How to survive coding interviews with a single line of Python code

Entering the world of coding interviews can be daunting. Whether you are a recent graduate with a bachelor’s or master’s degree in Computer Science or a senior industry professional, questions may arise with regards to which programming language to use and which strategy to follow before and during your coding interview.

This guide will give a simple answer out of the many possible one for that question.

To keep things simple, I recommend the Python programming language. And to keep our strategy even simpler, I recommend writing all your solutions in a single line.

Writing your code in one line can bring numerous benefits. Firstly, if there is a bug, you don’t need to worry about which line of code the error happened. Secondly, one line progams are concise. Combined with the fact that Python is similar to plain English, this strategy helps your interviewer understand your solution better. In fact, it is like reading one very long sentence with many commas.

Are you still stressed about the coding interview and in doubt on how to code in a single line? You need not worry, because I will provide examples and tips throughout the rest of this article.

Recursion is your friend

Recursion is a powerful concept that allows us to reutilize code. Hence, it fits perfectly with Python solutions using one line of code.

Let’s take for example the task of inverting a binary tree (link). This question is rumored to be a frequently asked questions for candidates that write package managers for macOS.

In the example above, recursion does most of the work. To invert a binary tree, we swap the left and right nodes and then keep repeating the process. Clear and concise.

Conditional Operators are helfpul

This detail was also in the last code snippet, but we can discuss it individually. Do not waste multiple lines writing if and else statements! In fact, Python lets you write conditional operators in a single line.

Consider the following code that computes the depth of a binary tree (link) .

Thanks to conditional operators, your code fits concisely in one line. The if and else statements let you handle correctly the base case for the recursion which is the empty tree node.

Lambdas are powerful

Lambdas, also known as anonymous functions, let us define Python functions without wasting unnecessary lines.

Take for example the task of finding the first palindrome in a list (link).

Thanks to the lambda statement, we didn’t need to define a separate function to check for palindromes (x == x[::-1]).

The Walrus is a must

I told you about the Walrus and me, man

You know that we’re as close as can be, man

- The Beatles

The walrus was Paul was introduced in PEP 572. Because the feature landed in Python 3.8, it is higly likely that you can use it as of 2024. The walrus lets us write less code, which is great.

The true power of the walrus, however, appears when it is combined with lambda functions, because it yields recursive lambdas. Now you don’t need to waste a separate line to define a recursive function!

Take for example the task to check if a binary tree is symmetric (link).

The lambda definition with the walrus operator (f := lambda) allows us to define a recursive function that solves the problem in a single line. The code does not need to waste lines in superfluous function definitions.

The standard library is one function call away

Python is known for its included batteries i.e. libraries that ship with Python. They can be useful for coding interviews as well.

There is a misconception that you need your import statement in a separate line. But that is not necessary due to the __import__ function.

I demonstrate that in the solution for calculating the majority element in an array (link) .

Python has a built-in Counter class that handles this exact case. By using the standard library, we reused existing code and made our solution simpler without having to sacrifice an extra line for the import.

The wheel can be reinvented

Sometimes your interviewer will explicitly forbid you from using the standard library. In those cases, it is likely take you can write similar code still in one line.

Take for example the task of sorting an array from scratch (link). We can fit a Randomized Quicksort in one (very long) line.

If the interviewer complains about using random.randint though, you can just read from /dev/random instead. And if they complain about opening /dev/random with the Python built-ins, you can use punched cards to execute code.

Believe in Yourself

You might find harder questions along your coding interview journey. Do not panic. Believe in yourself and keep going. Look at the following solution for calculating block sums in a matrix (link).

Sometimes I’ll start a line of code and I don’t even know where it’s going. I just hope I find it along the way. And pray there is no syntax error.


If you have reached the end of the article, I want to kindly remind you to read the date of the publication of this article. Happy Aрrіl Fօօls’ Day! Do not take the content of this post seriously, it is all a ʝοκе. The contents of this post do not reflect my employer’s opinion on coding interviews. In fact, they don’t even represent mine.

Ivan Carvalho
Software Developer

Always looking for a challenge!