[Tutor] recursive generator

[spir]

On Sun, 7 Mar 2010 17:20:07 +0100
Hugo Arts <hugo.yoshi at gmail.com> wrote:

> On Sun, Mar 7, 2010 at 1:58 PM, spir <denis.spir at 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
-- 
________________________________

la vita e estrany

spir.wikidot.com