Python iter() Function

In Python, the iter() function has two core purposes: first, to obtain an iterator from an iterable object, and second, to dynamically generate an iterator based on a callable object.

Function Syntax

iter(iterable)
iter(callable, sentinel)

iter() has two usage patterns. With one parameter:

• iterable: An iterable object.

Returns an iterator for this iterable object.

If an object is iterable, it implements the __iter__() method, which returns an iterator. The iterator returns elements one by one through the __next__() method and raises a StopIteration exception when exhausted.

With two parameters:

• callable: A function or callable object.
• sentinel: Stops iteration when the callable returns this value.

With two parameters, iter() creates a new iterator. When the iterator's __next__() method is called, it invokes callable() and checks if the return value equals sentinel. If it does, StopIteration is raised; otherwise, the value is returned.

iter() Function Examples

This example demonstrates how iterators work:

class MyRange:
    def __init__(self, start, stop):
        self.current = start
        self.stop = stop

    def __iter__(self):
        return self
    
    def __next__(self):
        if self.current >= self.stop:
            raise StopIteration

        value = self.current
        self.current += 1
        return value

# Use for loop to iterate
for num in MyRange(1, 5):
    print(num)

Simple usage of iter() function:

# Define a list
my_list = [1, 2, 3]

# Get iterator for my_list
# Actually calls my_list.__iter__()
my_iterator = iter(my_list)

# Get next values from iterator sequentially
# Actually calls my_iterator.__next__()
print(next(my_iterator))  # Output: 1
print(next(my_iterator))  # Output: 2
print(next(my_iterator))  # Output: 3

# Getting next value again raises StopIteration
print(next(my_iterator))  # Raises StopIteration exception

for loops internally use iter():

lst = [1, 2, 3]

for item in lst:
    print(item)

# Equivalent to
lst_iter = iter(lst)
while True:
    try:
        item = next(lst_iter)
        print(item)
    except StopIteration:
        break

Using the iter(callable, sentinel) form:

import random

# Returns a random number between 0 and 100
def get_number():
    return random.randint(0, 100)

# Create an iterator
# Each iteration value is generated by get_number
# Stops iteration when generated value is 0
num_iter = iter(get_number, 0)

for num in num_iter:
    print(num)