Python Iterator

Difference Between Iterator and Iterable

Understanding the difference between iterators and iterables is essential:

This table highlights the main differences and usage of iterators and iterables in Python.

Example

# Iterable
my_list = [1, 2, 3]

# Iterator
iterator = iter(my_list)

print(next(iterator))  # Works, output: 1
print(next(my_list))   # Error: 'list' object is not an iterator

You can also try this code with Online Python Compiler

Run Code

Explanation:
my_list is an iterable because it can be converted into an iterator using iter(). However, it is not an iterator itself because it lacks the __next__() method.

Python iter() Example

The iter() function is used to convert an iterable into an iterator. Let’s see an example:

# Using iter() on a string
my_string = "Hello"
string_iterator = iter(my_string)
print(next(string_iterator)) 
print(next(string_iterator))  
print(next(string_iterator)) 

You can also try this code with Online Python Compiler

Run Code

Output

H
e
I

Explanation:

1. The string my_string is an iterable.
    
2. The iter() function converts it into an iterator.
    
3. Using next(), we can retrieve each character one by one.
    

This functionality is particularly useful in loops, where iteration happens automatically. For instance:

# Using iter() in a loop
my_list = [10, 20, 30]
for item in iter(my_list):
    print(item)

You can also try this code with Online Python Compiler

Run Code

Output:

10
20
30

Creating an Iterator

In Python, you can create an iterator by defining a class that implements two key methods: __iter__() and __next__(). These methods allow the object to be iterated over, enabling it to produce a sequence of values one at a time.

__iter__() Method: This method returns the iterator object itself. It is necessary because Python expects the object to be iterable.
 

__next__() Method: This method is used to get the next item in the sequence. Once all items have been accessed, it should raise the StopIteration exception to signal the end of the sequence.

Example: Creating a Number Iterator

class MyIterator:
    def __init__(self, start, end):
        self.current = start
        self.end = end

    def __iter__(self):
        return self

    def __next__(self):
        if self.current > self.end:
            raise StopIteration
        else:
            self.current += 1
            return self.current - 1

# Using the iterator
my_iter = MyIterator(1, 5)

for num in my_iter:
    print(num)

You can also try this code with Online Python Compiler

Run Code

Output:

1
2
3
4
5

Explanation:

• The iterator starts at 1 and goes up to 5 (inclusive).
   
• The __next__() method increases the current value by 1 each time it's called and returns the previous value.
   
• Once the current value exceeds 5, the StopIteration exception is raised, stopping the loop.
   

Thus, the numbers 1, 2, 3, 4, and 5 are printed during the loop.

StopIteration Exception

In Python, the StopIteration exception is used to signal the end of an iteration. When you are using an iterator, the __next__() method is called to get the next item in a sequence. Once all items have been retrieved, Python raises the StopIteration exception to let you know there are no more items to process.

This is important because it prevents errors when trying to access items that do not exist. For example, when using a loop to iterate over a list, the loop will stop automatically when it reaches the end, thanks to the StopIteration exception.

Example

my_list = [1, 2, 3]
iterator = iter(my_list)
try:
    print(next(iterator))  
    print(next(iterator))  
    print(next(iterator)) 
    print(next(iterator)) 
except StopIteration:
    print("Iteration is complete.")

You can also try this code with Online Python Compiler

Run Code

Output

1
2
3

Key Points

• StopIteration is automatically handled in a for loop, making it a convenient tool for iteration.
   
• If you manually handle iterators, you need to account for this exception to prevent errors.

Practical Uses of Iterators

Reading Files:
Iterators are used to read large files line by line, avoiding memory overload.

with open('example.txt', 'r') as file:
    for line in iter(file.readline, ''):
        print(line.strip())

Infinite Streams:
Iterators can create infinite data streams, such as generating Fibonacci numbers.

Frequently Asked Questions

What is the difference between iter() and next()?

• iter() converts an iterable into an iterator.
   
• next() retrieves the next item from an iterator.

Why use custom iterators?

Custom iterators allow you to define unique ways to traverse data, such as generating sequences or processing elements dynamically.

What happens if StopIteration is not handled?

If StopIteration is not handled in a manual iteration, your program will throw an error. However, it is automatically managed in loops like for.

Conclusion

In this article, we discussed the concept of iterators in Python, their relationship with iterables, and how to use the iter() function. You also learned how to create custom iterators and handle the StopIteration exception. By mastering iterators, you can work efficiently with data streams and create more powerful, memory-efficient Python programs.

Recommended Readings:

• Itertools in Python
• Python Generator
• Iterate over a list in Python
• Iterators and Generators in Python