Introduction – Python One-Liners


With this book, I want to help you become a Python expert. To do this, we’re going to focus on Python one-liners: concise, useful programs packed into a single line of Python. Focusing on one-liners will help you read and write code faster and more concisely, and will improve your understanding of the language.

There are five more reasons I think learning Python one-liners will help you improve and are worth studying.

First, by improving your core Python skills, you’ll be able to overcome many of the small programming weaknesses that hold you back. It’s hard to make progress without a profound understanding of the basics. Single lines of code are the basic building block of any program. Understanding these basic building blocks will help you master high-level complexity without feeling overwhelmed.

Second, you’ll learn how to leverage wildly popular Python libraries, such as those for data science and machine learning. The book consists of five one-liner chapters, each addressing a different area of Python, from regular expressions to machine learning. This approach will give you an overview of possible Python applications you can build, as well as teach you how to use these powerful libraries.

Third, you’ll learn to write more Pythonic code. Python beginners, especially those coming from other programming languages, often write code in un-Pythonic ways. We’ll cover Python-specific concepts like list comprehension, multiple assignment, and slicing, all of which will help you write code that’s easily readable and sharable with other programmers in the field.

Fourth, studying Python one-liners forces you to think clearly and concisely. When you’re making every single code symbol count, there’s no room for sparse and unfocused coding.

Fifth, your new one-liner skill set will allow you to see through overly complicated Python codebases, and impress friends and interviewers alike. You may also find it fun and satisfying to solve challenging programming problems with a single line of code. And you wouldn’t be alone: a vibrant online community of Python geeks compete for the most compressed, most Pythonic solutions to various practical (and not-so-practical) problems.

Python One-Liner Example

The central thesis of this book is that learning Python one-liners is both fundamental to understanding more-advanced codebases and an excellent tool for improving your skills. Before understanding what’s going on in a codebase with thousands of lines, you must understand the meaning of a single line of code.

Let’s have a quick look at a Python one-liner. Don’t worry if you don’t fully understand it. You will master this one-liner in Chapter 6.

q = lambda l: q( [x for x in l[1:] if x <= l[0]]) + [l[0]] + q([x for x in l if x > l[0]]) if l else []

This one-liner is a beautiful and concise way of compressing the famous Quicksort algorithm, though the meaning may be difficult to grasp for many Python beginners and intermediates.

Python one-liners often build on each other, so one-liners will increase in complexity throughout the book. In this book, we’ll start with simple one-liners that will become the basis for more-complex one-liners later. For example, the preceding Quicksort one-liner is difficult and long, based on the easier concept of list comprehension . Here’s a simpler list comprehension that creates a list of squared numbers:

lst  = [x**2 for x in range(10)]

We can break this one-liner into even simpler one-liners that teach important Python basics, such as variable assignments, math operators, data structures, for loops, membership operators, and the range() function—all of which happens in a single line of Python!

Know that basic doesn’t mean trivial. All the one-liners we’ll look at are useful, and each chapter addresses a separate area or discipline in computer science, giving you a broad perspective on the power of Python.

A Note on Readability

The Zen of Python comprises 19 guiding principles for the Python programming languages. You can read it in your Python shell by entering import this:

>>> import this
The Zen of Python, by Tim Peters

Beautiful is better than ugly.
Explicit is better than implicit.
Simple is better than complex.
Complex is better than complicated.
Flat is better than nested.
Sparse is better than dense.
Readability counts.

According to The Zen of Python, “Readability counts.” One-liners are minimalistic programs to solve problems. In many cases, rewriting a piece of code as a Python one-liner will improve readability and make the code more Pythonic. An example is using list comprehension to reduce the creation of lists into a single line of code. Have a look at the following example:

squares = []

for i in range(10):
# [0, 1, 4, 9, 16, 25, 36, 49, 64, 81]

In this code snippet, we need five lines of code to create a list of the first 10 square numbers and print it to the shell. However, it’s much better to use a one-liner solution that accomplishes the same thing in a more readable and concise way:

print([i**2 for i in range(10)])
# [0, 1, 4, 9, 16, 25, 36, 49, 64, 81]

The output is the same, but the one-liner builds on the more Pythonic concept of list comprehension. It’s easier to read and more concise.

However, Python one-liners can also be hard to understand. In some cases, writing a solution as a Python one-liner isn’t more readable. But just as the chess master must know all possible moves before deciding which one is best, you must know all ways of expressing your thoughts in code so that you can decide on the best one. Going for the most beautiful solution is not a low-priority matter; it’s at the core of the Python ecosystem. As The Zen of Python teaches, “Beautiful is better than ugly.”

Who Is This Book For?

Are you a beginner- to intermediate-level Python coder? Like many of your peers, you may be stuck in your coding progress. This book can help you out. You’ve read a lot of programming tutorials online. You’ve written your own source code and successfully shipped small projects. You’ve finished a basic programming course and read a programming textbook or two. Maybe you’ve even finished a technical program in college, where you’ve learned about the basics of computer science and programming.

Perhaps you’re limited by certain beliefs, like that most coders understand source code much faster than you, or that you’re nowhere near the top 10 percent of programmers. If you want to reach an advanced coding level and join the top coding experts, you need to learn new applicable skills.

I can relate because when I started out studying computer science 10 years ago, I struggled with the belief that I knew nothing about coding. At the same time, it seemed that all my peers were already very experienced and proficient.

In this book, I want to help you overcome these limiting beliefs and push you one step further toward Python mastery.

What Will You Learn?

Here is an overview of what you will learn.

Chapter 1: Python Refresher Introduces the very basics of Python to refresh your knowledge.

Chapter 2: Python Tricks Contains 10 one-liner tricks to help you master the basics, such as list comprehension, file input, the functions lambda, map(), and zip(), the all() quantifier, slicing, and basic list arithmetic. You’ll also learn how to use, manipulate, and leverage data structures to solve various day-to-day problems.

Chapter 3: Data Science Contains 10 one-liners for data science, building on the NumPy library. NumPy is at the heart of Python’s powerful machine learning and data science capabilities. You’ll learn elementary NumPy basics such as array, shape, axis, type, broadcasting, advanced indexing, slicing, sorting, searching, aggregating, and statistics.

Chapter 4: Machine Learning Covers 10 one-liners for machine learning with Python’s scikit-learn library. You’ll learn about regression algorithms that predict values. Examples of these include linear regression, K-Nearest Neighbors, and neural networks. You’ll also learn classification algorithms such as logistic regression, decision-tree learning, support-vector machines, and random forests. Furthermore, you’ll learn about how to calculate basic statistics of multidimensional data arrays, and the K-Means algorithm for unsupervised learning. These algorithms and methods are among the most important algorithms in the field of machine learning.

Chapter 5: Regular Expressions Contains 10 one-liners to help you achieve more with regular expressions. You’ll learn about various basic regular expressions that you can combine (and recombine) in order to create more-advanced regular expressions, using grouping and named groups, negative lookaheads, escaped characters, whitespaces, character sets (and negative characters sets), and greedy/nongreedy operators.

Chapter 6: Algorithms Contains 10 one-liner algorithms addressing a wide range of computer science topics, including anagrams, palindromes, supersets, permutations, factorials, prime numbers, Fibonacci numbers, obfuscation, searching, and algorithmic sorting. Many of these form the basis of more-advanced algorithms and contain the seeds of a thorough algorithmic education.

Afterword Concludes this book and releases you into the real world, packed with your new and improved Python coding skills.

Online Resources

To enhance the training material in this book, I’ve added supplementary resources that you can find online at or The interactive resources include the following:

Python cheat sheets You can download those Python cheat sheets as printable PDFs and pin them to your wall. The cheat sheets contain essential Python language features, and if you study them thoroughly, you can refresh your Python skills and ensure that you’ve closed any knowledge gap you may have.

One-liner video lessons As part of my Python email course, I’ve recorded many Python one-liner lessons from this book, which you can access for free. Those lessons can assist you in your learning and provide a multimedia learning experience.

Python puzzles You can visit the online resources to solve Python puzzles and use the app for free to test and train your Python skills and measure your learning progress as you go through the book.

Code files and Jupyter notebooks You must roll up your sleeves and start working with code to make progress toward Python mastery. Take your time to play around with various parameter values and input data. For your convenience, I’ve added all Python one-liners as executable code files.