monkey_patch_exit.py

class CallsLeave:

    def __init__(self, cls, obj):
        self.cls = cls
        self.obj = obj
        
    def __call__(self, exc, val, tb):
        self.cls.__leave__(self.obj, val)
        
class CallsLeaveDescriptor:

    def __init__(self, cls):
        self.cls = cls

    def __get__(self, obj, objtype=None):
        return CallsLeave(self.cls, obj)
        
class ContextManagerUsingLeave:

    def __enter__(self):
        print("enter")
        return self

    def __leave__(self, exc):
        print("__leave__", self, exc)
        
# Patch in __exit__

ContextManagerUsingLeave.__exit__ = CallsLeaveDescriptor(ContextManagerUsingLeave)

with ContextManagerUsingLeave():
    pass

It should be the context manager class.

It's not so simple.

    def __get__(self, obj, objtype=None):
        return CallsLeave(self.cls, obj)

When we invoke this on a class and obj is None we need to do something else.