On Tue, 16 Feb 2010 21:09:27 -0800, John Nagle wrote:
> Yes, we're now at the point where all the built-in mutable types
> have "frozen" versions. But we don't have that for objects. It's
> generally considered a good thing in language design to offer, for user
> defined types, most of the functionality of built-in ones.
It's not hard to build immutable user-defined types. Whether they're
immutable enough is another story :)
>>> class FrozenMeta(type):
... def __new__(meta, classname, bases, classDict):
... def __setattr__(*args):
... raise TypeError("can't change immutable class")
... classDict['__setattr__'] = __setattr__
... classDict['__delattr__'] = __setattr__
... return type.__new__(meta, classname, bases, classDict)
...
>>>
>>> class Thingy(object):
... __metaclass__ = FrozenMeta
... def __init__(self, x):
... self.__dict__['x'] = x
...
>>>
>>>
>>> t = Thingy(45)
>>> t.x
45
>>> t.x = 42
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "<stdin>", line 4, in __setattr__
TypeError: can't change immutable class
It's a bit ad hoc, but it seems to work for me. Unfortunately there's no
way to change __dict__ to a "write once, read many" dict.
--
Steven
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