Context: I was thinking about having embedding Python in a REPL with live
preview of evaluation result of pure functions. Similar to nodejs REPL or
Chrome DevTools (which uses {throwOnSideEffect: true} under the hood).
This is somewhat similar to sandboxing. I'm aware of tricks like
`object.__subclasses__` to get "hidden" types, and `type(func)(...)` to
construct functions from bytecode. Currently I'm approaching it slightly
differently and I didn't find previous discussions.
If we define "side effects" as effects outside the current process (ex.
filesystem), and allow mutation of in-process global state (ex. `sys.modules`).
Then it seems all pure Python code is side-effect free, and only those C
functions (ex. `open`) need to be checked. In this world, we allow arbitrary
pure Python code including constructing a Python function from bytecode, as
long as native functions are checked.
Looking at existing APIs, I think `PyEval_SetProfile` can be abused to achieve
that kind of "throw on side effect" easily - when the profile function gets a
`PyTrace_C_CALL` event, it can examines the C function and decide to raise if
necessary. The missing piece is to define the whitelist of side-effect free
native functions, such as `sorted`, `join`, etc.
I wonder what people think about this approach, and whether this is worth some
sort of core support (ex. native functions whitelist per `PyThreadState`,
builtin function to run another function in restricted mode). I can envision
this to be useful in IPython, potentially interesting to other applications
wanting a "declarative", "pure" scripting language, especially when the
embedding applications can define their types and tweak the list of side-effect
free functions.
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