Hi, I just finished working on a project that plays around with ABCs and function annotations. The idea was to allow for more delcarative code by adding tools to describe Pytohn data in more detail. While I don't think the result is suitable for adding to Python (it's way too big a change, and it's not yet proven), the process of making something consistent involved working through a lot of ideas about "types in Python" that I recorded at http://www.acooke.org/pytyp.pdf Part of that paper (pages 8 and 9) describes some issues that caused particular problems, including: * The lack of annotations on type generators makes it hard to use annotations as a way of completely describing types. * There seems to be a missing ABC for __getitem__ (which unites lists, dicts and tuples). * As ever, mutability is complicated :o) If we had copy on write lists (which already exist), perhaps we could hash instances efficiently? (OK, this may be already discussed, but I had to mention it) * Given duck typing, shouldn't AttributeError be a TypeError (or vice versa?) * For this particular use-case, an __instancehook__ (which would work much like __subclasshook__) for ABCMeta would have been useful (as described in the paper, polymorphism in Python occurs at the instance level, so asking about the types of instances makes a surprising amount of sense, if done right). Anyway, apologies if some or all of this is old news or inapprorpiate. I just thought people here might find it interesting (you can do things like type check functions and use dynamic dispatch by type - all in a fairly pythonic way... (imho)) Cheers, Andrew PS The project home and more docs are at http://www.acooke.org/pytyp/ ; the code is at http://code.google.com/p/pytyp/ ; pypi page is http://pypi.python.org/pypi/pytyp