Implementation of iterator protocol without iter method in class

Question

I have tried with this code to implement iterator protocol, and every thing works fine.
Ideally it shouldn't as I have not implemeted iter().

class PowerofTwo():
    def __init__(self,maxi):
        self.max = maxi
        self.num = 0     

    def __next__(self):
        if self.num < self.max:
            result = 2 ** self.num
            self.num += 1
            return result
        else:
            raise StopIteration


myObj = PowerofTwo(5)

print(type(myObj)) #print <class '__main__.PowerofTwo'>
print(next(myObj)) #print 1

Even if assume that __iter__() method is available by default like __init__().
But not able to understand how can I directly call next() and skip calling iter(obj) at first place.
If I try same thing with list or other builtin iterator, I gets error that object is not an iterator.

Am I missing something basic here ?

__next__ is exactly the method that the next() function tries to call, and your class has a __next__ method. Why do you expect it not to work? — khelwood
– khelwood, Commented Apr 24, 2020 at 15:59
PowerofTwo is an iterator, but it is not iterable. An iterable is something you can pass to iter; an iterator is the thing that iter returns and can be passed to next. — chepner
– chepner, Commented Apr 24, 2020 at 16:06
Some things, like the objects returned by open, are both iterable and iterators. Others, like list, are iterable, but not iterators. iter([1,2,3]) returns not the list, but a value of type list_iterator. — chepner
– chepner, Commented Apr 24, 2020 at 16:07
@chepner What modifications are required to make PowerofTwo an iterable ? — user
– user, Commented Apr 24, 2020 at 16:10
You need to define __iter__, but it doesn't have to do anything other than return self. — chepner
– chepner, Commented Apr 24, 2020 at 16:10

chepner · Accepted Answer · 2020-04-24 16:19:51Z

1

Currently, PowerofTwo is an iterator, but it is not an iterable. To make it iterable, you need to define __iter__ to return an object that has a __next__ method. In this case, the instance of PowerofTwo itself will do.

class PowerofTwo:
    def __init__(self,maxi):
        self.max = maxi
        self.num = 0     

    def __iter__(self):
        return self

    def __next__(self):
        if self.num < self.max:
            result = 2 ** self.num
            self.num += 1
            return result
        else:
            raise StopIteration


for x in PowerofTwo(5):
    print(x)

outputs

It is considered good practice to always define

def __iter__(self):
    return self

in a class that defines __next__.

list is an example of a class whose __iter__ method returns an instance of another class, namely list_iterator.

edited Apr 24, 2020 at 16:19

answered Apr 24, 2020 at 16:14

chepner

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