What is a Python iterator and how to use it

Python Iterator

An iterator is an object that contains a countable number of values.

Iterators are iterable objects, which means you can iterate over all values.

Technically, in Python, an iterator is an object that implements the iterator protocol, which contains methods iter() and next().

Iterator VS Iterable object (Iterable)

Lists, tuples, dictionaries and sets are all iterable objects. They are iterable containers from which you can get an Iterator.

All these objects have iter() methods for getting iterators:

Instances

Return an iterator from a tuple, and print Each value:

mytuple = ("apple", "banana", "cherry")
myit = iter(mytuple)

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

Running instance

apple
banana
cherry

Even strings are iterable objects and can return iterators:

Instance

Strings are also iterable objects, containing a series of characters:

mystr = "banana"
myit = iter(mystr)

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

Running instance

b
a
n
a
n
a

Traversing iterator

We can also use a for loop Traverse the iterable object:

Instance

Iterate over the values ​​of the tuple:

mytuple = ("apple", "banana", "cherry")

for x in mytuple:
  print(x)

Run the instance

python_iterator_loop_1.py
apple
banana
cherry

Instance

Iterate over the characters in the string:

mystr = "banana"

for x in mystr:
  print(x)

Run the example

python_iterator_loop_2.py
b
a
n
a
n
a

Tip: The for loop actually creates an iterator object, and for each loop Execute next() method.

Creating an iterator

To create an object/class as an iterator, the iter() and next() methods must be implemented for the object.

As you learned in the Python Classes/Objects chapter, all classes have a function called init() which allows you to do some initialization when creating an object. The

iter() method works similarly, you can perform operations (initialization, etc.) but must always return the iterator object itself. The

next() method also allows you to perform operations and must return the next item in the sequence.

Example

Create an iterator that returns numbers, starting from 1, and each sequence will increase by 1 (returns 1, 2, 3, 4, 5, etc.) :

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

Running Example

python_iterator_create_1.py
1
2
3
4
5

StopIteration

If you have enough next() statements, or use them in a for loop, the above example will go on forever.

To prevent iteration from taking forever, we can use the StopIteration statement.

In the next() method, we can add a termination condition to raise an error if the iteration completes the specified number of times:

Example

Stop after 20 iterations:

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

  def __next__(self):
    if self.a <= 20:
      x = self.a
      self.a += 1
      return x
    else:
      raise StopIteration

myclass = MyNumbers()
myiter = iter(myclass)

for x in myiter:
  print(x)

Running instance

python_iterator_create_2.py
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20

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