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Comment by teraflop | Hacker News Reader

A caveat: this example shows that you can read the value of a local variable, not that you can modify it. If you try actually assigning to an element of f_locals, as opposed to just dereferencing it, you'll find that the modification doesn't take effect due to optimizations in CPython's bytecode interpreter.

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there's a secret hack to get your `f_locals` changes to propogate back

        exec(
            code_fragment,
            frame.f_globals,
            frame.f_locals
        )
        # update locals  
        ctypes.pythonapi.PyFrame_LocalsToFast(ctypes.py_object(frame), ctypes.c_int(0))

This sets the values back into the right place on the frame you were operating on.

https://pydev.blogspot.jp/2014/02/changing-locals-of-frame-f...

But tuples (and cells) are mutable in pure CPython if you believe strongly enough.

See: https://gist.github.com/llllllllll/b1ac68a6b77535a64c0b13bfd... https://gist.github.com/dutc/d2ff9f17a0520d233b2d197d1dafa1a... https://twitter.com/__qualname__/status/867928378040438784

That's a good point, I modified the object to which the variable was referring to (session), not the local variable itself.

Maybe they're running an alternative Python implementation? There are quite a few.

No, I'm saying that the OP's code isn't actually modifying a local variable, because it never assigns to an element of the f_locals dictionary.

To further clarify, it modifies the value of an object referenced by a local variable.

Or rather, it modifies an attribute on the object referenced by a local variable.

To further muddy things, it invokes the setter method on the object referenced by a local variable with a certain argument.

You were referring to CPython implementation details, which do not apply to Jython, IronPython, PyPy, and many more.

Alternative Python implementations do not follow all CPython implementation details.

In other words, if this is not part of the Python specification, it might work on other Python implementations even if it does not work on CPython.

> ...Jython...

Which is Java with Python syntax...

No. Jython is a full implementation of Python in Java.

Last I collided with it, it was terribly slow, but that is not the point here.

Some of these alternative implementations are very interesting. PyPy is JIT'ed, and insanely fast compared to CPython. IronPython, IIRC, has no GIL.

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