py3_interface_encapsulation_example.py

# Copyright 2023 Google LLC.
# SPDX-License-Identifier: Apache-2.0

# This snippet of Py3 code shows a way to pass a restricted object to a callee,
# so as to prevent the callee from wandering outside the walled garden of callbacks
# specified by a given mixin. It does not secure the original instance from tampering,
# but it removes all doubt about which methods are expected to be accessed and used
# by the callee.
#
# This assumes the lifetime of the original instance will exceed that of the
# restricted object. There may be other assumptions inherent here, as well.
# Use at your own risk. Constructive feedback welcome.

import inspect

def create_restricted_binding(instance, mixin):
    class RestrictedBinding(object):
        pass

    o = RestrictedBinding()

    for name, function in inspect.getmembers(mixin, predicate=inspect.isfunction):
        bound_method = getattr(instance, name)
        setattr(o, name, bound_method)

    return o


class PeasantMixin():
    def peasantMethod(self): return 0

class Snooty(PeasantMixin):
    def __init__(self, x): self._x = x
    def snootyMethod(self): return self._x

snoot = Snooty(3)
peasant = create_restricted_binding(snoot, PeasantMixin)

print(peasant.peasantMethod()) // Prints "0"
print(peasant.snootyMethod())  // Raises AttributeError, as desired