Op 2005-01-18, Steve Holden schreef <[EMAIL PROTECTED]>: > Antoon Pardon wrote: > >> 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. >> > 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.
Well I can dream, cant I? > 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? Maybe I was not clear enough. I meant to limit it to cases such as lst = list(genexp) tpl = tuple(genexp) Since in such cases the object is build in memory any way, I don't think it would be a problem of having them prebuilt in memory, or am I missing something? -- Antoon Pardon -- http://mail.python.org/mailman/listinfo/python-list
