__next__ and StopIteration
Charles Hixson
charleshixsn at earthlink.net
Mon Feb 9 23:33:57 EST 2015
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 at 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)
More information about the Python-list
mailing list