Python iter()

The iter() function creates an object which can be iterated one element at a time.

These objects are useful when coupled with loops like for loop, while loop.

The syntax of the iter() function is:

iter(object, sentinel)

iter() Parameters

The iter() function takes two parameters:

• object - object whose iterator has to be created (can be sets, tuples, etc.)
• sentinel (optional) - special value that is used to represent the end of a sequence

Return value from iter()

• The iter() function returns an iterator object for the given object.
• If the user-defined object doesn't implement __iter__(), and __next__() or __getitem()__, the TypeError exception is raised.
• If the sentinel parameter is also provided, iter() returns an iterator until the sentinel character isn't found.

Example 1: Working of Python iter()

# list of vowels
vowels = ['a', 'e', 'i', 'o', 'u']
vowels_iter = iter(vowels)

print(next(vowels_iter))    # 'a'
print(next(vowels_iter))    # 'e'
print(next(vowels_iter))    # 'i'
print(next(vowels_iter))    # 'o'
print(next(vowels_iter))    # 'u'

Output

a
e
i
o
u

Example 2: iter() for custom objects

class PrintNumber:
    def __init__(self, max):
        self.max = max

    def __iter__(self):
        self.num = 0
        return self

    def __next__(self):
        if(self.num >= self.max):
            raise StopIteration
        self.num += 1
        return self.num

print_num = PrintNumber(3)

print_num_iter = iter(print_num)
print(next(print_num_iter))  # 1
print(next(print_num_iter))  # 2
print(next(print_num_iter))  # 3

# raises StopIteration
print(next(print_num_iter))

Output

1
2
3
Traceback (most recent call last):
  File "", line 23, in 
File "", line 11, in __next__
StopIteration

Example 3: iter() with sentinel parameter

class DoubleIt:

    def __init__(self):
        self.start = 1

    def __iter__(self):
        return self

    def __next__(self):
        self.start *= 2
        return self.start

    __call__ = __next__
    
my_iter = iter(DoubleIt(), 16)

for x in my_iter:
    print(x)

Output

2
4
8

Here, we have implemented a custom iterable object without a StopIteration condition.

However, we can use the iter() method with the sentinel parameter to stop the iteration. If the value returned from __next__() is equal to sentinel, StopIteration will be raised, otherwise, the value will be returned.

Recommended Reading: Python Iterators