Iteratoration question
Robert Kern
robert.kern at gmail.com
Thu Apr 2 19:18:02 EDT 2009
On 2009-04-02 18:08, andrew cooke wrote:
> grocery_stocker wrote:
>>> in summary: iterator is bound to one instance of "it", while some_func()
>>> returns a new instance each time it is called.
>>>
>>> BUT
>>>
>>> while what you are doing is interesting, it is not the same as Python's
>>> iterators, which use "yield" from a function and don't require storing a
>>> value in a class. look for "yield" in the python docs. this comment
>>> may
>>> be irrelevant; i am just worried you are confusing the above (which
>>> apart
>>> from the mistake about instances is perfectly ok) and python's iterators
>>> (which use next(), yield, etc).
>>>
>> Okay, one last question for now
>>
>> When I have the follow class
>>
>>>>> class it:
>> ... def __init__(self):
>> ... self.count = -1
>> ... def next(self):
>> ... self.count +=1
>> ... if self.count< 4:
>> ... return self.count
>> ... else:
>> ... raise StopIteraton
>> ...
>>
>>
>>
>>>>> value = it()
>> How comes I can;t go over 'value' like in the following
>>
>>>>> for x in value:
>> ... print x
>> ...
>> Traceback (most recent call last):
>> File "<stdin>", line 1, in ?
>> TypeError: iteration over non-sequence
>>
>> But yet, I can do...
>>
>>>>> value.next()
>> 0
>>>>> value.next()
>> 1
>>>>> value.next()
>> 2
>>>>> value.next()
>> 3
>
> replace return with yield and it might work.
>
> i have to go eat, but if it doesn't read the python docs on iterators -
> for example http://docs.python.org/reference/expressions.html#index-1825
No, .next() needs to be a regular function that returns a value. What he needs
is an __iter__() method that returns self. Alternately, __iter__ could be a
generator, and you wouldn't implement a .next() at all.
In [1]: class it(object):
...: def __init__(self):
...: self.count = -1
...: def __iter__(self):
...: return self
...: def next(self):
...: self.count += 1
...: if self.count < 4:
...: return self.count
...: else:
...: raise StopIteration
...:
...:
In [2]: class it2(object):
...: def __init__(self):
...: self.count = -1
...: def __iter__(self):
...: self.count += 1
...: while self.count < 4:
...: yield self.count
...: self.count += 1
...:
...:
In [3]: list(it())
Out[3]: [0, 1, 2, 3]
In [4]: list(it2())
Out[4]: [0, 1, 2, 3]
--
Robert Kern
"I have come to believe that the whole world is an enigma, a harmless enigma
that is made terrible by our own mad attempt to interpret it as though it had
an underlying truth."
-- Umberto Eco
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