Python iter() Built-in Function

Python iter() creates an iterator from an iterable object (like a list or string), letting you access elements individually. It is often paired with next() to fetch items sequentially, which is useful for loops or custom iteration logic. Python iter() simplifies handling large datasets efficiently by processing elements on demand.

Table of Contents
Understanding Python iter() Function iter() Parameters iter() Return Value Python iter() for custom objects Python iter() with Sentinel Parameter	Python iter() with Callable Python iter() with Dictionary Python iter() with List Python iter() with Tuple

Understanding Python `iter()` Function

The iter() function is a fundamental part of working with iterators. It is used to obtain an iterator from an object. An iterator is an object that allows you to traverse through a sequence of values one at a time. Python iter() either takes a single argument, which should be an iterable object (like a list, tuple, or string), or it takes two arguments: a callable object and a sentinel value.

Syntax of Python `iter()`

iterator_object = iter(iterable)
# OR
iterator_object = iter(callable, sentinel)

Explanation

iterator_object: This variable will hold the iterator.
iter(): Built-in function to get an iterator.
iterable: Object that supports iteration (e.g., list, tuple, string).
callable: A function or other callable object (in the two-argument form).
sentinel: A value that signals the end of iteration (in the two-argument form).

Example of Python `iter()`

my_list = [1, 2, 3]
my_iterator = iter(my_list)
print(next(my_iterator))
print(next(my_iterator))

Explanation

my_list = [1, 2, 3]: Creates a list.
my_iterator = iter(my_list): Gets iterator from the list.
print(next(my_iterator)): Gets and prints the next item (1).
print(next(my_iterator)): Gets and prints the next item (2).

Output

1
2

`iter()` Parameters

Python iter() function has two distinct forms, each with different parameters. The first form takes a single parameter, an object, which must be iterable (something that supports iteration, like a list, tuple, string, set, or a custom object that implements the iterator protocol). Iter() takes two parameters in the second form: callable and sentinel. The callable must be a callable object (usually a function) that takes no arguments. The sentinel is a specific value; when callable returns this sentinel value, iteration stops.

Syntax

# Single-argument form
iterator = iter(iterable_object)

# Two-argument form
iterator = iter(callable_object, sentinel_value)

Explanation

iterator: Stores the resulting iterator object.
iter(): Obtains the iterator.
iterable_object: Must support iteration (single-argument form).
callable_object: A function or callable (two-argument form).
sentinel_value: Value that stops iteration (two-argument form).

Example

# Two-argument form example

class Counter:
    def __init__(self, limit):
        self.limit = limit
        self.current = 0

    def __call__(self):
        if self.current < self.limit:
            self.current += 1
            return self.current
        else:
            return -1  # Sentinel value

counter = Counter(3)
counter_iter = iter(counter, -1)

for val in counter_iter:
    print(val)

Explanation

class Counter:: Creates a custom class.
def __init__(self, limit):: Constructor.
self.limit = limit and self.current = 0: Initializes attributes.
def __call__(self):: Makes instances callable.
if self.current < self.limit:: Checks current value is smaller than limit.
self.current += 1 and return self.current: Increment and return if smaller than limit.
return -1: Sentinel value to stop.
counter = Counter(3): Creates object.
counter_iter = iter(counter, -1): Creates iterator using two-argument form.
for val in counter_iter:: Iterate and print the value.

Output

1
2
3

`iter()` Return Value

Python iter() function always returns an iterator object. This iterator is what you use to step through the values one by one, typically using the next() function. If you call iter() on an object that is not iterable (in the single-argument form) and does not have __iter__ method, you’ll get a TypeError. In the two-argument form, the iterator keeps calling the callable object until it returns the sentinel value.

Syntax

iterator = iter(something)

Explanation

iterator: Stores the returned iterator.
iter(): Creates or obtains iterator.
something: Either iterable or a callable and sentinel pair.

Example

my_string = "abc"
string_iterator = iter(my_string)
print(type(string_iterator))

Explanation

my_string = "abc": Creates a string (which is iterable).
string_iterator = iter(my_string): Gets an iterator for the string.
print(type(string_iterator)): Prints the type of the iterator.

Output

Python `iter()` for custom objects

You can make your classes iterable by implementing the iterator protocol. This involves defining two special methods: __iter__() and __next__(). The __iter__() method should return the iterator object (usually self), and the __next__() method should return the next value in the sequence. When no more values exist, __next__() should raise the StopIteration exception. When you call Python iter() on an instance of such a class, it automatically calls the __iter__() method.

Syntax

class MyIterable:
    def __iter__(self):
        # Return the iterator object (usually self)
        pass

    def __next__(self):
        # Return the next value, or raise StopIteration
        pass

Explanation

class MyIterable: Defines a custom iterable class.
def __iter__(self):: Returns the iterator object.
pass: Placeholder for __iter__ implementation.
def __next__(self):: Returns the next value or raises StopIteration.
pass: Placeholder for __next__ implementation.

Example

class MySequence:
    def __init__(self, data):
        self.data = data
        self.index = 0

    def __iter__(self):
        return self

    def __next__(self):
        if self.index < len(self.data):
            value = self.data[self.index]
            self.index += 1
            return value
        else:
            raise StopIteration

my_seq = MySequence([4, 5, 6])
my_iter = iter(my_seq)
print(next(my_iter))
print(next(my_iter))
print(next(my_iter))

Explanation

class MySequence:: Defines custom class.
def __init__(self, data):: Initializer.
self.data = data: Stores data in the instance.
self.index = 0: Initializes an index for iteration.
def __iter__(self):: Makes the class iterable.
return self: Return the object itself.
def __next__(self):: Defines how to get the next item.
if self.index < len(self.data):: Check the index is within bounds.
value = self.data[self.index]: Retrieves value at current index.
self.index += 1: Increases the index.
return value: Returns the value.
raise StopIteration: Signals end of iteration.
my_seq = MySequence([4, 5, 6]): Creates a MySequence object.
my_iter = iter(my_seq): Gets an iterator from my_seq.
print(next(my_iter)): Prints next value.
print(next(my_iter)): Prints next value.
print(next(my_iter)): Prints next value.

Output

4
5
6

Python `iter()` with Sentinel Parameter

The two-argument form of Python iter() provides a powerful way to create iterators from callable objects (like functions). In this form, iter(callable, sentinel), the callable is called repeatedly without arguments. Each time the callable returns a value, the iterator yields that value. When the callable returns the sentinel value, the iteration stops (a StopIteration exception is raised internally, ending the loop).

Syntax

iterator = iter(callable_object, sentinel_value)

Explanation

iterator: Variable that will hold iterator.
iter(): Creates iterator with a callable and a sentinel.
callable_object: A function (or other callable) that takes no arguments.
sentinel_value: Iteration stops when callable_object returns this value.

Example

import random

def random_until_five():
    return random.randint(1, 10)

random_iter = iter(random_until_five, 5)

for num in random_iter:
    print(num)

Explanation

import random: Imports random module.
def random_until_five():: Function that returns random integer between 1 and 10.
return random.randint(1, 10): Generates a random number.
random_iter = iter(random_until_five, 5): Creates an iterator which calls random function until 5 returned.
for num in random_iter:: Loops through the iterator.
print(num): Prints values.

Output

8
1
10
6
9
2

Python `iter()` with Callable

Using the callable parameter of the iter() function, the function continuously calls the method and returns the value until the sentinel value matches.

Syntax

iterator = iter(callable, sentinel)

Explanation

iterator: Variable which holds iterator.
iter(): Method to get the iterator.
callable: Method or object which needed to be called.
sentinel: Value, when callable returns this value, the loop will be terminated.

Example

class MyCounter:
  def __init__(self, limit):
    self.current_value = 0
    self.limit = limit
  def __call__(self):
    if(self.current_value < self.limit):
      self.current_value += 1
      return self.current_value
    return -1

my_counter = MyCounter(5)
for i in iter(my_counter, -1):
    print (i)

Explanation

class MyCounter: Creates a custom class.
def __init__(self, limit): Constructor to initilize instance variable.
self.current_value = 0 and self.limit = limit: Assign default values.
def __call__(self):: Method to make the class instance callable.
if(self.current_value < self.limit):: Check current value.
self.current_value += 1: Increment the value.
return self.current_value: Return incremented value.
return -1: Return sentinel value.
my_counter = MyCounter(5): Creates an instance of the class.
for i in iter(my_counter, -1):: Calls iter method.
print(i): Prints the returned value.

Output

1
2
3
4
5

Python `iter()` with Dictionary

When you use the iter() with a dictionary, it returns iterator of the keys.

Syntax

dictionary_iterator = iter(dictionary)

Explanation

dictionary_iterator: Variable to hold returned iterator.
iter(): To get the iterator.
dictionary: The dictionary object.

Example

dictionary = {"a":1, "b":2, "c":3}
for key in iter(dictionary):
  print(key)

Explanation

dictionary = {"a":1, "b":2, "c":3}: Define dictionary.
for key in iter(dictionary):: Iterate through the keys of the dictionary.
print(key): Prints the current key.

Output

a
b
c

Python `iter()` with List

When you use iter() with a list, it returns iterator of the list elements.

Syntax

list_iterator = iter(list)

Explanation

list_iterator: Variable to hold the iterator.
iter(): To get the iterator.
list: List variable.

Example

list1 = [1,2,3]
for element in iter(list1):
  print(element)

Explanation

list1 = [1,2,3]: Initialize list.
for element in iter(list1):: Get the iterator and iterate through it.
print(element): Prints the current element.

Output

1
2
3

Python `iter()` with Tuple

You can use iter() method with Tuple. It will return the iterator of the tuple elements.

Syntax

tuple_iterator = iter(tuple)

Explanation

tuple_iterator: Variable to store returned iterator.
iter(): To get the iterator.
tuple: Tuple variable.

Example

tuple1 = (4,5,6)
iterator = iter(tuple1)
print (next(iterator))
print (next(iterator))
print (next(iterator))

Explanation

tuple1 = (4,5,6): Assign tuple.
iterator = iter(tuple1): Get iterator from the tuple.
print (next(iterator)): Prints the first element using next().
print (next(iterator)): Prints the second element.
print (next(iterator)): Prints the third element.

Output

4
5
6

Conclusion

Python iter() function is used to get an iterator from an object. iter() can take either one or two arguments. In the one-argument form, it takes an iterable object and returns an iterator for that object. In the two-argument form, iter(callable, sentinel), it repeatedly calls the callable (without arguments) until the callable returns the sentinel value.

Also Read

Python issubclass ()

Python list()

Python Reference

Python iter()

Understanding Python iter() Function

Syntax of Python iter()

Example of Python iter()

iter() Parameters

iter() Return Value

Python iter() for custom objects

Python iter() with Sentinel Parameter

Python iter() with Callable

Python iter() with Dictionary

Python iter() with List

Python iter() with Tuple

Conclusion

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All Tutorial Topics

Understanding Python `iter()` Function

Syntax of Python `iter()`

Example of Python `iter()`

`iter()` Parameters

`iter()` Return Value

Python `iter()` for custom objects

Python `iter()` with Sentinel Parameter

Python `iter()` with Callable

Python `iter()` with Dictionary

Python `iter()` with List

Python `iter()` with Tuple