Iterators

At this point, you already know about for loops in Python that you can use to go through the elements in a list, for example. However, Python also has other similar tools with their own advantages, which we're going to learn about in this topic, so let's start!

Iterables and iterators

In Python, we call any object we can loop over an iterable. Very common examples of iterable objects are lists, strings, and dictionaries.

Iterables in Python implement the __iter__() method that returns an iterator, an object that traverses an iterable and returns its elements one by one. Iterators represent a stream of data. They implement the __next__() method, which returns the items of an iterable one by one.

You can create an iterator passing an iterable to the built-in iter() function.

# This is a list...
my_list = [1, 2, 3]

# ... and this is how we create an iterator from it
my_iterator = iter(my_list)
print(my_iterator)

# <list_iterator object at 0x000001F06D792B70>

Each time you want to get the actual values, you need to pass iterator to the next() function:

print(next(my_iterator))
# 1

print(next(my_iterator))
# 2

print(next(my_iterator))
# 3

print(next(my_iterator))
# StopIteration exception

Note that when we call next() for the fourth time, we get a StopIterationexception. It's because our list contains just three elements, and iterator can only pass them once.

But do you always have to call next() manually? Not if you create and use iterators in for loop statements using the following syntax:

for item in iterable:
    ...

Python for loop will automatically create an iterator from a given iterable and get its elements one by one with the help of the next method until the iterable is exhausted. Thus, to print out the elements of my_list defined above, we can simply write the following:

for item in my_list:
    print(item)

# 1
# 2
# 3

zip()

Now you know how to create an iterator from a single iterable. What if you need to look over the elements of not one but multiple lists at the same time? Well, then the built-in zip() comes in very handy.

Suppose, for example, that you have two lists with the first and last names of the employees, and you need to print out the full names. With zip(), this can be easily done as follows:

first_names = ['John', 'Anna', 'Tom']
last_names = ['Smith', 'Williams', 'Davis']

for name, last_name in zip(first_names, last_names):
    print(name, last_name)

# John Smith
# Anna Williams
# Tom Davis

zip() takes several iterables and returns an iterator of tuples, where each tuple contains one element from each of the given iterables. Note that if zip() gets iterables of different lengths, iteration will stop as soon as the shortest iterable is exhausted:

short_list = [1, 2, 3]
long_list = [10, 20, 30, 40]

for a, b in zip(short_list, long_list):
    print(a, b)

# 1 10
# 2 20
# 3 30

However, in Python 3.10 zip() has an optional boolean keyword parameter strict. When set to True, if one of the arguments is exhausted before the others, a ValueError will be raised:

my_pets = ['Pistachio', 'Fluffy', 'Emperor']
your_pets = ['Claws', 'Grumpy', 'Mr. Cat', 'Naughty', 'Blue Paws']

for pet1, pet2 in zip(my_pets, your_pets):
    print(pet1, pet2)

# Pistachio Claws
# Fluffy Grumpy
# Emperor Mr. Cat

for pet1, pet2 in zip(my_pets, your_pets, strict=True):
    print(pet1, pet2)

# Pistachio Claws
# Fluffy Grumpy
# Emperor Mr. Cat

Traceback (most recent call last):
   File "<pyshell#4>", line 1, in <module>
     for pet1, pet2 in zip(my_pets, your_pets, strict=True):
 ValueError: zip() argument 2 is longer than argument 1

This new option allows you to explicitly require all iterables to have an equal length and will help you to quickly detect bugs in case they don't.

enumerate()

Another very useful tool is the built-in enumerate() function, which takes an iterable and returns its elements one by one along with their indexes. For instance, the code below prints out the names of the months (stored in a list) along with their numbers:

months_list = ['Jan', 'Feb', 'Mar', 'Apr', 'May', 'Jun', 
               'Jul', 'Aug', 'Sep', 'Oct', 'Nov', 'Dec']

for n, month in enumerate(months_list):
    print(n + 1, month)

# 1 Jan
# 2 Feb
# 3 Mar
# 4 Apr
# 5 May
# 6 Jun
# etc.

Note that by default the counter starts at 0, but you can actually explicitly specify any starting point:

for n, month in enumerate(months_list, start=1):
    print(n, month)

# 1 Jan
# 2 Feb
# 3 Mar
# etc.

Summary

• In Python, iterator objects traverse an iterable, e.g., a list.
• There are several built-in functions to create iterators, for example, iter(), zip() and enumerate().
• zip() performs parallel iteration over several iterables.
• enumerate() returns elements of an iterable along with their indexes one by one.