File sketch to implement a "nonlocals()" call in Python 3.x (analog to "globals()" and "locals()" )

nonlocals.py

import sys
from collections.abc import MutableMapping

class DictFilteredView(MutableMapping):
    def __init__(self, viewed, allowed):
        self.viewed = viewed
        self.allowed = set(allowed)
    def __getitem__(self, item):
        if not item in self.allowed:
            raise KeyError(str(item))
        return self.viewed[item]
    
    def __setitem__(self, item, other):
        if not item in self.allowed:
            raise KeyError(str(item))
        self.viewed[item] = other
    
    def __delitem__(self, item):
        raise NotImplementedError
    
    def __iter__(self):
        for key in self.allowed:
            if key in self.viewed:
                yield self.viewed[key]
 
    def __len__(self):
        return len(self.allowed.intersection(self.viewed.keys()))

def nonlocals():
    frame = sys._getframe().f_back
    return DictFilteredView(frame.f_locals, frame.f_code.co_freevars)


# function for testing the thing on the console:
                                                                                                                                                                          
def a():                                                                                                                                                                  
    b = 0                                                                                                                                                                 
    def c():                                                                                                                                                              
        nonlocal b                                                                                                                                                        
        b += 1                                                                                                                                                            
        e = nonlocals()                                                                                                                                                   
        import ipdb; ipdb.set_trace()                                                                                                                                     
        e["b"] = 5                                                                                                                                                        
    def d():                                                                                                                                                              
        nonlocal b                                                                                                                                                        
        print(b)                                                                                                                                                          
    return c, d