[Lots of proposals snipped]
90% of my gripes with range disappeared with the addition of enumerate.
However, if there's going to be another round of range literal proposals
I might as well throw out what seems (to me anyway) like the only
halfway obvious choice in the context of Python.
1. a:b:c is equivalent to slice(a,b,c) with the usual rules that missing
values are mapped to None: Parentheses would be required where this
construct would otherwise be ambiguous.
2. slice(a,b,c) grows an __iter__ method that returns a generator that
produces an appropriate sequence of integers. This is easy to define for
positive steps, but tricky for negative steps (more on that in a second).
Thus:
for i in (1::2): print i,
=> 1 3 5 ...
and
for i in (:10:3): print i,
=> 0 3 6 9
The tricky part is deciding what to with negative steps when the start
is undefined. It's tempting to use 0 as the start, but I think -1 is
actually better if slightly odd. The reason is that I think the
following invariant should hold.
newlist = []
for i in someslice:
try:
newlist.append(origlist[i])
except IndexError:
break
assert(newlist == origlist[slice])
For that to work out, you want:
for i in (::-2): print i,
=> -1 -3 -5 ...
etc. I realize that this is quite similar to the rejected PEP 204
syntax, but the addition of generators has reduced many of the problems
that that PEP had.
I have no great need for range literals, but it seems that if there were
to be one, one that fit together with the existing syntax would be
better than making up something new.
-tim
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