("banana") # FAKE METHOD: items.amount() # Should return 3 How do I get the number of elements in the list items?

Read More