Top Python Interview Questions
Updated 20 December 2020
What Is a Python Interview?
If you are applying for a job that requires coding and programming, then a solid working knowledge of programming languages will be essential.
While there are many programming languages in use for different functions, one of the most popular – and most sought after – seems to be Python.
It is popular as a programming language because it uses simple syntax and can be used on multiple platforms – Windows, Mac, Linux, Raspberry Pi, etc.
Python seems to be the top programming language in emerging data science fields, so experienced Python coders are future-proofed for the development of the Internet.
Python can be used for:
- Web development, on the back end/server-side
- Software development, like software apps
- Mathematical processes, such as big data computations
- Creating system scripts and instructions
Many big tech firms, including Google and Facebook, use Python in their stacks, and it could be used by beginner coders, web and mobile app developers, software engineers, data scientists... the list goes on.
Software engineers, programmers and coders are in high demand – thousands of job opportunities exist in big tech firms and startups alike. However, this doesn’t make recruitment any less competitive.
Successful applicants need to be passionate as well as qualified, and need to demonstrate their ability throughout the interview process.
Unlike some other positions, interviews for Python positions tend to be more like an exam – technical and testing – with live demonstrations of your ability.
This means that Python interviews can be tricky. Not necessarily because the tasks themselves are hard, but because you need to demonstrate not only the finished task but also the process you took to get there.
You can make the most of your coding abilities by practicing, taking on tasks and timing them, and looking to learn new functions that could make a task easier to complete.
As Python is open-source, it is a free resource that can be used, modified and developed by anyone – with plenty of tutorials, practice sessions and mentors available online.
How to Prepare for a Python Interview
There are usually three stages to a Python interview, like most programming and coding interviews.
The first is usually over the phone, with a call from the recruiter.
This technical phone screen is a combination of checking your qualifications and assessing your enthusiasm – they want to know what you can offer them, so make sure you tailor your answers to the job and the provided job description.
Try to convey soft skills here, so the recruiter can get a feel for who you are.
You might be offered a remote coding challenge (or this could be a coding challenge as part of an in-person interview, which we discuss below).
This may be sent to you via a link that you can complete in your own time (usually within 48 hours) or as a ‘live’ challenge over Skype or a phone call.
Whichever way you must approach this, make sure that you take your time, plan and write out the steps you use, and be prepared to justify your choices.
These remote coding challenges aren’t meant to be complicated or too rigorous – but that doesn’t mean that you should cut corners in your coding. Recruiters love it when you go ‘above and beyond’, so make sure that you complete any bonus challenges.
Remote challenges are usually part of the screening process, so they are meant to weed out those who aren’t qualified or are not qualified enough.
If successful, you will be invited to an in-person interview.
Attending an interview, whether over Skype or in person, means presenting yourself well.
For many tech companies, attire is casual, so business casual should be fine. You can find out what the dress code is by looking at photographs on the company website or by asking the recruiter.
As with other interview styles, there are a few things that you need to remember.
You will likely still have to answer typical background questions and discuss what is on your resume, so make sure that you have a great project on there that you can talk through.
Use the job description as much as you can throughout the interview process.
Although they are looking for a programmer who is confident and fluent in Python, they might also have a ‘nice to have’ list that might contain other programming languages.
If you don’t know these, it might not be a dealbreaker, but if you can get an overview and understanding of the processes and benefits of them, it will make a good impression. Conveying your willingness and ability to learn other programming languages will help.
Basic interview techniques like researching the company (what tech do they use, what markets or domains do they operate in) should be part of your pre-application preparation.
This makes it easier for you to tailor not only your resume and covering letter to the company and the specific position but means that when you do get to the interview stage, you can answer the questions in the way that the recruiter wants.
Recruiters want to see skills like:
- Confidence – Naturally, consistent eye contact, be energetic and enthusiastic.
- Body language – Firm handshake, good posture, read and recognize body language cues
- Personable – Make it a conversation, use the interviewer’s name, demonstrate good communication skills.
The Coding Test
The coding challenge is an important part of the interview process. It usually takes one of two forms – either an online test using a provided computer or completed in front of them on a whiteboard or on paper.
You will usually be presented with a timed exercise that will demonstrate your coding skills and the way you think about problems – creative, analytic and with an understanding of the bigger picture of how systems interact.
Preparing for the Coding Test
Like every interview, preparation is key. When it comes to the coding test, alongside general interview skills it is a great idea to practice specific coding skills, such as data structures, trees, sorting and search algorithms, arrays, and the most used functions.
When you practice coding challenges, make it a realistic testing scenario.
Give yourself a time limit and, even if the problem is simple, spend time planning and coding by hand on paper or a whiteboard.
You should develop a reliable process to deconstruct the question – in the test you might be presented with a challenge that is not simple or straightforward. A great process will make sure that even if the test is difficult, you won’t be flustered.
The best thing you can do to prepare for a coding interview is to know your programming. Make sure that you practice using Python wherever you can and find online practice challenges.
During the Coding Test
When you are presented with your challenge, take some time to think about what you are going to do.
The first thing you need to do is ask some questions to check your assumptions.
Asking thoughtful questions about the test will help you ensure that you are on the right track, as well as getting helpful hints from the interviewer.
Throughout the challenge, remember that the interviewer wants to understand your process. Document the processes and workflows by hand and talk them through too.
Articulating your thoughts will demonstrate that you are aware that the journey is as important as the destination – and it means that you can try something new if your original plan doesn’t work.
As long as you remember that you need to show, not tell, even if you can’t produce the correct answer, much like in your school math tests, you will get partial credit for showing your working.
When discussing the problem out loud, be clear and concise with your phrasing. Pay attention to language, use formal names and avoid ambiguity.
Precise use of the right terms demonstrates your mastery and avoids any confusion: use 'output' instead of 'result', for example.
Interviewers want you to be structured and systematic in your application of knowledge. Being able to discuss the pros and cons of the method you are using, the tradeoff between space and time, performance and optimization.
They want the code you produce to be clear and easy to read. They want you to clearly demonstrate your creative problem-solving skills, and to see how you respond to feedback.
After the Coding Test
You might need to complete a few tests, depending on the company you are interviewing for.
There might be further questions or discussion, and you may be invited to ask the interviewer some questions – think of interesting questions to ask that are related to the specific business but also demonstrate your enthusiasm for the role.
- Find out about opportunities for personal development and advancement
- Ask about a new initiative
- Find out what the interviewer loves about the company or how they came to work there
When you leave the interview, remember that any result is a good result. If you get the job, excellent – but even if you don’t, it is a great opportunity to develop and practice your interview and coding skills.
To make sure that you are top of mind after the interview, send an email to thank the team.
Mention those that helped you by name – this demonstrates great attention to detail.
If you don’t have any contact from the interviewer within a week, send a brief follow-up email to ask for the next steps and reiterate your interest.
Core Python Interview Questions
Although there are potentially hundreds of interview questions that could be asked, there are some that are core questions.
Having good answers to these questions is important – it is a great way to demonstrate your knowledge and mastery of Python and the functions that are needed to write good, useful code.
1. 'Why Is Python an Interpreted Language?'
Python, like PHP and Ruby, is known as an interpreted language because the code does not have to be compiled before it can run.
Compilation is used by languages like C and C++, translating code that can be understood by humans into code that can be read by machines. Compilation creates an executable file.
Although Python is an interpreted language, there is some level of compilation – it is compiled to bytecode which is executed by the interpreter on a virtual machine rather than a CPU.
Dynamic typing allows the interpreter to check the validity of every line of code as you write it.
2. 'What Are the Benefits of Using Python?'
Python is a high-level language and the English-like syntax makes it easy to read and understand. With fewer lines of code needed, increased productivity through simplicity is what makes Python an incredibly attractive programming language.
The code is directly executed, line by line – so if there are any errors in the code, it will stop execution.
The creation of Python as a free, open-source language has allowed for so many functions and external libraries to be created. It can be downloaded and modified for the end-user, and the standard library has almost all the functions that you need, as well as huge swathes of online support from other programmers and the Python team.
As a programming language, Python is portable. The use of a virtual machine means that the code does not need to be changed to run on different platforms – making it useful whether it needs to run on a Mac or Windows system.
Knowing the benefits is only part of the answer. Understanding the limitations of Python is important so that you can find effective ways to manage them.
Python is sometimes difficult to debug and run time errors can be difficult to pin down. Some people recognize that Python can be slow; the interpreter must work in between each line of code, making the speed a problem in some cases.
3. 'How Is Memory Managed in Python?'
Python’s Memory Manager allocates space for objects and data structures, and the inbuilt garbage collector recycles unused memory.
The memory allocator works in small blocks with a specific strategy. There are three main parts to this allocation:
Arenas are the largest chunks of memory, and Python gives them 256 kb as an assumed size
Within the arenas are pools, which are smaller (4 kb).
Inside the pools are blocks. All blocks in a pool are the same size, and the memory allocator decides where the data should go depending on the size of it.
4. 'What Tools Could You Use to Find Bugs?'
As an open-source resource, several tools are available to check for problems. Three main tools that are used include:
PyChecker – This software development tool executes the code as it checks it, performing a static analysis of the structural elements of the code, through the modules, classes, functions and executable statements.
Pylint – This source code, bug and quality checker doesn’t execute the code as it heuristically checks through the lengths of lines and that the variable names are well formed. It also checks that all the declared interfaces are truly implemented.
Pyflakes – Sometimes described as ‘Pylint lite’, Pyflakes is a quick checker that can check the source file for errors without executing the code.
5. 'Write a Code to Sort a Numerical List in Python'
Two well-known functions will sort a list:
Knowing the difference is important, and although this is a simple task, you could make an error if you do not know what the desired output is.
The way you create this code will depend on whether the result is to change the way the list is presented originally or to create a new, sorted list without affecting the original list.
Asking the interviewer what they want demonstrates that you know and understand there is a difference.
Numbers.sort() will return the list, sorted into numerical order
Numbers.sorted() will keep the original list and also provide a new list in numerical order
For both these functions, creating an ascending order numerical list works with the reverse=true function.
6. 'What Is the Python GIL?'
One of the most frustrating parts of the Python code is the Global Interpreter Lock (GIL).
This makes sure that only one thread of code can control the interpreter, and only one thread can be executed at a time.
The reason the GIL is necessary in Python is as a pragmatic solution to multi-threading and the cost to memory and runtime, as well as the risk of leaked memory and the associated bugs and crashes.
Without the GIL, multiple locks can prevent memory leaks but can cause deadlocks and a massive decrease in performance as they open and close. By putting a single lock on the interpreter itself, Python can make any CPU-bound program single threaded.
For those who want to run multiple threads, it is possible for multi-processing – each Python iteration can have its own interpreter and therefore its own GIL.
Many alternative interpreters can be used, such as Jython, IronPython and PyPy.
Having knowledge and experience in the Python programming language opens doors to jobs from small tech startups to major software and development employers.
Succeeding in the market means knowing your language inside and out – while also presenting yourself as the best fit for the business and the role.
Software development and tech roles do not exist in a vacuum – with multiple projects needing different expertise, your application will be judged not only on your Python ability but also your transferable skills, your commitment and passion for the work, and how your values align with the company’s goals.
Interviewers want to know you have the soft skills to become a part of their team.
There are a lot of things to consider and learn when you are interviewing for a Python role. Remember that as long as you know your programming language and you practice under exam conditions (timed, articulate your process), then you have every chance of impressing the recruitment team and winning that role.