UserList - which methods needs to be overriden?

Nagy László Zsolt gandalf at
Fri Jun 10 04:04:38 EDT 2016

2016.06.10. 0:38 keltezéssel, Michael Selik írta:
> On Thu, Jun 9, 2016 at 5:07 AM Nagy László Zsolt <gandalf at
> <mailto:gandalf at>> wrote:
>     I would like to create a collections.UserList subclass that can notify
>     others when the list is mutated.
> Why not subclass MutableSequence instead? The ABC will tell you which
> methods to override (the abstract ones). The mixin methods rely on
> those overrides.
I'm not sure wich one is the best. Peter wrote that UserList was left in
collections only for backward compatiblity. This might be a point
against using UserList. The thing with MutableSequence is that it is
quite inefficient. For example, it implements clear() by calling pop()
in a loop. It implements extend() by calling append() in a loop. And we
all know that the built-in extend() method of the list object is much
more efficient. It is true that by overriding the abstract methods, I'll
get a functional list like object, but we all know that

 There should be one-- and preferably only one --obvious way to do it.

Which one is that? :-)

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