Dynamically patch a Python function's source code at runtime
58 comments
·August 24, 2025breuleux
diggan
> Also, why is every damn post these days somehow framed in an AI context? It's exhausting.
Every 5/10-year segment of my life has somehow had one or two "This is the future"-hypes running concurrently with my life. Previously it was X, now it's Y. And most of the times, everything else is somehow connected to this currently hyped subject, no matter if it's related or not.
The only thing I've found to be helpful is thinking about and changing my perspective and framing about it. I read some article like this which is just tangentially related to AI, but the meat is about something else. So mentally I just ignore the other parts, and frame it in some other way in my head.
Suddenly people can write their articles with attachments to the hyped subject but I don't mind, I'm reading it for other purposes and get other takeaways that are still helpful. A tiny Jedi mind-trick for avoiding that exhaustion :)
Twirrim
AI, block chain, rust, go, serverless, nosql, ruby on rails..... The list goes on and on :-)
Some of it gets really annoying on the business side, because companies like Gartner jump on the trends, and they have enough influence that businesses have to pay attention. When serverless was a thing, every cloud provider effectively had to add serverless things even if it made zero sense and no customers were asking for it, simply to satisfy Gartner (and their ilk) and be seen as innovating and ahead of the curve. Same thing happened with block chain, and is currently happening with AI.
quantum_state
wheels get reinvented again again again and again … this is quite unique to info tech … imagine mathematicians do the same, the world would be in chaos …
dkdcio
I also find it useful to keep in mind (often more junior) people are learning new things and expressing their joy, which is a good thing. and most (junior) people learning things in tech right now are doing so in the context of AI, for better or worse
(idk if this author is “junior” per se, mostly just agreeing the shift in perspective is helpful to not get burnt out by things like this)
sczi
Oh that is really interesting, I was just aware of IPython's autoreload extension, I hadn't found your library. I'm also working on hot reload for python as I'm working on a development environment for python that aims to give it a development experience closer to lisp: https://codeberg.org/sczi/swanky-python/
Some minor details. You currently aren't updating functions if their freevars have changed, you can actually do that by using c-api to update __closure__ which is a readonly attribute from python:
ctypes.pythonapi.PyFunction_SetClosure.argtypes = [ctypes.py_object, ctypes.py_object]
ctypes.pythonapi.PyFunction_SetClosure(old, new.__closure__)
Rather than using gc.get_referrers I just maintain a set for each function containing all the old versions (using weakref so they go away if that old version isn't still referenced by anything). Then when a function updates I don't need to find all references, all references will be to some old version of the function so I just update that set of old functions, and all references will be using the new code. I took this from IPython autoreload. I think it is both more efficient than gc.get_referrers, and more complete as it solves the issue of references "decorated or stashed in some data structure that Jurigged does not understand". The code for that is here: https://codeberg.org/sczi/swanky-python/src/commit/365702a6c...
hot reload for python is quite tricky to fully get right, I'm still missing plenty parts that I know about and plan on implementing, and surely plenty more that I don't even know. If you or anyone else that's worked on hot reload in python wants to talk about it, I'm happy to, just reach out, my email is visible on codeberg if you're signed in.
breuleux
Thanks for the tips, I'll try to look into these when I get some time! Didn't know you could modify the closure pointer.
I'm not sure what you mean by "maintaining a set of old versions". It's possible I missed something obvious, but the issue here is that I have the code objects (I can snag them from module evaluation using an import hook)... but I do not have the function objects. I never had them in the first place. Take this very silly and very horrible example:
def adder(x):
def inner(y):
return x / y
return inner
adders = {}
def add(x, y):
adders.setdefault(x, adder(x))
return adders[x](y)
modeless
Jurigged is awesome. It works really well and saves me tons of time. Thank you for making it!
I do wish there were callbacks I could subscribe to that would notify me whenever my file changed or whenever any code changed, so I could re-run some init.
My other feature request would be a way to replace function implementations even when they are currently on the stack, as some other hot reload implementations can. But I certainly understand why this would be difficult.
frou_dh
In some respects that's even nicer to use than a typical editor-integrated live-coding REPL, because one doesn't have to think about what code needs (re)sending from the source to the REPL. Just save the file and it'll be figured out which parts meaningfully changed.
Esophagus4
> why is every damn post these days somehow framed in an AI context? It's exhausting.
It’s even in the real world now - most of my conversations with people in tech end up at AI eventually.
It kind of reminds me of the 2010s when non-tech people would ask me about crypto at social events.
johnfn
Jurigged is really cool - thanks for the tool!
zipy124
jurigged is great, love using it for quick GUI prototyping with imgui!
gjvc
I use jurigged in conjunction with cmd2 to make command line driven tools. The combination works well because I can issue a command, watch the output, make a change, hit up-return and see the change just like that.
Thank you a bazillion for making it. It works quietly in the background without fuss, and I'm grateful for it every time I use it.
aeonik
And wouldn't it be nice if that Python code, instead of a string, was just more python? Then you could use your existing Python code to append, or transform sections of your code!
That's what Lisp is!
Once you see how cool that is, then you can begin to appreciate why Lisp was the defacto standard for AI programing all the way back in the 1960s!
almostgotcaught
diggan
Ah, so in Python, you have "normal code" then you have AST code. Imagine that they were exactly the same, and whenever you're writing "normal code", you're at the same time writing AST code and vice-versa.
So whenever you want, you can start using "normal code" for manipulating the "normal code" itself, and hopefully now we have yet another perspective on the same thing, for why Lisps are so awesome :)
oefrha
S-expressions are a nice gimmick, but it’s not the fundamental reason why some Lisps support dynamic patching of code at runtime. Indeed you can easily imagine Lisp written in M-expressions and still support that. Or you can imagine other dynamic languages with reflection gaining this capability with a bit of will, like… check notes… Python.
breuleux
I think there is potential to do something like that with template strings [1] (upcoming feature in Python 3.14). The choice of {} for interpolation isn't ideal because any code with dict and set literals becomes super awkward, but besides that, it could be super interesting for codegen.
heap_perms
I get that it works, and it's actually pretty cool that you can do this. But honestly, it feels like it would good, readable code into a tangled mess pretty fast.
charcircuit
There is not 2 different kinds of code. That's like saying Lisp has normal code and list code.
westurner
Is there a good way to verify self-modifying code - in Lisp, or Combo (MOSES), or Python - at runtime against a trusted baseline at loader time?
Dynamic metaprogramming is flexible but dangerous. Python is also "dynamic", meaning that code can be changed at runtime instead of only being able to accidentally pass null function pointers.
Python's metaclasses function similarly to Lisp's macros but in a consistent way: most Python code uses the standard metaclasses so that the macros don't vary from codebase to codebase. The Django "magic removal" story was about eliminating surprise and non-normative metaclassery, for example.
Does this tool monkey patch all copies of a function or just the current reference? There are many existing monkey patching libraries with tests
almostgotcaught
Ok now do the part where lisp is actually used for any remotely useful project today...
miohtama
Also check the history of monkey patching
zekrioca
Other than Lisp, is it possible to do this in other languages?
OskarS
Several! You're correct that Lisps is the most famous, and there's also languages like Erlang that have this as a core functionality. But it's also used in things like game engines for C/C++. You do have your "updateAndRenderFrame()" function in a dynamic library, having it take a pointer to the full game state as an argument. When you want to reload, you recompile your dynamic library, and the main loop can swap out the implementation, and the game keeps running with the new code. I don't see a reason why you couldn't do this in Rust, though I imagine it's trickier.
woadwarrior01
Greenspun's tenth rule, restated for Python instead of C/Fortran.
duttish
I used live patching of the function byte code to enforce type safety in python as an experiment. It was quite fun, took about a weekend or so :) not something for production though, due to the performance hit.
ath3nd
There is a reason similar approaches are called 'monkey patching'.
Just cause you can do something doesn't mean you should. I send thoughts and prayers for the people debugging programs where this is in place.
dfee
Different, but you may find interest in changing function signatures, too: https://github.com/dfee/forge
zzzeek
I really couldn't follow the use case. it looked like, they have a chain of method calls with some kind of `mark_circle().encode().properties()`, OK, so, if you want to make those methods do something different, you reach into OOP wisdom from the 1980s and write an appropriate "impl" used by whatever `alt.Chart()` is.
Someone explain to me, an old, aging programmer old enough to know UML, why this isn't some we presume very young person who has no idea how to write OOP coming up with some horrible convoluted way to do something routine?
porridgeraisin
> aging ... UML
Oh don't worry... they still cram that down our throats in CS undergrad in one of the courses.... Forget which one. I did my UG from 2020-24
;)
tovej
Ah, self-modifying code, the more things change the more they stay the same.
Wasn't SMC one of the LISP-associated AI fields a few decades ago? iirc it's been mostly abandoned due to security issues, but some of it survives in dynamic compilation.
thrown-0825
this will be the next big breakthrough for agents lmao
edem
wait a sec ... i thought you could monkey patch python code. you can only do it by using this technique?
null
rob_c
Aka I just learned how to modify python code whilst it's running.
Next step, here's how to load modules, resolve a dependency. Handle capabilities and dynamically inject more functionality 'live'.
Patching running machine code in memory for compiled objects is the same but you just need to work around the abstraction that is introduced by languages trying to make the whole stack human parseable.
Yeah, I've been using this trick to implement a hot reload library, to only update the specific functions that are changed without re-executing modules: https://github.com/breuleux/jurigged
I also use it in a multiple dispatch library (https://github.com/breuleux/ovld) to replace the entry point by specialized dispatch code in order to cut some overhead.
It's fun.
Also, why is every damn post these days somehow framed in an AI context? It's exhausting.