[Tutor] __iter__

[Rinzwind]

danny: my apologies for sending this to your email instead of the list!!
(There I was looking at the list going: WTF DOES IT TAKE SO LONG when
it hit me...).

*cuts in* No I don't :-)

With this:

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

Where/how/when is 'def next(self(:' called?
Oh and if someone has some newby tut's on this I'd appreciate it. All
I find are not-so-newby-friendly websites regarding to classes :(*

Wim

> On 1/17/06, Danny Yoo <dyoo at hkn.eecs.berkeley.edu> wrote:
> >
> >
> > 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!
> >
> > _______________________________________________
> > Tutor maillist  -  Tutor at python.org
> > http://mail.python.org/mailman/listinfo/tutor
> >
>