A Python Interpreter Written in Python

BoppreH•about 4 hours ago

> Byterun is a Python interpreter written in Python. This may strike you as odd, but it's no more odd than writing a C compiler in C.

I'm not so sure. The difference between a self-hosted compiler and a circular interpreter is that the compiler has a binary artifact that you can store.

With an interpreter, you still need some binary to run your interpreter, which will probably be CPython, making the new interpreter redundant. And if you add a language feature to the custom interpreter, and you want to use that feature in the interpreter itself, you need to run the whole chain at runtime: CPython -> Old Interpreter That Understand New Feature -> New Interpreter That Uses New Feature -> Target Program. And the chain only gets longer, each iteration exponentially slower.

Meanwhile with a self-hosted compiler, each iteration is "cached" in the form a compiled binary. The chain is only in the history of the binary, not part of the runtime.

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Edit since this is now a top comment: I'm not complaining about the project! Interpreters are cool, and this is genuinely useful for learning and experimentation. It's also nice to demystify our tools.

gwerbin•about 3 hours ago

PyPy handled this by implementing PyPy in a restricted minimal subset of Python that they called RPython, and that seemed to work out well for them.

SJC_Hacker•about 2 hours ago

This is the case only if the new interpreter does not simply include the layer that the old interpreter has for translating bytecode to native instructions. Once you have that, you can simply bootstrap any new interpreters from previous ones. Even in the case of supporting new architectures, you can still work at the Python level to produce the necessary binary, although the initial build would have to be done on an already supported architechture.

anitil•about 8 hours ago

Oooh it's a bytecode interpreter! I was wondering how they'd fit a parser/tokenizer in 500 lines unless the first was `import tokenizer, parser`. And it looks like 1500ish lines according to tokei

I think because python is a stack-based interpreter this is a really great way to get some exposure to how it works if you're not too familiar with C. A nice project!

blueybingo•about 5 hours ago

the article glosses over something worth pausing on: the `getattr` trick for dispatching instructions (replacing the big if-elif chain) is actaully a really elegant pattern that shows up in a lot of real interpreters and command dispatchers, not just toy ones -- worth studying that bit specifically if you're building anything with extensible command sets.

throwpoaster•32 minutes ago

How does it define ‘\n’? ;)

cf: https://www.cs.cmu.edu/~rdriley/487/papers/Thompson_1984_Ref...

vachanmn123•about 4 hours ago

Very well written! Everyone used to tell me during Uni that stacks are used for running programs, never ACTUALLY understood where or how.

tekknolagi•about 8 hours ago

bjoli•about 8 hours ago

And, in some ways, PyPy. I still think it is the sanest way to implement Python.

It makes me sad that I have to write C to make any meaningful changes to Python. Same goes for ruby. Rubinius was such a nice project.

Hacking on schemes and lisps made me realize how much more fun it is when the language is implemented in the language itself. It also makes sure you have the right abstractions for solving a bunch of real problems.

anitil•about 8 hours ago

> And, in some ways, PyPy

What do you mean by that? I'm not familiar with PyPy

nxpnsv•about 8 hours ago

PyPy is python implemented in python. It is fast.

actionfromafar•about 5 hours ago

Well, one could rewrite Python (perhaps piece by piece?) in Shedskin.

Shedskin is very nearly Python compatible, one could say it is an implementation of Python.

gield•about 2 hours ago

(2012)

woadwarrior01•about 6 hours ago

aka A Metacircular Interpreter

mapontosevenths•about 2 hours ago

Do you think God stays in heaven because he too lives in fear of what he's created?

andltsemi3•about 5 hours ago

"Yaw dog I heard you liked python, so I put python in your python so you can interpret python while you interpret python"

hcfman•about 7 hours ago

Just wondering why you stopped there? Why not a python interpreter for a python interpreter for python ?

dnnddidiej•about 6 hours ago

It already is that.

A Python Interpreter Written in Python

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