[Python-Dev] Python and Coercion

[Patrick Maupin]

Sorry about the HTML.  I _hate_ the way they
configure things at work -- I have to remember
to force text, and can't get around the stoopid
legal disclaimer at the bottom.

I'll try to remember not to post from here again.

Best regards,
Pat

-----Original Message-----
From: Guido van Rossum [mailto:guido at esi.elementalsecurity.com]
Sent: Monday, October 13, 2003 3:39 PM
To: Patrick Maupin
Cc: pyython-dev at python.org
Subject: RE: [Python-Dev] Python and Coercion


This is a feature.  It is mentioned in passing in http://www.python.org/2.2.2/descrintro.html :

 """Note that while in general operator overloading works just as for classic classes, there are some differences. (The biggest one is the lack of support for __coerce__; new-style classes should always use the new-style numeric API, which passes the other operand uncoerced to the __add__ and __radd__ methods, etc.) """

PS Next time don't post HTML.
--Guido van Rossum (home page: http://www.python.org/~guido)

-----Original Message-----
From: python-dev-bounces at python.org [mailto:python-dev-bounces at python.org] On Behalf Of Patrick Maupin
Sent: Monday, October 13, 2003 1:08 PM
To: python-dev at python.org
Subject: [Python-Dev] Python and Coercion


Dear developers: 
__coerce__ does not seem to work in new-style classes, e.g. 
class foo: 
    def __int__(self): return 1 
    def __coerce__(self,other):  return int(self), int(other) 
x = foo() 
print 1+x 
works fine, but if foo is derived from object, it fails with: 
TypeError: unsupported operand type(s) for +: 'int' and 'foo' 


After finding this difference, I could not figure out if this was 
an interpreter error or a documentation error. 
http://www.python.org/doc/current/ref/coercion-rules.html 
states that: "In Python 3.0, coercion will not be supported." 
so I thought maybe this was the first round of removing this 
support. 
I googled around for awhile trying to find supporting documentation for 
this -- it appears it might have to do with PEP 228, but I'm not really 
sure, so I was hoping someone could point me at a reference point for 
this statement. 
Regards, 
Pat Maupin 


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