Returning a list/dict as "read-only" from a method ?

[Sean Ross]

or, you might try this:

class ReadOnlyDict(dict):
    def __setitem__(self, key, value):
        self.setdefault(key, value)
    def update(self, _dict):
        for key, val in _dict.items():
            self[key] = val

This subclass of dict will allow full access, but modification is restricted
to adding new key, value pairs.

For instance,

m = ReadOnlyDict()
m['hello'] = 'world'
m['you'] = 'me'
m['hello'] = 'universe'

for key in m:
    print "m[%s]: %s" % (key, m[key])

will give you:

m[you]: me
m[hello]: world

and,

b = {'hello':'universe', 'hey':'there'}
m.update(b)
for key in m:
    print "m[%s]: %s" % (key, m[key])

will give you:

m[hey]: there
m[you]: me
m[hello]: world

You'll notice that I did not raise an Exception when trying to modify an
existing key's value.
You probably should raise one in __setitem__():
    def __setitem__(self, key, value):
        if not self.setdefault(key, value) == value:
            raise Exception("Error: ReadOnlyDict attributes cannot be
modified")

(Note: if you don't want to allow people to add new key, value pairs, then
you'll need to make a constructor
and __setitem__() and update() should probably raise exceptions whenever
they are called)

ok. hope that's useful,
sean