Iterators in Python

Iterator in python is any python type that can be used with a ‘for in loop’. Python lists, tuples, dicts and sets are all examples of inbuilt iterators. These types are iterators because they implement following methods. In fact, any object that wants to be an iterator must implement following methods.

__iter__ method that is called on initialization of an iterator. This should return an object that has a next or __next__ (in Python 3) method.
next ( __next__ in Python 3) 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.

Below is a simple Python program that creates iterator type that iterates from 10 to given limit. For example, if limit is 15, then it prints 10 11 12 13 14 15. And if limit is 5, then it prints nothing.

# A simple Python program to demonstrate 
# working of iterators using an example type 
# that iterates from 10 to given value 
  
# An iterable user defined type 
class Test: 
  
    # Cosntructor 
    def __init__(self, limit): 
        self.limit = limit 
  
    # Called when iteration is initialized 
    def __iter__(self): 
        self.x = 10
        return self
  
    # To move to next element. In Python 3, 
    # we should replace next with __next__ 
    def next(self): 
  
        # Store current value ofx 
        x = self.x 
  
        # Stop iteration if limit is reached 
        if x > self.limit: 
            raise StopIteration 
  
        # Else increment and return old value 
        self.x = x + 1; 
        return x 
  
# Prints numbers from 10 to 15 
for i in Test(15): 
    print(i) 
  
# Prints nothing 
for i in Test(5): 
    print(i) 

Output :

Examples of inbuilt iterator types.

# Sample built-in iterators 
  
# Iterating over a list 
print("List Iteration") 
l = ["geeks", "for", "geeks"] 
for i in l: 
    print(i) 
      
# Iterating over a tuple (immutable) 
print("
Tuple Iteration") 
t = ("geeks", "for", "geeks") 
for i in t: 
    print(i) 
      
# Iterating over a String 
print("
String Iteration")     
s = "Geeks"
for i in s : 
    print(i) 
      
# Iterating over dictionary 
print("
Dictionary Iteration")    
d = dict()  
d['xyz'] = 123
d['abc'] = 345
for i in d : 
    print("%s  %d" %(i, d[i])) 

Output :

List Iteration
geeks
for
geeks

Tuple Iteration
geeks
for
geeks

String Iteration
G
e
e
k
s

Dictionary Iteration
xyz  123
abc  345

Generators in Python

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