Making The Case For A (Semi) Formal Specification Of CPython

Summary

The CPython implementation has grown and evolved significantly over the past ~25 years. In that time there have been many other projects to create compatible runtimes for your Python code. One of the challenges for these other projects is the lack of a fully documented specification of how and why everything works the way that it does. In the most recent Python language summit Mark Shannon proposed implementing a formal specification for CPython, and in this episode he shares his reasoning for why that would be helpful and what is involved in making it a reality.

Announcements

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• Your host as usual is Tobias Macey and today I’m interviewing Mark Shannon about his efforts to create a formal specification for the CPython interpreter

Interview

• Introductions
• How did you get introduced to Python?
• Can you start by describing the current state of how the Python language and the CPython runtime are defined?
• What is your motivation in advocating for a specification?
  • After ~25 years of the language, why is now the time to pursue this effort?
  • How does the history of the language and the scope of the ecosystem and community impact the effort required to make this a reality?
• What is involved in creating the specification and where would it be located once complete?
  • What are some examples of languages that are formally specified?
• What are the possible benefits of creating a specification for the CPython virtual machine?
  • What is the distinction between a specification for the VM as opposed to a specification for the language?
• What are some potential downsides to having a (semi-)formal specification become part of the definition of the interpreter?
• Can you describe the process of doing the work to create the specification?
• How are you approaching the actual definition of the specification (e.g. prose vs programmatic)?
  • What are the tradeoffs of prose vs. an executable specification (e.g. TLA+, Alloy)?
• How does this work tie into your goals of improving the speed of the CPython interpreter?
• What are some of the most interesting, unexpected, or challenging aspects of your efforts to bring this specification to CPython?
• How can the community contribute to this effort?

Keep In Touch

• markshannon on GitHub
• Website

Picks

• Tobias
  • American Gods book and TV series
• Mark
  • Roadside Picnic
  • In Death (VR game) –On Steam

Closing Announcements

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Links

• CPython
• PyPy
• PEP 380 yield from
• Language Summit
• RustPython
• Jython
• C++
• ML programming language
• Java
• Python Formal Semantics git repository
• CPython PEG Parser Episode with Pablo Galindo and Lysandros Nikolaou
• IETF RFCs

The intro and outro music is from Requiem for a Fish The Freak Fandango Orchestra / CC BY-SA