Restricting methods in derived classes

Mark McEahern marklists at
Thu Sep 12 19:17:31 EDT 2002

[Huaiyu Zhu]
> I was hoping for a way to confirm wanted methods, because there are more
> methods that I don't want.  But the above is good enough for preventing
> obvious mistakes.  Thank you.

Have you considered a metaclass solution?  Here's a half-baked attempt that
doesn't work and merely serves to demonstrate how weak my grasp of Python
is.  If Alex Martelli is in the house, perhaps he can shed some light.  :D

Here's some classic Martelli speak on metaclasses for ya:

Meanwhile, my lame attempt...

#!/usr/bin/env python

import inspect

def make_unwanted(cls, method_name):
    def unwanted(self, *args, **kwargs):
        raise AttributeError(cls.__name__, method_name)
    return unwanted

class Restricted(type):

    def __init__(cls, name, bases, dict):
        for base in bases:
            for method in inspect.getmembers(base,
                if not method[0].startswith('__') and method[1] not in
                    dict[method[0]] = make_unwanted(cls, method[0])
        super(Restricted, cls).__init__(name, bases, dict)

class RestrictedDict(dict):

    __metaclass__ = Restricted

    wanted_methods = [dict.update]

d = RestrictedDict()
d['a'] = 1

# I expect the following to blow up, but alas, they don't...
for key in d.keys():
    print key

d.update({'a': 2})

print d.items()

# Will this give me any clue?
print dir(d)

# Time to go listen to some good music...

// m

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