[Tutor] __iter__

[Danny Yoo]

On Mon, 16 Jan 2006, Christopher Spears wrote:

> I'm not sure if I understand __iter__.  You use it to create an object
> that iterates through itself using a next menthod ?

Hi Chris,

Yes, that's one application.


But __iter__() doesn't necessarily have to return 'self'.  For example,
here's a useless toy class that might help explain what can happen if we
do so without thinking:

################################
class MyListOfNumbers:
    def __init__(self, data):
        self.data = data
    def __iter__(self):
        return Pointer(self)


class Pointer:
    def __init__(self, numbers):
        self.numbers = numbers
        self.offset = 0
    def next(self):
        if self.offset == len(self.numbers.data):
            raise StopIteration
        element = self.numbers.data[self.offset]
        self.offset = self.offset + 1
        return element
################################

Let's see how this might work:

#######
>>> nums = MyListOfNumbers([0, 1])
>>> for x in nums:
...     for y in nums:
...         print x, y
...
0 0
0 1
1 0
1 1
#######



Now imagine what might happen if we didn't return a separate Pointer
iterator:

################################################
class MyListOfNumbers2:
    def __init__(self, data):
        self.data = data
        self.offset = 0

    def __iter__(self):
        return self

    def next(self):
        if self.offset == len(self.data):
            raise StopIteration
        element = self.data[self.offset]
        self.offset = self.offset + 1
        return element
####################################################

On a glance, this also looks reasonable:

######
>>> nums = MyListOfNumbers2([3, 1, 4, 1, 5])
>>> for n in nums:
...     print n
...
3
1
4
1
5
######


But, of course, you know that there has to be SOMETHING wrong here.
*grin*

And here's one example that shows a problem:

######
>>> nums = MyListOfNumbers2([0, 1])
>>> for x in nums:
...     for y in nums:
...         print x, y
...
0 1
>>>
######

We expected to see all pairs of 0-1 combinations, but came up way short.
Do you know why?


Best of wishes!