[Python-Dev] Re: PEP 322: Reverse Iteration
Guido van Rossum
guido at python.org
Wed Nov 5 19:30:56 EST 2003
> > I'm okay with adding reversed() as a builtin that works for sequences
> > only but I'm not okay with adding the __reversed__ protocol.
> But, doesn't this effectively take the PEP back to the original proposal
> of a sequence method that it drifted away from?
No, because making it a sequence method would require every sequence
implementation to support it. Making it a builtin makes it work for
all sequences (everything that supports __len__ and __getitem__ with
random access, really).
> With the restriction to sequences, reversed() is then likely to be
> implemented as a thin wrapper around seq.somerevmethod() which could then
> return either a new reversed sequence, an iterable, or an iterator
> depending upon efficiency, implementation, thread-safety, etc.
No. reversed() should *never* return a new sequence; it should return
> Since reversed() is turning out not be generally applicable anyway,
> perhaps going back to the original idea of a sequence method would be a
> good thing?
No. The feedback on that was pretty uniformly negative. The PEP is
95% about reversed() on sequences and only a timy bit about
__reversed__, so little is lost.
--Guido van Rossum (home page: http://www.python.org/~guido/)
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