A python implementation of the sealer/unsealer pattern from "E in a Walnut"

sealer_unsealer.py

# Returns two classes, a "Sealer", and an "Unsealer".
# Lets call the Sealer "s" and the Unsealer "u".
# For any pair of s and u returned from makeBrandPair, s.seal(value)
# (where "value" can be any python object) returns a boxed version of
# value that cannot be extracted unless you have a reference to the
# unsealer u (not 100% sure on this, but it seems to be true). 
# When you call u.unseal(box), the value originally sealed into the 
# box is returned. A separate call to makeBrandPair will return
# a different sealer/unsealer pair that only work with eachother.
def makeBrandPair():
    shared = [None]

    def makeSealedBox(obj):
        # This class holds a reference to the stored value
        class Box(object):
            @classmethod
            def unbox(kls):
                shared[0] = obj
        return Box()

    class Sealer(object):
        @classmethod
        def seal(kls, obj):
            return makeSealedBox(obj)

    class Unsealer(object):
        @classmethod
        def unseal(kls, box):
            # Make sure we're not accesing a value unboxed before
            shared[0] = None
            # Unbox our value
            box.unbox()
            # Make sure the box has a value
            if shared[0] is None: raise Exception("invalid box")
            # Get the value we just shared
            contents = shared[0]
            # Kill the shared value 
            shared[0] = None
            # return a reference to the shared value
            return contents

    return Sealer, Unsealer

s, u = makeBrandPair()
box = s.seal("Hello World")
print u.unseal(box)

s2, u2 = makeBrandPair()
print "Should throw exception: u2.unseal(box)"
print u2.unseal(box)