iter()

iter(object[, sentinel])

Returns: iterator · Updated March 13, 2026 · Built-in Functions

built-in iteration iterator sentinel

The iter() function returns an iterator object from an iterable. It is the standard way to obtain an iterator in Python, converting any iterable into an object that can be traversed one element at a time using next().

Syntax

iter(object[, sentinel])

Parameters

Parameter	Type	Default	Description
object	iterable or callable	—	An iterable to convert to an iterator, or a callable to call repeatedly
sentinel	any	—	If provided, object must be a callable. iter() calls object() repeatedly until the sentinel value is returned

Examples

Basic usage with an iterable

my_list = [1, 2, 3]
my_iter = iter(my_list)

print(next(my_iter))
# 1

print(next(my_iter))
# 2

print(next(my_iter))
# 3

Using iter() with a string

text = "hello"
char_iter = iter(text)

print(next(char_iter))
# h

print(next(char_iter))
# e

Using iter() with a sentinel value

The two-argument form of iter() is powerful for creating custom iterators from callable functions:

class LineReader:
    def __init__(self, lines):
        self.lines = lines
        self.index = 0
    
    def __call__(self):
        if self.index >= len(self.lines):
            return ""
        line = self.lines[self.index]
        self.index += 1
        return line

lines = ["line 1", "line 2", ""]
reader = LineReader(lines)

for line in iter(reader, ""):
    print(line)
# line 1
# line 2

Creating an iterator from a callable

import random

def dice_roller():
    return random.randint(1, 6)

for roll in iter(dice_roller, 6):
    print(f"Rolled: {roll}")

Common Patterns

Reading file contents line by line

with open("data.txt") as f:
    for line in iter(f.readline, ""):
        if line.strip():
            print(line.strip())

This pattern reads lines until an empty string is returned.

Using iter() with class objects

class Counter:
    def __init__(self, limit):
        self.current = 0
        self.limit = limit
    
    def __iter__(self):
        return self
    
    def __next__(self):
        if self.current >= self.limit:
            raise StopIteration
        self.current += 1
        return self.current - 1

for num in iter(Counter(5)):
    print(num)
# 0
# 1
# 2
# 3
# 4

Error Handling

TypeError: object is not iterable

iter(123)
# TypeError: object is not iterable

TypeError: iter() with sentinel requires a callable

iter([1, 2, 3], 5)
# TypeError: iter() with sentinel requires a callable