"""Bastionification utility.

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A bastion (for another object -- the 'original') is an object that has

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the same methods as the original but does not give access to its

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instance variables. Bastions have a number of uses, but the most

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obvious one is to provide code executing in restricted mode with a

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safe interface to an object implemented in unrestricted mode.

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The bastionification routine has an optional second argument which is

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a filter function. Only those methods for which the filter method

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(called with the method name as argument) returns true are accessible.

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The default filter method returns true unless the method name begins

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with an underscore.

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There are a number of possible implementations of bastions. We use a

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'lazy' approach where the bastion's __getattr__() discipline does all

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the work for a particular method the first time it is used. This is

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usually fastest, especially if the user doesn't call all available

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methods. The retrieved methods are stored as instance variables of

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the bastion, so the overhead is only occurred on the first use of each

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method.

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Detail: the bastion class has a __repr__() discipline which includes

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the repr() of the original object. This is precomputed when the

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bastion is created.

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"""

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from warnings import warnpy3k

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warnpy3k("the Bastion module has been removed in Python 3.0", stacklevel=2)

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del warnpy3k

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__all__ = ["BastionClass", "Bastion"]

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from types import MethodType

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class BastionClass:

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"""Helper class used by the Bastion() function.

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You could subclass this and pass the subclass as the bastionclass

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argument to the Bastion() function, as long as the constructor has

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the same signature (a get() function and a name for the object).

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"""

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def __init__(self, get, name):

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"""Constructor.

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Arguments:

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get - a function that gets the attribute value (by name)

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name - a human-readable name for the original object

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(suggestion: use repr(object))

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"""

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self._get_ = get

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self._name_ = name

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def __repr__(self):

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"""Return a representation string.

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This includes the name passed in to the constructor, so that

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if you print the bastion during debugging, at least you have

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some idea of what it is.

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"""

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return "<Bastion for %s>" % self._name_

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def __getattr__(self, name):

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"""Get an as-yet undefined attribute value.

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This calls the get() function that was passed to the

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constructor. The result is stored as an instance variable so

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that the next time the same attribute is requested,

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__getattr__() won't be invoked.

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If the get() function raises an exception, this is simply

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passed on -- exceptions are not cached.

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"""

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attribute = self._get_(name)

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self.__dict__[name] = attribute

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return attribute

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def Bastion(object, filter = lambda name: name[:1] != '_',

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name=None, bastionclass=BastionClass):

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"""Create a bastion for an object, using an optional filter.

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See the Bastion module's documentation for background.

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Arguments:

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object - the original object

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filter - a predicate that decides whether a function name is OK;

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by default all names are OK that don't start with '_'

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name - the name of the object; default repr(object)

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bastionclass - class used to create the bastion; default BastionClass

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"""

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raise RuntimeError, "This code is not secure in Python 2.2 and later"

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# Note: we define *two* ad-hoc functions here, get1 and get2.

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# Both are intended to be called in the same way: get(name).

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# It is clear that the real work (getting the attribute

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# from the object and calling the filter) is done in get1.

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# Why can't we pass get1 to the bastion? Because the user

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# would be able to override the filter argument! With get2,

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# overriding the default argument is no security loophole:

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# all it does is call it.

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# Also notice that we can't place the object and filter as

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# instance variables on the bastion object itself, since

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# the user has full access to all instance variables!

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def get1(name, object=object, filter=filter):

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"""Internal function for Bastion(). See source comments."""

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if filter(name):

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attribute = getattr(object, name)

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if type(attribute) == MethodType:

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return attribute

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raise AttributeError, name

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def get2(name, get1=get1):

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"""Internal function for Bastion(). See source comments."""

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return get1(name)

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if name is None:

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name = repr(object)

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return bastionclass(get2, name)

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def _test():

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"""Test the Bastion() function."""

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class Original:

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def __init__(self):

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self.sum = 0

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def add(self, n):

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self._add(n)

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def _add(self, n):

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self.sum = self.sum + n

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def total(self):

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return self.sum

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o = Original()

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b = Bastion(o)

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testcode = """if 1:

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b.add(81)

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b.add(18)

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print "b.total() =", b.total()

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try:

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print "b.sum =", b.sum,

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except:

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print "inaccessible"

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else:

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print "accessible"

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try:

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print "b._add =", b._add,

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except:

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print "inaccessible"

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else:

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print "accessible"

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try:

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print "b._get_.func_defaults =", map(type, b._get_.func_defaults),

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except:

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print "inaccessible"

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else:

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print "accessible"

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\n"""

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exec testcode

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print '='*20, "Using rexec:", '='*20

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import rexec

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r = rexec.RExec()

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m = r.add_module('__main__')

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m.b = b

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r.r_exec(testcode)

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if __name__ == '__main__':

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_test()

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