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Date: Mon, 09 Feb 2015 20:33:57 -0800
From: Charles Hixson <charleshixsn@earthlink.net>
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To: python-list@python.org
Subject: Re: __next__ and StopIteration
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On 02/09/2015 03:56 PM, Ian Kelly wrote:
> On Mon, Feb 9, 2015 at 4:30 PM, Steven D'Aprano
> <steve+comp.lang.python@pearwood.info> wrote:
>> The way you write iterators is like this:
>>
>>
>> Method 1 (the hard way):
>>
>> - Give your class an __iter__ method which simply returns self:
>>
>>      def __iter__(self):
>>          return self
>>
>> - Give your class a __next__ method (`next` in Python 2) which *returns* a
>> value. You will need to track which value to return yourself. It must raise
>> StopIteration when there are no more values to return. Don't use yield.
>>
>>      def __next__(self):
>>          value = self.value
>>          if value is None:
>>              raise StopIteration
>>          self.value = self.calculate_the_next_value()
>>          return value
>>
>> Your class is itself an iterator.
> This is an anti-pattern, so don't even suggest it. Iterables should
> never be their own iterators. Otherwise, your iterable can only be
> iterated over once!
>
> The proper version of the "hard way" is:
>
> 1) The __iter__ method of the iterable constructs a new iterator
> instance and returns it.
>
> 2) The __iter__ method of the *iterator* simply returns itself.
>
> 3) The __next__ method of the iterator tracks the current value and
> returns the next value. Note that the iterator should never store the
> iterable's data internally, unless the iterable is immutable and the
> calculation is trivial (e.g. a range object). Instead, it should
> determine the next value by referring to its source iterable.
So if I'm understanding this correctly, I should implement as an 
internal class within Grid something like:
     class GridIter(Iterator):
         """ A class to iterate over the cells of a Grid instance
         Vars:
           row  ::  The row currently being iterated over.
           col  ::  Yhe column currently being iterated over.
           grid ::  The grid instance being iterated over.
         """

         def __init__ (self, grid):
             """
             Params:
               grid  ::  The instance of the grid to iterate over.
             """
             self.row    =    -1
             self.col    =    -1
             self.grid    =    grid
         #end    __init__

         def __iter__ (self):
             return    self

         def __next__ (self):
             if self.row == -1 and self.col == -1:
                 self.row    =    0
                 self.col    =    0
             elif self.col >= grid.cols():
                 self.row    =    self.row + 1
                 self.col    =    0
             if self.row > grid.rows():
                 raise    StopIteration
             if not    [row, col] in self.grid:
                 return    self.__next__()
             return    grid(row, col)