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Mon Feb 16 09:37:38 EST 2004 · http://mail.python.org/pipermail/python-dev/2004-February/042578.html

[Tim Peters]
> >> Type objects are also singletons (e.g., type(True) returns the same
> >> object as type(False); ditto type("True") and type("False") and
> >> type("xyz"); etc).

> [Guido]
> > But types are not immutable, so their semantics are different anyway.

> I never thought of type objects as being mutable [...]

I'm not sure of the meaning of "immutability of a type".  Looking
around, I found out this other message from Guido (quoted in its
entirety in case the context changes the wanted meaning of words):

---------------------------------------------------------------------->
From: Guido van Rossum <guido at python.org>
Date: Thu, 18 Oct 2001 21:00:28 GMT
Subject: Re: hashval and Numpy
To: python-list at python.org
Newsgroups: comp.lang.python

> Michael Hudson <mwh at python.net> wrote:
> 
> |>>> a = Numeric.array([1,2])
> |>>> hash(a)
> | 56
> |>>> d = {a:1} # a will be in the 56%8-th slot (in 2.2, anyway)
> |>>> a[0] = 2
> |>>> hash(a)
> | 57
> |>>> d[a] # looks in 57%8-th slot - which is empty
> | KeyError
> |>>> b = Numeric.array([1,2])
> |>>> hash(b)
> | 56
> |>>> d[b] # looks in 56%8-th slot - but what's there is not __eq__ to b!
> | KeyError
> 
> | Moral: don't use mutable objects as dictionary keys.

Hm.  The Numeric.array object should not have defined __hash__ or
tp_hash.  It's wrong if a mutable object defines a hash function, for
exactly this reason.
----------------------------------------------------------------------<

However, all types derive from object, and object has `__hash__'.
Consequently, I would be tempted to think that under the new system,
everything deriving from object is immutable by construction, unless
tricks are used against it, like maybe, an intermediate class overriding
`__hash__' with some function with the specific goal of raising an
exception.  Is that so?

What means a "mutable type" then?

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[Python-Dev] bool does not want to be subclassed?