[Python-Dev] PEP 279

2002年3月28日 16:55:15 -0500

PEP 279 proposes three separate things. Comments on each:
1. New builtin: indexed()
 I like the idea of having some way to iterate over a sequence and
 its index set in parallel. It's fine for this to be a builtin.
 I don't like the name "indexed"; adjectives do not make good
 function names. Maybe iterindexed()?
 I don't like the start and stop arguments. If I saw code like
 for i, j in iterindexed("abcdefghij", 5, 10): print i, j
 I would expect it to print
 5 f
 6 g
 7 h
 8 i
 9 j
 while the spec in the PEP would print
 5 a
 6 b
 7 c
 8 d
 9 e
 Very confusing. I propose to remove the start/stop arguments, *or*
 change the spec to:
 def iterindexed(sequence, start=0, stop=None):
 i = start
 while stop is None or i < stop:
 try:
 item = sequence[i]
 except IndexError:
 break
 yield (i, item)
 i += 1
 This reduces the validity to only sequences (as opposed to all
 iterable collections), but has the advantage of making
 iterindexed(x, i, j) iterate over x[i:j] while reporting the index
 sequence range(i, j) -- not so easy otherwise.
 The simplified version is still attractive because it allows
 arbitrary iterators to be passed in:
 def iterindexed(collection):
 i = 0
 it = iter(collection)
 while 1:
 yield (i, it.next())
 i += 1
2. Generator comprehensions
 I don't think it's worth the trouble. I expect it will take a lot
 of work to hack it into the code generator: it has to create a
 separate code object in order to be a generator. List
 comprehensions are inlined, so I expect that the generator
 comprehension code generator can't share much with the list
 comprehension code generator. And this for something that's not
 that common and easily done by writing a 2-line helper function.
 IOW the ROI isn't high enough.
3. Generator exception passing
 This is where the PEP seems weakest. There's no real motivation
 ("This is a true deficiency" doesn't count :-). There's no hint as
 to how it should be implemented. The example has a "return log"
 statement in the generator body which is currently illegal, and I
 can't figure out to where this value would be returned. The
 example looks like it doesn't need a generator, and if it did, it
 would be easy to stop the generator by setting a global "please
 stop" flag and calling next() once more. (If you don't like
 globals, make the generator a method of a class and make the stop
 flag an instance variable.)
--Guido van Rossum (home page: http://www.python.org/~guido/)