Since we already have the islice iterator, what's the point?
I'd like to see iterators become as easy to work with as lists are. At the moment, anything that returns an iterator forces you to use the relatively cumbersome itertools.islice mechanism, rather than Python's native slice syntax.
In the example below (printing the first 3 items of a sequence), the fact that sorted() produces a new iterable list, while reversed() produces an iterator over the original list *should* be an irrelevant implementation detail from the programmer's point of view.
However, the fact that iterators aren't natively sliceable throws this detail in the programmer's face, and forces them to alter their code to deal with it. The conversion from list comprehensions to generator expressions results in similar irritation - the things just aren't as convenient, because the syntactic support isn't there.
Py> lst = "1 5 23 1234 57 89 2 1 54 7".split() Py> lst ['1', '5', '23', '1234', '57', '89', '2', '1', '54', '7']
Py> for i in sorted(lst)[:3]: ... print i ... 1 1 1234
Py> for i in reversed(lst)[:3]: ... print i ... Traceback (most recent call last): File "<stdin>", line 1, in ? TypeError: unsubscriptable object
Py> from itertools import islice Py> for i in islice(reversed(lst), 3): ... print i ... 7 54 1
It seems to me that introducing this notation would mostly lead to confuse users, since in most other places a slice produces an independent *copy* of the data.
Well, certainly everything I can think of in the core that currently supports slicing produces a copy. Slicing on a numarray, however, gives you a view. The exact behaviour (view or copy) really depends on what is being sliced.
For iterators, I think Raymond's islice exemplifies the most natural slicing behaviour. Invoking itertools.tee() behind the scenes (to get copying semantics) would eliminate the iterator nature of the approach in many cases.
I don't think native slicing support will introduce any worse problems with the iterator/iterable distinction than already exist (e.g. a for loop consumes an iterator, but leaves an iterable unchanged. Similarly, slicing does not alter an iterable, but consumes an iterator).
Continuing the sorted() vs reversed() example from above:
Py> sortlst = sorted(lst) Py> max(sortlst) '89' Py> len(sortlst) 10
Py> revlst = reversed(lst) Py> max(revlst) '89' Py> len(revlst) 0
Regards, Nick.
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Nick Coghlan | [EMAIL PROTECTED] | Brisbane, Australia
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http://boredomandlaziness.skystorm.net
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