Python – Iterators

After reading this Python Iterators topic, you will know how to implement an iterator and use a loop over an iterator in Python, you will understand iterator two methods __iter()__ and __next()__,  & stopiteration statement in Python.

In python, an iterator is an object and iterator object is generally associated with two methods as __iter()__ and __next()__. Lists, tuples, dictionaries, sets, etc. all are examples of iterable objects. The object said to be iterable when it gives iterator and for doing this the object used the method __iter()__. The __next()__ method is used to visit each item of lists, tuples, dictionaries, sets, etc. in python.

Examples:

Program (1): To show how to return an iterator from a tuple and print each item in Python.

# storing items 2,3 and 4 in the tuple
x = (2, 3, 4)
# using the method iter()
y = iter(x)
# print each item of tuple using method next()
print(next(y))
print(next(y))
print(next(y))

Output(1)

2
3
4

Program (2): To show how to return an iterator from string and print each character in Python.

# storing the string
x = "python"
# using the method iter()
y = iter(x)
# print each characer of string using method next()
print(next(y))
print(next(y))
print(next(y))
print(next(y))
print(next(y))
print(next(y))

Output(2)

p
y
t
h
o
n

Using loop over the iterator

Using loop over the iterator means we move through an iterable object and visit each item of it.

Examples:

Program (1): To show how to iterate the characters of a string and print each character in Python.

# storing the string
x = "python"
# use for loop to iterate through an iterable object as string
for i in x:
print(i)

Output(1)

p
y
t
h
o
n

Program (2): To show how to iterate the items of a tuple and print each item in Python.

# storing items 2,3 and 4 in the tuple
x = (2,3,4)
# use for loop to iterate through an iterable object as tuple
for i in x:
print(i)

Output(2)

2
3
4

How to build an iterator in Python

For building iterator as an object or class in python use two methods are given as __iter__() and __next__().

The object said to be iterable when it gives iterator and for doing this the object used the method __iter()__.

The __next()__ method is used to visit and return each item in the sequence.

Example:

Program (1): To create an iterator that returns numbers in decreasing order as 5, 4, 3, 2 and 1.

Consider the statements in python is given by

class dec_numbers:
  def __iter__(self):
    self.a = 5
    return self
  def __next__(self):
    x = self.a
    self.a -= 1
    return x
my_classname = dec_numbers()
# using the method iter() 
start_iter = iter(my_classname)
print(next(start_iter))
print(next(start_iter))
print(next(start_iter))
print(next(start_iter))
print(next(start_iter))

Output(1)

5
4
3
2
1

StopIteration statement in Python

We already discussed next() method in the topic, how to build an iterator in python. The StopIteration statement used to run next() method for a particular predefined number of iterations.

Example:

Program (1): To create an iterator that returns numbers in decreasing order as 10, 9, 8, 7, 6, 5

class dec_numbers:
def __iter__(self):
   self.a = 10
   return self
def __next__(self):
   #to stop itterations as number 5 print
   if self.a >= 5:
     x = self.a
     self.a -= 1
     return x
   else:
       raise StopIteration
my_classname = dec_numbers()
# using the method iter()
start_iter = iter(my_classname)
for x in start_iter:
print(x)

Output(1)

10
9
8
7
6
5