Iterators

Iterators

An iterator is an object in Python that allows you to loop through a collection (like lists, tuples, etc.) one element at a time.

Two key methods:

• __iter__() → returns the iterator object itself.
• __next__() → returns the next value from the collection.

Example using a built-in iterator (like a list)

# A list (iterable)
numbers = [1, 2, 3]

# Getting an iterator from the list
my_iter = iter(numbers)

# Accessing elements one by one using next()
print(next(my_iter))  # Output: 1
print(next(my_iter))  # Output: 2
print(next(my_iter))  # Output: 3
# print(next(my_iter))  # Will raise StopIteration error if uncommented

Explanation:

Step 1:

# A list (iterable)
numbers = [1, 2, 3]

• numbers is a list, which is an iterable (can be looped through).
• It contains three elements: 1, 2, and 3.

Step 2:

# Getting an iterator from the list
my_iter = iter(numbers)

• We use the iter() function to convert the list into an iterator object.
• my_iter is now an iterator, and it remembers its current position.

Step 3:

# Accessing elements one by one using next()
print(next(my_iter))  # Output: 1
print(next(my_iter))  # Output: 2
print(next(my_iter))  # Output: 3

The next() function gets the next value from the iterator:

• 1st call → returns 1
• 2nd call → returns 2
• 3rd call → returns 3

Creating a Custom Iterator:

Let’s create our own iterator that returns numbers from 1 to 5.

class CountToFive:
    def __iter__(self):
        self.num = 1
        return self

    def __next__(self):
        if self.num <= 5:
            val = self.num
            self.num += 1
            return val
        else:
            raise StopIteration

# Using the custom iterator
counter = CountToFive()
for i in counter:
    print(i)

Output:

1
2
3
4
5

Explanation:

Step1: Class Definition

class CountToFive:

This defines a class named CountToFive. This class will behave like an iterator.

Step 2: __iter__() Method

    def __iter__(self):
        self.num = 1
        return self

• This method is called when iteration starts (e.g., in a for loop).
• It initializes the counter self.num to 1.
• It returns the object itself, which is also an iterator.

Step 3:__next__() Method

    def __next__(self):
        if self.num <= 5:
            val = self.num
            self.num += 1
            return val
        else:
            raise StopIteration

• This method returns the next value in the sequence each time it is called.
• If num is less than or equal to 5:
  • It stores the current value in val.
  • Increments num by 1.
  • Returns val

• If num is greater than 5, it raises StopIteration to signal that the iteration is finished.

Step 4: Using the Iterartor

counter = CountToFive()
for i in counter:
    print(i)

• counter = CountToFive() creates an instance of the class.
• for i in counter: starts a loop:
  • It automatically calls __iter__() to get the iterator.
  • Then it keeps calling __next__() to get values until StopIteration is raised.

✅ Real-life Analogy:

Imagine a TV remote (iterator) going through channels (iterable).
You press next to go to the next channel.
Once you’re on the last channel, pressing next shows nothing or error — just like StopIteration.