Issue 1562: Decimal can't be subclassed useful

The Decimal class doesn't use lookups through self to construct results 
in functions like __add__ to generate the resulting object. This makes 
subclassing Decimal more or less senseless since all methods have to be 
wrapped instead of overriding the __new__ and __init__ methods, which 
could be enough for immutable type.
Currently I'm implementing a Money class which is more or less a 
Decimal with addition currency information. Because resulting Types 
generated with something like return Decimal(something) instead of 
self.__new__(...)

Can you create a patch that replaces Decimal with self.__class__ and the
string "Decimal" with "%s ..." % self.__class__.__name__? 

Thanks :)

It's not clear to me that this would be the right behaviour.  Unless I'm 
missing something, Decimal behaves in just the same way as types like 
int, float and str in this respect:

>>> class myint(int): pass
... 
>>> a = myint(2)
>>> b = myint(3)
>>> a+b
5
>>> type(_)
<type 'int'>

Tim Peters had something to say on this subject at:

http://mail.python.org/pipermail/python-list/2005-January/300791.html

Mark is right, the current behaviour is correct. See Tim post for a
longer explanation.

As general help, for the money class take a look of this:
http://sourceforge.net/projects/pymoney

There you'll see that the approach was to store the value, not subclass
Decimal, as the other currency info makes math ugly. For example:
  
  Money("15.30", currency="USD") + Money("2.33", currency="ARS")

There's a lot of discussion for a Money data type, also, in python-list
and python-dev (I even proposed a pre-PEP), but then Decimal was
implemented; check those lists at the fourth quarter of 2003 for these
posts.

Regards,