On Wed, 2009-05-20 at 11:35 -0700, Jan wrote:
> Wouldn't it be easy for Python to implement generating functions so
> that the iterators they return are equipped with a __reset__() method?
>
> Here is the context of this question.
>
> Python documentation defines a "iterator" as an object ITERATOR
> having methods __next__() and __iter__() such that the call
> ITERATOR.__iter__() returns the object itself, and once a call
> ITERATOR. __next__() raises StopIteration every such subsequent call
> does the same.
You don't need a reset method. There is no hard and fast rule that
__iter__ must return the object itself. It just needs to return an
iterator. For example:
>>> l = [1,2,3]
>>> l.__iter__()
<listiterator object at 0x7fd0da315850>
>>> l is l.__iter__()
False
Just create a class with an __iter__ method that returns a reset
iterator object.
class X(object):
def __init__(self, max=3):
self.counter = 0
self.max = max
def __iter__(self):
return self
def next(self):
if self.counter < self.max:
self.counter += 1
return self.counter
else:
raise StopIteration
class Y(object):
def __iter__(self):
return X()
In this setup, X has the problem you are trying to avoid, but Y behaves
as a resettable iterable.
>>> x = X()
>>> for c in x:
... print c
...
1
2
3
>>> for c in x:
... print c
...
>>> y = Y()
>>> for c in y:
... print c
...
1
2
3
>>> for c in y:
... if c < 3:
... print c
...
1
2
>>> for c in y:
... print c
...
1
2
3
Cheers,
Cliff
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