[Tutor] Iterators and Generators...

[Gregor Lingl]

Marc Barry schrieb:

> Dear All:
>
> I have a class which is an iterator, but I can't figure out why it is 
> not working.  I have included a pointless example to illutsrate what 
> is happening.  I have created a class called list_iterator that 
> implements the iterator protocol over a list.  I know that that this 
> is already done for lists, but it shows the problem I am having.  Here 
> is the Python code:
>
> #-----
>
> class list_iterator:
>
>     def __init__(self, a_list):
>         self.a_list = a_list
>
>     def __iter__(self):
>         return self
>
>     def next(self):
>         for i in self.a_list:
>             yield i
>         raise StopItertion

To define an iterator you must or need not use generators, i. e.
the keyword yield. (This makes next a generator, therefore your strange 
output)
I think, one should try to understand itrators and generators als
different concepts.

Instead this might work:

class list_iterator:

    def __init__(self, a_list):
        self.a_list = a_list
        self.i = 0

    def __iter__(self):
        return self

    def next(self):
        try:
            value = self.a_list[self.i]
        except:
            raise StopIteration
        self.i += 1
        return value
   
 >>> a_list_iterator = list_iterator([1,2,3,4,5,6,7,8,9,10])
 >>> for v in a_list_iterator:
    print v,

   
1 2 3 4 5 6 7 8 9 10

Regards, Gregor

>
> a_list_iterator = list_iterator([1,2,3,4,5,6,7,8,9,10])
>
> for i in a_list_iterator:
>     print i
>
> #-----
>
> When I run the above, I expected the following output:
>
> 1
> 2
> 3
> 4
> 5
> 6
> 7
> 8
> 9
> 10
>
> Instead, I get something similar to the following:
>
> <generator object at 0x007EA328>
> <generator object at 0x007EA418>
> <generator object at 0x007EA328>
> <generator object at 0x007EA418>
> <generator object at 0x007EA328>
> <generator object at 0x007EA418>
> <generator object at 0x007EA328>
> <generator object at 0x007EA418>
> <generator object at 0x007EA328>
> <generator object at 0x007EA418>
> <generator object at 0x007EA328>
> <generator object at 0x007EA418>
> <generator object at 0x007EA328>
> <generator object at 0x007EA418>
> <generator object at 0x007EA328>
> <generator object at 0x007EA418>
> <generator object at 0x007EA328>
> ...  Well you get the point, it just loops forever.  Also, my yeild 
> statement seems to be retruning a generator object instead of the 
> number.  Also, it never reaches the 'raise' statement.  Obviously, 
> there is a flaw in my understanding of how to use a generator and the 
> iterator protocol.  Can anyone clear this up for me?
>
> Regards,
>
> Marc
>
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