Implementing iter()

Lesson 4 of 13 in Coddy's Python Iterators course.

Creating a custom iterator in Python involves implementing the __iter__() method in a class. This method is a crucial part of the iterator protocol and allows you to define how your custom object behaves as an iterator.

The iter() Method

The __iter__() method has two main purposes:

It makes an object iterable.
It returns the iterator object itself.

Here's the basic structure of a class with an __iter__() method:

class MyIterator:
    def __init__(self, data):
        self.data = data
        self.index = 0

    def __iter__(self):
        return self

Implementing iter()

To implement __iter__() in your custom class:

Define the __iter__() method within your class.
The method should return the iterator object itself (usually self).

Here's an example of a simple custom iterator:

class CountDown:
    def __init__(self, start):
        self.start = start

    def __iter__(self):
        # Return the iterator object (self)
        return self

    def __next__(self):
        if self.start <= 0:
            raise StopIteration
        self.start -= 1
        return self.start + 1

# Using the custom iterator
countdown = CountDown(5)
for num in countdown:
    print(num)

In this example, the __iter__() method returns self, making the CountDown object both an iterable and its own iterator.

Key Points

The __iter__() method is called when you use the iter() function on an object or when you use it in a for loop.
Returning self in __iter__() is common when the class implements both __iter__() and __next__().
If your class only implements __iter__(), it should return a separate iterator object that has a __next__() method.

By implementing __iter__(), you make your custom objects iterable, allowing them to be used in for loops and with other iterator-based operations in Python.

Test yourself

This lesson includes a short quiz. Start the lesson to answer it and track your progress.

Test yourself

This lesson includes a short quiz. Start the lesson to answer it and track your progress.

Test yourself

This lesson includes a short quiz. Start the lesson to answer it and track your progress.

Challenge

Easy

Create a custom iterator class called AlphabetIterator that generates uppercase letters of the English alphabet. The iterator should start from 'A' and continue up to a specified letter.

You are provided with the following:

A single uppercase letter as input (e.g., 'E', 'K', 'Z')

Your AlphabetIterator class should:

Implement the __iter__() method to make the object iterable
Generate uppercase letters starting from 'A'
Stop when it reaches the specified input letter (inclusive)

After implementing the AlphabetIterator class, create an instance of it using the input letter, and print each generated letter on a new line.

The input will be provided as a string containing a single uppercase letter.

Try it yourself

# Read input
end_letter = input().strip()

class AlphabetIterator:
    def __init__(self, end_letter):
        self.end_letter = end_letter
        self.current_letter = 'A'

    def __iter__(self):
        return self

    # TODO: Implement the __next__() method

# Create an instance of AlphabetIterator
alphabet_iter = AlphabetIterator(end_letter)

# TODO: Iterate through the AlphabetIterator and print each letter

# Note: Make sure to print each letter on a new line