An iterator is an object that contains a countable number of values and can be iterated upon, meaning that all the values can be traversed through.

Lists, tuples, dictionaries, and sets are all iterable objects, which you can get an iterator from. All these objects have a iter() method, which is used to get an iterator.

The following returns an iterator from a tuple and prints each value.

mytuple = ("Ferrari", "Maserati", "Alfa Romeo")
myit = iter(mytuple)

print(next(myit))
print(next(myit))
print(next(myit))

Output:

Ferrari
Maserati
Alfa Romeo

The following returns an iterator from a string, which is a sequence of characters, and prints each character.

mystring = "Alfa Romeo"
myit = iter(mystring)

print(next(myit))
print(next(myit))
print(next(myit))
print(next(myit))
print(next(myit))
print(next(myit))
print(next(myit))
print(next(myit))
print(next(myit))
print(next(myit))

Output:

A
l
f
a
 
R
o
m
e
o

To create an object/class as an iterator, the methods __iter__() and __next__() must be added to the object.

class MyNumbers:
    def __iter__(self):
        self.a = 1
        return self

    def __next__(self):
        x = self.a
        self.a += 1
        return x

myclass = MyNumbers()
myiter = iter(myclass)

print(next(myiter))
print(next(myiter))
print(next(myiter))

Output:

1
2
3

The examples above would continue forever if there were enough __next__() statements, or if it was used in a for loop.

To prevent the iteration from going on forever, the StopIteration statement is used.

class MyNumbers:
    def __iter__(self):
        self.a = 1
        return self

    def __next__(self):
        if self.a <= 5:
            x = self.a
            self.a += 1
            return x
        else:
            raise StopIteration

myclass = MyNumbers()
myiter = iter(myclass)

for x in myiter:
    print(x)

Output:

1
2
3
4
5

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