PyDev of the Week: Jelle Zijlstra : Mike

PyDev of the Week: Jelle Zijlstra
by: Mike
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This week, we welcome Jelle Zijlstra as our PyDev of the Week! Jelle is a core developer of the Python programming language. If you’d like to see what exactly Jelle is working on, you should check out his GitHub profile, Quora, or the Python Packaging Index.

Let’s take a few moments to get to know Jelle better!

Can you tell us a little about yourself (hobbies, education, etc):

I grew up in the Netherlands wanting to be a biologist, but decided to go to the United States for college. Then I took a computer science class, took a few more, and decided that software engineering offered better career prospects than biology. I still graduated with a degree in biology, but found a job as a software engineer.

Still, biology remains one of my main hobbies: I maintain a big database of biological nomenclature at I started out doing this in spreadsheets, and now it is a SQLite database. Learning to program has been hugely valuable for building up a high-quality, interconnected database.

Before the pandemic I used to walk much of the way to work, but then I was stuck at home, so I decided to go on walks anyway to exercise. That quickly spiraled into ridiculously long walks around the San Francisco Bay Area, up to 57 miles in a day. I write about my walks at

But the reason you’re interviewing me is my open source work. At my job a few years ago I got to start a new service that was going to be in Python 3, so I got to use shiny new tools like typing and asyncio. I ran into some minor issues with typing, started contributing to the relevant open source projects, and got sucked in. Now I help maintain several important parts of the Python ecosystem, including typing-extensions, typeshed, Black, mypy, and CPython.

Why did you start using Python?

The first language I learned in college was C. I’m still not sure whether I think C is a good first language to teach to students, but it’s definitely been valuable in giving me an understanding of how basic concepts such as pointers and memory work. Later in that class, we did a little bit of web programming in PHP. PHP isn’t a great language, but for my personal programming projects (such as maintaining my mammal database), it was still a better fit than C—no segfaults and much more library support. But I had to use Python in another class, and I quickly realized it was a lot better than PHP. Then I started a job mostly using Python, so it remained my main language.

What other programming languages do you know and which is your favorite?

When I interviewed for my current job, during each interview I picked the language that felt like the best fit for the question, and at the end of the day it turned out I had used a different language in each interview! I think it was C, Python, JavaScript, and OCaml. More recently, the main non-Python languages I’ve used have been TypeScript and C++, plus a bit of Swift. I think languages are valuable when they teach you something new about programming. OCaml was the most valuable language I learned in college because it taught me a completely new style of programming. Among languages I have dabbled in since, Haskell and Rust have been most useful.

What projects are you working on now?

My most recent major open-source contribution has been implementing PEP 695 for Python 3.12, which is coming out in October. It’s a major syntactic change that makes it a lot easier to write generics in Python. I wrote up a detailed account of the implementation at

At my job, I am now working on Poe, an AI chat app. Outside of work, I’ve been focusing recently on my mammal database instead of open-source software.

Which Python libraries are your favorite (core or 3rd party)?

typing. It’s been an invaluable tool to keep our huge Python codebase at work manageable.

How did you get into doing core Python development?

I attended a CPython sprint at PyCon in Portland and Cleveland before the pandemic and contributed a few things; for example, you can thank me for `@contextlib.asynccontextmanager`. However, the CPython core always felt a little remote and hard to get into, with the unusual workflow at the time (e.g., using Gerrit) and long release cycle. But then one day, Guido van Rossum emailed me to ask whether I was interested in becoming a core dev, so I said yes and after spending a few months becoming more familiar with the workflow (which is now a lot less unusual), I was voted in as a core dev. Guido asked me because at the time he was basically maintaining by himself, and obviously he is very busy. Now, we have several other people helping out in that area.

What are some new features in Python that you’re excited about?

I’m excited about the new syntax for generics and type aliases that I helped get into Python 3.12. Longer term, Python 3.13 should ship with deferred evaluation of annotations (PEP 649), which will make a lot of code that uses type annotations more ergonomic. We’re also likely to ship support for inline TypedDict types, another nice usability improvement.

Thanks so much for doing the interview, Jelle!

The post PyDev of the Week: Jelle Zijlstra appeared first on Mouse Vs Python.

September 11, 2023 at 06:00PM
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