Change what dictionary serves as a function's global scope

Question

I want to make an @pure decorator for Python, part of this is being able to selectively disallow access to the global scope of the function.

Is there a way to programmatically change which dictionary thing serves as a function's global/external scope?

So for instance in the following I want to be able to intercept the access to f in h and throw an error, but I want to allow access to g because it's a pure function.

def f():
    print("Non-pure function")

@pure
def g(i):
    return i + 1

@pure
def h(i):
    f()
    return g(i)

Answer 1

You would have to create a new function object from the old one:

newfunc = type(h)(h.__code__, cleaned_globals, h.__name__, h.__defaults__, h.__closure__)

Here, cleaned_globals is a dictionary that is to be used as the global namespace for the newly created function object. All other arguments echo the original function's.

cleaned_globals could be based on a copy of h.__globals__, of course.

Demo:

>>> def h(i):
...     f()
...     return g(i)
... 
>>> def g(i):
...     return i + 1
... 
>>> def f():
...     print("Non-pure function")
... 
>>> h(1)
Non-pure function
2
>>> cleaned_globals = {'g': g}
>>> newfunc = type(h)(h.__code__, cleaned_globals, h.__name__, h.__defaults__, h.__closure__)
>>> newfunc(1)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 2, in h
NameError: global name 'f' is not defined
>>> cleaned_globals['f'] = lambda: print('Injected function')
>>> newfunc(1)
Injected function
2