Ben Finney <[EMAIL PROTECTED]> wrote: > How can a (user-defined) class ensure that its instances are > immutable, like an int or a tuple, without inheriting from those > types?
You can make a good start by defining __setattr__, __delattr__ (and __setitem__ and __delitem__ if your class is a container) to raise exceptions. Of course, these restrictions can be easily worked around by a sufficiently determined attacker... but if you have to think of the user of your code as an attacker, you've got worse problems than this trifling one. Do not define any of the in-place operator special methods, such as __iadd__ and friends (or, if you're inheriting from a class that does define them, override them to raise exceptions). > What caveats should be observed in making immutable instances? Remember that your redefined __setattr__ IS "in place" even when you're initializing your istance, so remember to delegate attribute setting to the superclass (the other special methods mentioned above are less likely to byte you). You will probably want to define __hash__ and __eq__ if you're going to the trouble of making instances immutable. Alex -- http://mail.python.org/mailman/listinfo/python-list
