Iterators in python

Iterator is a python object which can be iterated upon. Python lists, tuples, dictionaries and sets are all examples of built-in iterators. Python iterator object must implement two methods, __iter__() and __next__() .

1. __iter__ : method that is called on initialization of an iterator. This should return an object that has a next or __next__ method.

2. __next__ : The iterator next method should return the next value for the iterable. When an iterator is used with a ‘for in’ loop, the for loop implicitly calls next() on the iterator object. This method should raise a StopIteration to signal the end of the iteration.

Iterating through iterators

Now, we will use these two methods to iterate through iterator:

list_of_numbers=[1,2,3,4]
# get an iterator using iter()
iterator = iter(list_of_numbers)
## iterate through it using next()
print(next(iterator))          #prints 1
print(next(iterator))          #prints 2
print(iterator.__next__())     #prints 3
print(iterator.__next__())     #prints 4

next(iterator)

Output:
1
2
3
4
Traceback (most recent call last):
  File "/home/main.py", line 31, in <module>
    next(iterator)
StopIteration

Using iter() we can get an iterator. Using next or __next__() we are fetching the next value in the list.

Last line will result in error because there is no more data to be returned.

In simplest way, we can use for loop to print the entire list:

list_of_numbers=[1,2,3,4]
for i in list_of_numbers:
    print(i)
Output:
1
2
3
4

What does it mean? It means, for loop uses iterator internally and automatically will iterate through the list and return it.

how for loop executes internally ?

Syntax for for loop:

for element in iterable:
    # coding part

Let’s see how exactly it executes:

1. for loop will create an object from the iterable() .

iterable_obj = iter(iterable)

2. then will go to an infinite loop where it calls next() method to get the next element and executes the for loop body with this value as shown below:

while True:
    try:
        element = next(iterable_obj)   #to get the next item
        
    except StopIteration:
        # if StopIteration is raised, break from loop
        break

After all elements visited, StopIteration will be raised and loop ends.

Creating an iterator

Here, we are calculating a square of numbers by creating our own iterator. For this you have to use only two methods, __iter__() and __next__() .

class square:
    """Class to implement an iterator
    of square of a number"""
def __init__(self,x=0):
        self.x = x                   #x will be assigned as 3
def __iter__(self):
        self.n = 0                   #n is initialized to 0        
        return self
def __next__(self):
        if self.n <= self.x:         #calculate square if <=3
            result = self.n*self.n   #squaring a number
            self.n += 1              #increment number
            return result
        else:
            raise StopIteration     #if n>3, throw exception
            
a=square(3)
i = iter(a)
print(next(i))
print(next(i))
print(next(i))
print(next(i))
print(next(i))
Output:
0
1
4
9
Traceback (most recent call last):
  File "/home/main.py", line 34, in <module>
    print(next(i))
  File "/home/main.py", line 26, in __next__
    raise StopIteration
StopIteration

Here, we are creating an __iter__() function to create an iterable object and then __next__() method is called to iterate over a list and return desired result.

If all the elements in the list are covered and if you tried to call __next__() , an exception will be raised since there is no element left to return in the list.

We can use for loop for above example as follows:

class square:
    """Class to implement an iterator
    of square of a number"""
def __init__(self,x=0):
        self.x = x                   #x will be assigned as 3
def __iter__(self):
        self.n = 0                   #n is initialized to 0        
        return self
def __next__(self):
        if self.n <= self.x:         #calculate square if <=3
            result = self.n*self.n   #squaring a number
            self.n += 1              #increment number
            return result
        else:
            raise StopIteration     #if n>3, throw exception
for i in square(3):
    print(i)
Output:
0
1
4
9

That’s it!