What is iter and next in Python: Simple Explanation and Example

iter creates an iterator from an iterable object, allowing you to loop through its items one by one. next retrieves the next item from that iterator, raising an error if there are no more items.

How It Works

Imagine you have a book and you want to read it page by page. The iter function is like opening the book and preparing it so you can read one page at a time. It turns a collection (like a list or a string) into an iterator, which remembers where you are.

The next function is like turning to the next page in the book. Each time you call next, you get the next item from the iterator. When you reach the end of the book, next will tell you there are no more pages by raising a special error called StopIteration.

Example

This example shows how to use iter and next to go through a list of fruits one by one.

python

fruits = ['apple', 'banana', 'cherry']
iterator = iter(fruits)

print(next(iterator))  # apple
print(next(iterator))  # banana
print(next(iterator))  # cherry

# Uncommenting the next line will raise StopIteration error
# print(next(iterator))

Output

apple
banana
cherry

When to Use

Use iter and next when you want to manually control looping through items, especially when you don't want to use a for loop. This is helpful when you need to pause and resume reading items, or when you want to handle the end of the sequence yourself.

For example, reading lines from a file one by one, processing data streams, or implementing custom loops where you decide when to stop.

Key Points

• iter turns an iterable into an iterator that keeps track of the current position.
• next gets the next item from the iterator.
• When no items remain, next raises StopIteration.
• These functions give you fine control over looping.

Key Takeaways

iter creates an iterator from any iterable object.

next retrieves the next item from the iterator or raises StopIteration if done.

Use them to manually control looping through data.

They are useful for reading data step-by-step or pausing loops.

Always handle StopIteration to avoid errors when using next.