[Python-ideas] Make len() usable on a generator

[Stephen J. Turnbull]

Nick Coghlan writes:

 > len(container) by contrast, is an O(1) operation - the time it takes
 > is independent of the length of the input.

I think a better way to put this is that len(container) is *designed*
to return an attribute that is computed as the container is (de-)
populated, so is O(1) for access.

The fact that some "containers" that wrap external objects (eg,
Scott's example of Maildir) have to use O(N) algorithms for accuracy
(since these objects are shared with other processes) doesn't really
contradict the argument from "design", it just suggests one should be
careful when giving those containers a __len__.

Also, at least for the Maildir example, len(maildir) doesn't access
the Maildir; it accesses the OS's directory listing facility, which
has a "C" so small that I suppose it's effectively O(1) compared to
sum(1 for m in maildir).  I don't know if that (pseudo?) logic applies
to other such examples, though.