warn when deprecated dict key is accessed

sample.py

class Foo(object):
    def __init__(self):
        self.data = DelegateDict()
        self.data.set_delegation('py:class', 'py:class_stack')

warn.py

#: dict wrapper to warn deprecation
class DelegateDict(dict):

    def __init__(self, *args, **kw):
        super(DelegateDict, self).__init__(*args, **kw)
        self._delegation = {}
        self._warns = {}

    def set_delegation(self, from_, to_, warn=DeprecationWarning):
        self._delegation[from_] = to_
        self._warns[from_] = warn

    def _delegate(self, key):
        new_key = self._delegation.get(key, key)
        warn = self._warns.get(key)
        if warn:
            if True:  # debug
                print('access to {} from {}'.format(
                    key, sys._getframe().f_back.f_back.f_code.co_name))
            warnings.warn('Use {} instead of {}'.format(new_key, key),
                          self._warns[key], stacklevel=3)
        return new_key
    _d = _delegate

    def __delitem__(self, key):
        super(DelegateDict, self).__delitem__(self._d(key))

    def __getitem__(self, key):
        return super(DelegateDict, self).__getitem__(self._d(key))

    def __setitem__(self, key, value):
        return super(DelegateDict, self).__setitem__(self._d(key), value)

    def get(self, key, default=None):
        return super(DelegateDict, self).get(self._d(key), default)

    def setdefault(self, key, default=None):
        return super(DelegateDict, self).setdefault(self._d(key), default)

    def update(self, e=None, **f):
        raise NotImplementedError