On Sun, 7 Mar 2010 11:58:05 pm spir wrote:
> Hello,
>
> Is it possible at all to have a recursive generator?
Yes.
>>> def recursive_generator(n):
... if n == 0:
... return
... yield n
... for i in recursive_generator(n-1):
... yield i
...
>>> it = recursive_generator(5)
>>> it.next()
5
>>> it.next()
4
>>> it.next()
3
>>> it.next()
2
>>> it.next()
1
>>> it.next()
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
StopIteration
> 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
__iter__ should be an ordinary function, not a generator. Something like
this should work:
# Untested.
def __iter__(self):
''' Iteration on (key,value) pairs. '''
def inner():
print '*', # Side effects bad...
if self.holdsEntry:
yield (self.key,self.value)
for child in self.children:
print "<",
child.__iter__()
print ">",
raise StopIteration
return inner()
This means that your class won't *itself* be an iterator, but calling
iter() on it will return a generator object, which of course is an
iterator.
If you want to make your class an iterator itself, then you need to
follow the iterator protocol. __iter__ must return the instance itself,
and next must return (not yield) the next iteration.
class MyIterator(object):
def __init__(self, n):
self.value = n
def next(self):
if self.value == 0:
raise StopIteration
self.value //= 2
return self.value
def __iter__(self):
return self
See the discussion in the docs about the iterator protocol:
http://docs.python.org/library/stdtypes.html#iterator-types
> 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.
That's exactly what generators do: when they reach a yield, execution is
halted, but the state of the generator is remembered. Then when you
call the next() method, execution resumes.
> (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.
I don't understand what you mean.
--
Steven D'Aprano
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