Python iter()

The python iter() function helps to return an iterator. An iterator is an object which means it contains a number of values that can be countable. An iterator can traverse through all the values. These objects are useful with loops like for loop, while loop etc. The iter() object iterates one element at a time.

iter(object, sentinel) # object can be sets,tuples etc 

iter() Parameters:

Takes 2 parameters where a first parameter is an object that can be sets, tuples, etc.

iter() Return Value

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.

Examples of iter() method in Python

# 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

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

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