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#434 Most of OpenAI’s tech stack runs on Python

29 min • 2 juni 2025

Topics covered in this episode:

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Brian #1: Making PyPI’s test suite 81% faster

Alexis Challande
The PyPI backend is a project called Warehouse
It’s tested with pytest, and it’s a large project, thousands of tests.
Steps for speedup
- Parallelizing test execution with pytest-xdist
  - 67% time reduction
  - --numprocesses=auto allows for using all cores
  - DB isolation - cool example of how to config postgress to give each test worker it’s on db
  - They used pytest-sugar to help with visualization, as xdist defaults to quite terse output
- Use Python 3.12’s sys.monitoring to speed up coverage instrumentation
  - 53% time reduction
  - Nice example of using COVERAGE_CORE=sysmon
- Optimize test discovery
  - Always use testpaths
  - Sped up collection time. 66% reduction (collection was 10% of time)
  - Not a huge savings, but it’s 1 line of config
- Eliminate unnecessary imports
  - Use python -X importtime
  - Examine dependencies not used in testing.
  - Their example: ddtrace
    - A tool they use in production, but it also has a couple pytest plugins included
    - Those plugins caused ddtrace to get imported
    - Using -p:no ddtrace turns off the plugin bits
Notes from Brian:
- I often get questions about if pytest is useful for large projects.
- Short answer: Yes!
- Longer answer: But you’ll probably want to speed it up
- I need to extend this article with a general purpose “speeding up pytest” post or series.
- -p:no can also be used to turn off any plugin, even builtin ones.
  - Examples include
    - nice to have developer focused pytest plugins that may not be necessary in CI
    - CI reporting plugins that aren’t needed by devs running tests locally

Michael #2: People aren’t talking enough about how most of OpenAI’s tech stack runs on Python

Original article: Building, launching, and scaling ChatGPT Images
Tech stack: The technology choices behind the product are surprisingly simple; dare I say, pragmatic!
- Python: most of the product’s code is written in this language.
- FastAPI: the Python framework used for building APIs quickly, using standard Python type hints. As the name suggests, FastAPI’s strength is that it takes less effort to create functional, production-ready APIs to be consumed by other services.
- C: for parts of the code that need to be highly optimized, the team uses the lower-level C programming language
- Temporal: used for asynchronous workflows and operations inside OpenAI. Temporal is a neat workflow solution that makes multi-step workflows reliable even when individual steps crash, without much effort by developers. It’s particularly useful for longer-running workflows like image generation at scale

Michael #3: PyCon Talks on YouTube

Some talks that jumped out to me:
- Keynote by Cory Doctorow
- 503 days working full-time on FOSS: lessons learned
- Going From Notebooks to Scalable Systems
  - And my Talk Python conversation around it. (edited episode pending)
- Unlearning SQL
- The Most Bizarre Software Bugs in History
- The PyArrow revolution in Pandas
  - And my Talk Python episode about it.
- What they don't tell you about building a JIT compiler for CPython
  - And my Talk Python conversation around it (edited episode pending)
- Design Pressure: The Invisible Hand That Shapes Your Code
- Marimo: A Notebook that "Compiles" Python for Reproducibility and Reusability
  - And my Talk Python episode about it.
- GPU Programming in Pure Python
  - And my Talk Python conversation around it (edited episode pending)
- Scaling the Mountain: A Framework for Tackling Large-Scale Tech Debt

Brian #4: Optimizing Python Import Performance

Mostly pay attention to #'s 1-3
This is related to speeding up a test suite, speeding up necessary imports.
Finding what’s slow
- Use python -X importtime <the reset of the command
- Ex: python -X importtime ptyest
Techniques
- Lazy imports
  - move slow-to-import imports into functions/methods
- Avoiding circular imports
  - hopefully you’re doing that already
- Optimize __init__.py files
  - Avoid unnecessary imports, heavy computations, complex logic
Notes from Brian
- Some questions remain open for me
  - Does module aliasing really help much?
- This applies to testing in a big way
  - Test collection imports your test suite, so anything imported at the top level of a file gets imported at test collection time, even if you only are running a subset of tests using filtering like -x or -m or other filter methods.
  - Run -X importtime on test collection.
  - Move slow imports into fixtures, so they get imported when needed, but NOT at collection.
See also:
- option -X in the standard docs
- Consider using import_profile

Extras

Brian:

PEPs & Co.
- PEP is a ‘backronym”, an acronym where the words it stands for are filled in after the acronym is chosen. Barry Warsaw made this one up.
- There are a lot of “enhancement proposal” and “improvement proposal” acronyms now from other communities
pythontest.com has a new theme
- More colorful. Neat search feature
- Now it’s excruciatingly obvious that I haven’t blogged regularly in a while
  - I gotta get on that
- Code highlighting might need tweaked for dark mode

Michael:

Joke: There is hope.

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