The future of "frozen" types as the number of CPU cores increases

Steven D'Aprano steven at
Wed Feb 17 08:35:49 CET 2010

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
>>> 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.


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