An Iterator in Python is an Object which contains a countable number of values or elements. You can use this Python iterator to Traverse all those elements.
The Python iterator uses the special methods called __iter__() and __next__() methods to iterate the object elements.
Python Iterables vs Iterator
If we can get an iterator from it, then we call that object as iterable. So far, we have seen the iterables called Lists, Tuples, Sets, and Dictionaries. From these objects, you can call iterator or Iter.
This section explains how to use the Iterator in Python and create our own iterators and infinity iterators with practical examples.
Python String Iterator Example
In python, strings are iterable objects. Here, we use for loop to iterate each character in a string and returns that character.
By default, or say, For loop implements the Iterator concept without your knowledge.
# Python Iterator Example string = 'tutorialgateway' for x in string: print(x, end = ' ')
Python Iterators For loop Example 2
In this example, we used both for loop and iterators. In python, we have to use the next() function to return an element or value from an iterator.
Remember, the Python next() function returns one item at a time. So, for n number of items, you have to use n next() statements. Here, we used the next() for one time. So, it returns the first character from a string.
string = 'tutorialgateway'
for x in string:
print(x, end = ' ')
print("\n**** Iterator Example *****")
itr = iter(string)
print(next(itr))
Let me use next() to iterate all the characters in a string. This time, it writes the completed string.
string = 'tutorialgateway'
for x in string:
print(x, end = ' ')
print("\n**** Iterator Example *****")
itr = iter(string)
print(next(itr))
print(next(itr))
print(next(itr))
print(next(itr))
print(next(itr))
print(next(itr))
print(next(itr))
print(next(itr))
print(next(itr))
print(next(itr))
print(next(itr))
print(next(itr))
print(next(itr))
print(next(itr))
print(next(itr))
Python List Iterator
In this example, we show how to iterate List items and prints them.
numbers = [10, 20, 30, 40, 50] itr = iter(numbers) print(next(itr)) print(next(itr)) print(next(itr)) print(next(itr)) print(next(itr))
In this list iterator example, we used the __next__() special method. If you observe the last statement, we are trying to return the next element that doesn’t exist. That’s why Python Iterator is calling StopIteration.
numbers = [10, 20, 30, 40, 50] itr = iter(numbers) print(itr.__next__()) print(itr.__next__()) print(itr.__next__()) print(itr.__next__()) print(itr.__next__()) print(itr.__next__())
Python Set Iterator
You can use the Iterator to iterate the set items too. This program iterates the numbers in a Set and prints each one of them.
mySet = {1, 2, 3, 4, 5}
ittr = iter(mySet)
print(ittr.__next__())
print(ittr.__next__())
print(ittr.__next__())
print(ittr.__next__())
print(ittr.__next__())
Tuple Iterator
Python also allows you to use this on Tuples as well. This program iterates Tuple items and prints each item in a tuple.
fruits = ('Apple', 'Orange', 'Grape', 'Banana', 'Kiwi')
fruit_itr = iter(fruits)
print(next(fruit_itr))
print(next(fruit_itr))
print(next(fruit_itr))
print(next(fruit_itr))
print(next(fruit_itr))
Create Own Iterator in Python
It is a simple class called Counter. First, we declared a maximum value within the __init__ method.
The __iter__ method is to initialize the value of an iterable object. For now, we set the number to 0. It means Iterator starts at 0.
The __next__ method is to select the next element from an Object. Within this method, we used the If Else Statement to check whether the next number is greater than the maximum value or not. If True, StopIteration error raised. Otherwise, the number incremented.
class Counter: def __init__(self, maximum): self.maximum = maximum def __iter__(self): self.number = 0 return self def __next__(self): number = self.number if number > self.maximum: raise StopIteration else: self.number = number + 1 return number numbers = Counter(10) a = iter(numbers) print(next(a)) print(next(a)) print(next(a))
Python Own Iterator Example 2
The above iterator code is displaying numbers from 0 to 2. It is because we used next(a) for three times only.
In this program, we used for loop to iterate the Counter class where the maximum value is 10. This prints the values from 0 to 10. __iter__() starts at 0, and __next__() print elements up to 10.
class Counter: def __init__(self, maximum): self.maximum = maximum def __iter__(self): self.number = 0 return self def __next__(self): number = self.number if number > self.maximum: raise StopIteration else: self.number = number + 1 return number for t in Counter(10): print(t)
Python Infinity Iterator
While you create your own iterator, you should always be careful with Infinity loops. If you forget to raise the StopIterator error, then you end up in infinity loops.
It is a simple example of an infinity iterator. It is the same as above, but we removed the If statement.
class Counter: def __iter__(self): self.number = 0 return self def __next__(self): number = self.number self.number = number + 1 return number numbers = Counter() a = iter(numbers) print(next(a)) print(next(a)) print(next(a))
Although the above example is an infinite iterator, you might not have noticed it. It is because we used the next() for three times only. So, let me use for loop to an iterator the Counter class.
class Counter: def __iter__(self): self.number = 0 return self def __next__(self): number = self.number self.number = number + 1 return number for t in Counter(): print(t)
Now you can see the Python infinity iterator. This iterator executes continuously. Please refer to Python Dictionary article.