__name__ on new-style classes

[Guido van Rossum]

"Gonçalo Rodrigues" wrote:
> 
> On Mon, 01 Apr 2002 02:35:47 GMT, Guido van Rossum <guido at python.org>
> wrote:
> 
> >It's a feature.  The reason for the change is that I've never seen an
> >example of a use for it (as opposed to changing an instance's __class__).
> >If you have one, let us know.
> 
> I bumped into this "feature" a few days ago trying to understand
> metaclasses in Python. And to understand them there is nothing better
> than actually coding one, so I'll explain my problem/solution and let
> you judge if this is a good use case for having a writable __name__.
> 
> My problem is modelling finite arithmetic in Python. Here by finite
> arithmetic I mean all the quotients of the integer ring Z. There is a
> countable number of them, one for each n > 1 (n=0 is Z and n=1 is the
> trivial ring {0}. If n is negative then it is the same as -n). If n is
> prime then we actually have a field.
> 
> The obvious way (to me, that is) to do this is to let each finite ring
> Z_n be a class, inheriting from int. Since there are an infinite number
> of these we have to have a metaclass, call it Field, that has as
> intances the Z_n rings, e.g. Z_n would be Field(n). The skeleton of this
> metaclass would be something like:
> 
> class Field(type):
>     """The Finite Field metaclass."""
> 
>     def __new__(cls, n):
>         if isinstance(n, int) and n > 1:
>             #The template class.
>             class Z(int):
>                 pass
> 
>             #Generate proper name for the instance class, e.g. "Z_n"
>             Z.__name__ = "Z_%d" % n
> 
>             #Return class.
>             return Z
>         else:
>             raise TypeError, "%s is not an integer > 1." % str(n)
> 
> If I cant write to __name__ then all the instances will have the same
> name... Not A Good Thing, IMHO.

But you're doing the meta-instantiation all wrong.  Rather than using a
class
statement, you should be using cls.__new__(cls, "Z_%d"%n, (int,), {...}).

--Guido van Rossum (home page: http://www.python.org/~guido/)