[Python-Dev] re: Sets BOF / for in dict

[M.-A. Lemburg]

Ka-Ping Yee wrote:
> 
> On Mon, 5 Feb 2001, M.-A. Lemburg wrote:
> > Two things:
> >
> > 1. the proposed syntax key:value does away with the
> >    easy to parse Python block statement syntax
> 
> Oh, come on.  Slices and dictionary literals use colons too,
> and there's nothing wrong with that.  Blocks are introduced
> by a colon at the *end* of a line.

Slices and dictionary enclose the two parts in brackets -- this
places the colon into a visible context. for ... in ... : does
not provide much of a context.
 
> > 2. why can't we use the old 'for x,y,z in something:' syntax
> >    and instead add iterators to the objects in question ?
> >
> >    for key, value in object.iterator():
> >       ...
> 
> Because there's no good answer for "what does iterator() return?"
> in this design.  (Trust me; i did think this through carefully.)
> Try it.  How would you implement the iterator() method?

The .iterator() method would have to return an object which
provides an iterator API (at C level to get the best performance).
For dictionaries, this object could carry the needed state
(current position in the dictionary table) and use the PyDict_Next()
for the internals. Matrices would have to carry along more state
(one integer per dimension) and could access the internal
matrix representation directly using C functions.

This would give us: speed, flexibility and extensibility
which the syntax hacks cannot provide; e.g. how would you
specify to iterate backwards over a sequence using that notation
or diagonal for a matrix ?
 
> The PEP *is* suggesting that we add iterators to the objects --
> just not that we explicitly call them.  In the 'for' loop you've
> written, iterator() returns a sequence, not an iterator.

No, it should return a forward iterator.

-- 
Marc-Andre Lemburg
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