Understanding Iterators and Iterables in python

Question

I am trying to understand iterators and iterables. I have the python program below.

class fib():
    def __init__(self):
        self.prev = 0
        self.curr = 1

    def __iter__(self):
        return self

    def __next__(self):
        value = self.curr
        self.curr += self.prev
        self.prev = value
        return value

f = fib()
print(type(f))


z = iter(f)
item = z.next()
print(z)
item = z.next()
print(z)

I get the following error

   item = z.next()
AttributeError: 'fib' object has no attribute 'next'
1

What am I doing wrong and why am I getting this error.As per my understanding calss fib() is an iterable and hence should return an iterator.Calling next() on that iterator should return the items.

`fib()` should inherit from `object`. It doesn't fix everything, but at least you get a more specific error. — John Gordon, Dec 04 '15 at 22:30
I tried his example using python 2.7 and got the same error, so I assume he's not using python 3. — John Gordon, Dec 04 '15 at 22:32
In Python 2, classes don't inherit from `object` by default, and you can use `.next` on an iterator. In Python 3, classes do inherit from `object` by default, but you can't use `obj.next()`; you need to use `next(obj)`. — BrenBarn, Dec 04 '15 at 22:32
@JohnGordon - tried it.doesn't seem to help.It looks like it does inherit from object by default.I am using Python 3.5 — liv2hak, Dec 04 '15 at 22:32
It's `next(z)`, not `z.next()`. You didn't define a `next` method, after all. `z.next()` is the old style. — user2357112, Dec 04 '15 at 22:35
@BrenBarn - calling next(obj) does seem to work fine.Could you explain why that is ? — liv2hak, Dec 04 '15 at 22:35
@user2357112 - what is the difference between next() and `__next__` that I do define.? — liv2hak, Dec 04 '15 at 22:36
you don't have a next method defined. So you are not able to call it — wolendranh, Dec 04 '15 at 22:37
Also see [here](http://stackoverflow.com/questions/29578469/how-to-make-an-object-both-a-python2-and-python3-iterator). — TigerhawkT3, Dec 04 '15 at 22:37
@TigerhawkT3 - So just to clarify, next(obj) calls `__next__` method inside the object? — liv2hak, Dec 04 '15 at 22:41
Not only does it not get called, it doesn't even need to exist. — TigerhawkT3, Dec 04 '15 at 22:44
Although it [looks like](http://stackoverflow.com/questions/19151/how-to-make-class-iterable) custom classes need to define a `__next__` and then `next(f)` will call it. — TigerhawkT3, Dec 04 '15 at 22:52

Understanding Iterators and Iterables in python

0 Answers0