On Sun, 7 Mar 2010 17:20:07 +0100
Hugo Arts <hugo.yo...@gmail.com> wrote:
> On Sun, Mar 7, 2010 at 1:58 PM, spir <denis.s...@gmail.com> wrote:
> > Hello,
> >
> > Is it possible at all to have a recursive generator? I think at a iterator
> > for a recursive data structure (here, a trie). The following code does not
> > work: it only yields a single value. Like if child.__iter__() were never
> > called.
> >
> > def __iter__(self):
> > ''' Iteration on (key,value) pairs. '''
> > print '*',
> > if self.holdsEntry:
> > yield (self.key,self.value)
> > for child in self.children:
> > print "<",
> > child.__iter__()
> > print ">",
> > raise StopIteration
> >
> > With the debug prints in code above, "for e in t: print e" outputs:
> >
> > * ('', 0)
> > < > < > < > < > < > < > < > < >
> >
> > print len(t.children) # --> 9
> >
> > Why is no child.__iter__() executed at all? I imagine this can be caused by
> > the special feature of a generator recording current execution point. (But
> > then, is it at all possible to have a call in a generator? Or does the
> > issue only appear whan the callee is a generator itself?) Else, the must be
> > an obvious error in my code.
> >
> >
> > Denis
>
> remember that child.__iter__ returns a generator object. Merely
> calling the function won't do anything. To actually get elements from
> a generator object, you need to call next() on the returned iterator,
> or iterate over it in another way (e.g. a for loop). I would write
> this method more or less like so:
>
> from itertools import chain
>
> def __iter__(self):
> this_entry = [(self.key, self.value)] is self.holds_entry else []
> return chain(this_entry, *[iter(c) for c in self.children])
>
> (Warning! untested! I'm pretty sure chain will work like this, but you
> might have to do a little tweaking)
@ Hugo & Steven
Thank you very much. I'll study your proposals as soon as I can, then tell you
about the results.
In the meanwhile, I (recursively) constructed the collection of entries and
returned iter(collection). [This works, indeed, but I also want do know how to
properly build a recursive iterator.]
Denis
--
________________________________
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