Steven D'Aprano wrote: > On Wed, 22 Jun 2011 15:28:23 -0400, Neal Becker wrote: > >> AFAICT, the python iterator concept only supports readable iterators, >> not write. Is this true? >> >> for example: >> >> for e in sequence: >> do something that reads e >> e = blah # will do nothing >> >> I believe this is not a limitation on the for loop, but a limitation on >> the python iterator concept. Is this correct? > > Have you tried it? "e = blah" certainly does not "do nothing", regardless > of whether you are in a for loop or not. It binds the name e to the value > blah. >
Yes, I understand that e = blah just rebinds e. I did not mean this as an example of working code. I meant to say, does Python have any idiom that allows iteration over a sequence such that the elements can be assigned? ... > * iterators are lazy sequences, and cannot be changed because there's > nothing to change (they don't store their values anywhere, but calculate > them one by one on demand and then immediately forget that value); > > * immutable sequences, like tuples, are immutable and cannot be changed > because that's what immutable means; > > * mutable sequences like lists can be changed. The standard idiom for > that is to use enumerate: > > for i, e in enumerate(seq): > seq[i] = e + 42 > > AFAIK, the above is the only python idiom that allows iteration over a sequence such that you can write to the sequence. And THAT is the problem. In many cases, indexing is much less efficient than iteration. -- http://mail.python.org/mailman/listinfo/python-list
