Is it possible to access exec-provided globals dictionary from within a function?

Question

Is it possible to access exec-provided globals dictionary from within a function, if the function was defined outside of the exec-ed code (and thus already bound to different __globals__)?

In other words, is there a way to make the following example work?

def f():
    log("Hi")

exec('f()', {'f': f, 'log': print})

In general, is it possible to substitute the __globals__ of a function?

Answer 1

This is a pretty weird thing to do, but it's doable.

Your exec call executes the statement f() in the provided globals. It does not execute the body of f in the provided globals. The provided globals are being used in the wrong stack frame. To access those globals from f, you can use stack inspection:

import inspect

def f():
    log = inspect.currentframe().f_back.f_globals['log']
    log('Hi')

exec('f()', {'f': f, 'log': print})

If you want to execute the body of f with the provided globals rather than just gaining access to the globals, you need to make a copy of f with your own custom globals:

import types
my_f = types.FunctionType(f.__code__,
                          {'log': print},
                          f.__name__,
                          f.__defaults__,
                          f.__closure__)
my_f()

The function type constructor is sort of documented; it's not in the online docs, but it is documented in the function type's docstring:

function(code, globals[, name[, argdefs[, closure]]])

Create a function object from a code object and a dictionary.
The optional name string overrides the name from the code object.
The optional argdefs tuple specifies the default argument values.
The optional closure tuple supplies the bindings for free variables.