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Python iter() Function

The iter() method returns an iterator for the given object.

note

An iterator object allows you to iterate over a collection of data, such as lists, tuples, dictionaries, and sets, in a memory-efficient manner.

Syntax​

iter(object, sentinel)

iter() Parameters​

Python iter() function parameters:

ParameterConditionDescription
objectRequiredAn iterable object like list, set, tuple, etc.
sentinelOptionSpecial value that is used to represent the end of a sequence

iter() Return Value​

Python iter() function returns an iterator object for the given argument until the sentinel character is found.

danger

It raises TypeError for a user-defined object that doesn't implement __iter__(), and __next__() or __getitem()__.

Examples​

Example 1: Iterating over a List​

Let's iterate over a list:

my_list = [1,  2,  3,  4,  5]
my_iter = iter(my_list)

print(next(my_iter))  # Output:  1
print(next(my_iter))  # Output:  2
print(next(my_iter))  # Output:  3

output

1
2
3

Example 2: Iterating Using a Sentinel Value​

Let's use a sentinel value to stop the iteration. Just specify the value as second parameter of the iter() function

def counter():
    count =  0
    def increase():
        nonlocal count
        count +=  1
        return count
    return increase

cnt = counter()
iterator = iter(cnt,  5)

for count in iterator:
    print(count)  # Output:  1,  2,  3,  4

output

1
2
3
4
note

The value of the sentinel parameter here is 5 so the program will stop when the value from the __next__() method is equal to this number!

Example 3: Implement an Iterable User-Defined Class​

class CounterIterator:
    def __init__(self, fn, sentinel):
        self.fn = fn
        self.sentinel = sentinel

    def __iter__(self):
        return self

    def __next__(self):
        current = self.fn()
        if current == self.sentinel:
            raise StopIteration
        return current

cnt = counter()
iterator = CounterIterator(cnt,  5)

for count in iterator:
    print(count)  # Output:  1,  2,  3,  4

output

1
2
3
4

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