Antoon Pardon wrote:
Since it doesn't yet optimize 2+5 to a constant-folded 7 you should realize that you are suggesting a large increase in the compiler's analytical powers.Op 2005-01-18, Nick Coghlan schreef <[EMAIL PROTECTED]>:
Raymond Hettinger wrote:
[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 .
"""The situation slightly more involved whenever an object allows length mutation during iteration. """
Ouch. Nice understatement.
It's rather unfortunate that we can't make use of the length information even when the source *doesn't* mutate, though. I'll have to think some more to see if I can come up with any concrete ideas for you to shoot down :)
Something else I was thinking about. I think it would be nice if the python compilor could figure out whether a genexp in a list or tuple expression always generates the same list or tuple and then instead of generating code would generate the list or tuple in place.
I agree it would be nice under certain circumstances, but don't forget that unlike list comprehensions (for which it would be even nicer) the whole point of generator expressions is often to defer the generation of the individual items until they are required and thereby relieve stress on memory.
As an edge case to demonstrate the point, what about a constant but infinite sequence?
regards Steve -- Steve Holden http://www.holdenweb.com/ Python Web Programming http://pydish.holdenweb.com/ Holden Web LLC +1 703 861 4237 +1 800 494 3119 -- http://mail.python.org/mailman/listinfo/python-list
