Access the locals available in the previous stack frame

Question

I have a debug context manager where I would like to access the locals() at the time the context manager was initiated, without giving the locals as an argument. Is this possible?

I would like to do this in the general case, so that my Debug context manager can be used from any file importing Debug, not just in the tinkertoy example below.

Here is my minimal example:

import inspect

class Debug:
    def __init__(self):

        frames = inspect.stack()

        for frame in frames:
            line = frame.code_context[0]
            if "Debug" in line:
                break

        # I want to get the locals() at the time debug was called here!
        # give me i_will_be_in_the_locals
        raise Exception()

    def __enter__(self):
        pass

    def __exit__(self, exc_type, exc_val, exc_tb):
        pass


if __name__ == "__main__":

    i_will_be_in_the_locals = 42
    with Debug():
        "hi"

What's wrong with what you have already tried,? Is above the complete code? — DisappointedByUnaccountableMod, Apr 15 '19 at 22:39
I would like to access the locals() where Debug() was called where the Exception is now thrown. For that, you need to use insepect somehow, I'm sure, but I have been unable to figure out how. — The Unfun Cat, Apr 15 '19 at 22:43

score 4 · Accepted Answer · answered Apr 15 '19 at 22:55

The frame object is inside the "frame" variable you defined. To get the local variables for a frame object, you can call its f_locals attribute like this:

import inspect

class Debug:
    def __init__(self):

        frames = inspect.stack()

        for frame in frames:
            line = frame.code_context[0]
            if "Debug" in line:
                break

        # I want to get the locals() at the time debug was called here!
        # give me i_will_be_in_the_locals
        from pprint import pprint
        pprint(frame.frame.f_locals)

    def __enter__(self):
        pass

    def __exit__(self, exc_type, exc_val, exc_tb):
        pass


if __name__ == "__main__":

    i_will_be_in_the_locals = 42
    with Debug():
        "hi"

The returned value is:

{'Debug': <class '__main__.Debug'>,
 '__builtins__': <module 'builtins' (built-in)>,
 '__cached__': None,
 '__doc__': None,
 '__file__': '/home/user1/main-projects/overflow/file.py',
 '__loader__': <_frozen_importlib_external.SourceFileLoader object at 0x7f7bbb44f7f0>,
 '__name__': '__main__',
 '__package__': None,
 '__spec__': None,
 'i_will_be_in_the_locals': 42,
 'inspect': <module 'inspect' from '/usr/lib/python3.5/inspect.py'>}

Wonderful. Will look into it and see if this works in the general case and then accept :) — The Unfun Cat, Apr 16 '19 at 05:03
Thanks. I did try `f_locals` yesterday, but must have been confusing myself with the double `frame` (`frame.frame`). I probably did `frame.f_locals` XD. — The Unfun Cat, Apr 16 '19 at 05:48

Access the locals available in the previous stack frame

1 Answers1