Python Iterable and Iterator

As you know everything in python is an object. Python has a special protocol defined for iterating through objects. This concept of protocol is a set of standard defined so as to call an object an iterator (which can be iterated using python standard defined next() method or using loops).

In Python, iterator is any object which can be iterated using standard next() method or using any loop which internally calls next() method. Any custom class defined can be made iterator by defining next()  method(in python 2.7+) or __next__()  method (in python 3.4+) .

Next() method uses class’s internal variable to track the current position of index value to be returned when next() method is called on the class’s object.

class MyClass1(object):

    lst = []
    index = 0

    def __init__(self):
        self.lst = []

    def __init__(self,l):
        self.lst.extend(l)


    def next(self):
        try:
            self.index = self.index+1
            if self.index>=len(self.lst):
                raise StopIteration
            else:
                return self.lst[self.index-1]
        except:
            print("error")
   
b = MyClass1([1,2,4])

try:
    print(next(b))

    print(next(b))
except:
    print('b is not yet iterator')

Output:

1

2

However python’s list, sets etc are not an iterator. But it still allows to loop through them. This is because these are iterables which can be converted to iterators by calling python’s standard iter() method and passing the object for list or sets through it. Internally list and set function implements __iter__() method. Calling standard iter() method internally calls __iter__() method of the class which returns an iterator object.

class MyClass1(object):

    lst = []
    index = 0

    def __init__(self):

        self.lst = []

    def __init__(self,l):

        self.lst.extend(l)


    def __iter__(self):

        return iter(self.lst)

    def next(self):

        try:
            self.index = self.index+1
            if self.index>=len(self.lst):
                raise StopIteration
            else:
                return self.lst[self.index-1]
        except:
            print("error")



b = MyClass1([1,2,4])



try:
    x = iter(b) 
    print(next(x))
    print(next(x))
except:
    print('b is not yet iterable')

Output:

1

2