Discussion

500 Lines or LessA Python Interpreter Written in Python

tekknolagi: See also https://github.com/nedbat/byterun and https://github.com/rocky/x-python

bjoli: 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: > And, in some ways, PyPyWhat do you mean by that? I'm not familiar with PyPy

nxpnsv: PyPy is python implemented in python. It is fast.

anitil: 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 tokeiI 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!

notpushkin: https://pypy.org/Not in a great shape right now: https://news.ycombinator.com/item?id=47293415But you can help! https://pypy.org/howtohelp.htmlhttps://opencollective.com/pypy

Doxin: PyPy is python implemented in RPython, which is technically a python subset. It's so restricted it might as well be a different language though.

bjoli: It is restricted in a way that you would restrict yourself to write high speed software in most languages, and I found it is not that restrictive compared to C that you would have to use if you were to write a fast Python library.

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

dnnddidiej: It already is that.

andltsemi3: "Yaw dog I heard you liked python, so I put python in your python so you can interpret python while you interpret python"

blueybingo: 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.

Doxin: oh for sure, but I still feel like telling people pypy is written in python is misleading. it's written in something significantly like python, but it's not python.

actionfromafar: 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.

BoppreH: > 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 compiler, each execution is "cached" in the form a compiled binary. The chain is only in the history of the binary, not part of the runtime.

vachanmn123: Very well written! Everyone used to tell me during Uni that stacks are used for running programs, never ACTUALLY understood where or how.

mjmas: > technically a python subsetSo it can just run under CPython? If so, then that isn't too misleading.

gwerbin: 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.

bjoli: Yes. It can run under Cpython (2.7).

gield: (2012)

SJC_Hacker: 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.

woadwarrior01: aka A Metacircular Interpreter

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

wyldfire: The fact that it's written in python is often brought up in order to explain its name. But really, it's much less interesting than the fact that it has a tracing JIT. If it were called PyJIT I'd bet it would be clearer and more obvious that it's fast. And people would prob get less hung up on the distinction between python/rpython.

throwpoaster: How does it define ‘\n’? ;)cf: https://www.cs.cmu.edu/~rdriley/487/papers/Thompson_1984_Ref...

jgbuddy: one liner:eval(str)

johndough: Are you a bot? All your recent comments point out a thing in an article and contain LLM-isms.

bdangubic: you asking a bot if it is a bot? :)

PhunkyPhil: I can do you one better:```python3from openai import OpenAIimport sysclient = OpenAI()response = client.chat.completions.create( model="gpt-4", messages=[{ "role": "user", "content": f"generate valid python byte code this program compiles to: {sys.argv[1]}" }] )print(response.choices[0].message.content)```Actually, probably not better.

direwolf20: Interpreters don't translate bytecode to native instructions.

SJC_Hacker: The usual understanding of "interpreter" in a CS context is program that executes source code directly without a compilation step. However the binary that translates an intermediate bytecode to native machine code is at least sometimes called a "bytecode interpreter".https://doc.pypy.org/en/latest/interpreter.html

em-bee: actually it was published as a chapter in "500 lines or less" in 2016: https://news.ycombinator.com/item?id=11796253the text is based on python 3.5 which was released in 2015other discussions:https://news.ycombinator.com/item?id=16795049https://news.ycombinator.com/item?id=12455104https://news.ycombinator.com/item?id=11796253

gield: Oops, I went by the publication date of the book

paulddraper: The key difference is that RPython is actually a compiled language.I.e. PyPy DOESN'T have an interpreter written in an interpreted language.

nasretdinov: Went into comments looking for this exact comment. Wasn't disappointed

_blk: Great minds think alike ;)

cestith: The article contrasts Python to Perl, saying Perl is purely interpreted while Python has compilation. This is factually incorrect.Perl is transformed into an AST. Then that is decorated into an opcode tree. The thing runs code nearly as fast as C in many instances, once the startup has completed and the code is actually running.

genxy: They do now https://github.com/bytecodealliance/weval

ghusbands: That's a partial evaluator, not an interpreter, and it converts an interpreter into compiler, which are different things.

blueybingo: haha no im not a bot, but starting to realise i sound like one. need to be less cynical.

bdangubic: exactly what the bot would say lol :)

genxy: > Interpreters don't translate bytecode to native instructions.> That's a partial evaluator, not an interpreter, and it converts an interpreter into compiler, which are different things.https://old.reddit.com/r/Compilers/comments/1sm90x5/retrofit...

ghusbands: Yes, that's another great example of the same kind of thing - creating a JIT from an interpreter. It remains true that interpreters do not directly generate machine code.

nxpnsv: PyRPy is just less catchy sounding

genxy: Reading the comments and understanding that transitively, weval turns interpreters into compilers, allowing interpreters to generate machine code.

direwolf20: If you turn milk into cheese it isn't milk any more, and it doesn't prove that milk is a yellow solid.

blueybingo: damn brain is becoming bot mush