damn, you start thinking 'wow a new kewl feature that does not break anything' and suddenly stuff is broken. Life just sux
newstream = lambda p,stream: (n for n in stream if p(n))
head = stream.next()
p = lambda a, b=head: not divisible(a, b)
stream = newstream(p,stream)
Genexp are meant to be passed to a function for
immediate consumption, at least for Guido.
So this is beyond that scope.
That this problem would show up in this kind of usage was known. Lambda itself has a similar problem (workaround b=head).
Other capture rules were also suggested.
The PEP tracks some of this.
""" damn, you start thinking 'wow a new kewl feature that does not break anything' and suddenly stuff is broken."""
It doesn't break existing code, AFAIK...
And yes, this was a known problem, but nevertheless it's a bit disappointing, that genexps cannot be used for this purpose. Not that it's really important, but this lazy name lookup rule makes it difficult to do interesting stuff with genexps.
Just looking at the first example, in python2.3 you can already get essentially the same functionality:
odd_numbers = itertools.ifilter(lambda x: x%2, numbers())
Consequently, I don't really see the need for generator expressions...
All in all the following version is not bad, I think:
def newstream( head, stream):
return (n for n in stream if not divisible(n, head))
head = stream.next()
stream = newstream(head, stream)
sieve 2 ( and 3 ) non recursieve ???
Try calculating the first 1000 primes!
RuntimeError: maximum recursion depth exceeded
Hm, yes, they are just as recursive... except in Scheme-land (where the original examples came from) this is considered "iterative". I guess it doesn't make much difference for Python.
This blog entry might be useful: http://goldenspud.com/webrog/archives/2004/09/12/ah-so-thats-how-they-work/
Maybe you could adapt it to use that (admittedly hackishly ugly) trick?
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