generator expressions: performance anomaly?

[Raymond Hettinger]

[Delaney, Timothy C]
> Nick's other suggestion - that genexps propagate __len__ - might
> still be interesting. Of course, it would only be applicable for
> unconditional genexps(i.e. no if clause).

Length transparency for iterators is not as general as one would expect.  I once
spent a good deal of effort exploring where it made sense, and I was surprised
to find that it only rarely works out.  Length transparency is an unexpectedly
thorny subject with many dead-ends which precludes a fully general solution such
as that proposed by Nick.

For a recap of my research, see the docstring for Lib/test/test_iterlen.py .


Raymond Hettinger