Python iter() Function

The iter() function in Python is used to create an iterator from an iterable (like a list, tuple, or string). It plays a key role in enabling for-loops, comprehensions, and lazy evaluation.

Syntax

iter(iterable)

or

iter(callable, sentinel)

Parameters:

• iterable – Any object capable of returning its elements one at a time (like a list, string, tuple, etc.)
• callable, sentinel – An alternate form used to create an iterator from a function until a specific value (sentinel) is returned.

Returns:

• An iterator object.

Example: Using iter() with a List

my_list = [10, 20, 30]
it = iter(my_list)

print(next(it))  # 10
print(next(it))  # 20
print(next(it))  # 30

10
20
30

Example: Looping with iter()

words = ["hello", "world"]
it = iter(words)

for word in it:
    print(word)

hello
world

Example: Using iter() with Callable and Sentinel

# Read input until the user types 'stop'
def get_input():
    return input("Enter value: ")

for val in iter(get_input, 'stop'):
    print("You entered:", val)

Explanation: This form of iter() calls the function repeatedly until it returns the sentinel value.

Use Case: Why Use iter()?

• Used behind the scenes in for loops
• Manual control over iteration using next()
• Custom data streaming or lazy evaluation

Common Mistakes

• Trying to use next() on an object that is not an iterator
• Forgetting that iter() does not copy the object; it returns a pointer-like object

Interview Tip

When asked about iterables vs iterators, remember: iter() converts an iterable into an iterator, which then maintains internal state across next() calls.

Summary

• iter() is used to create an iterator from an iterable or callable
• Works with lists, strings, tuples, dictionaries, and more
• Returns an object compatible with next()

Practice Problem

Create your own list of numbers and use iter() with next() to print each number one-by-one.

nums = [1, 2, 3, 4]
it = iter(nums)

while True:
    try:
        print(next(it))
    except StopIteration:
        break