[Python-bugs-list] [ python-Bugs-479698 ] __coerce__ not working with new classes

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Bugs item #479698, was opened at 2001-11-08 13:53
Message generated for change (Comment added) made by rhettinger
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Category: Documentation
Group: Python 2.2
>Status: Closed
>Resolution: Fixed
Priority: 5
Submitted By: Nobody/Anonymous (nobody)
Assigned to: Fred L. Drake, Jr. (fdrake)
Summary: __coerce__ not working with new classes

Initial Comment:
I converted a numeric-like class from the "old-style"
Python class to the "new-style" Python class by
inheriting from "object".  Numerical operations like
"__mul__" and "__div__" failed with the "new-style"
class.  After tracking the problem, I found that the
"__coerce__" special method was called when performing
mixed-type arithmetic, as expected, with the
"old-style" class, but not with the "new-style" class. 
The arithmetic operations failed because the expected
type conversion was not performed with the mixed-type
arithmetic.  As best I can tell, without examining the
Python source code, the following is taking place:

1.  The documentation for the "__coerce__" special
method states that it is invoked in the case of
mixed-type arithmetic only if at least one of the
arguments in an arithmetic operation is a class
instance.

2.  An object created with an "old-style" class is of
type "instance", as verified by the "type" function,
i.e.
    <type 'instance'>

3.  An object created with a "new-style" class is of a
different type, as also verified by the "type function,
i.e.
    <class '.....'>

Evidently, because of this change in type, the
"__coerce__" function is not longer invoked for
"new-style" Python classes.

Gary H. Loechelt
ON Semiconductor


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>Comment By: Raymond Hettinger (rhettinger)
Date: 2003-07-11 21:01

Message:
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Fred fixed this in the docs on June 4, 2002.
See Doc/ref/ref3.tex 1.190.
Marking a closed.
Thanks for the bug report and thorough analysis.

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Comment By: Guido van Rossum (gvanrossum)
Date: 2001-12-03 14:09

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Encouraged by the submittor's comment, I've decided not to
fix this.  New-style class instances cannot be old-style
numbers, and the conversion from old-style class to
new-style class will have to involve changing any numeric
operators to do their own coercions.

I'll leave this as a documentation item, at a lower priority
(I'll add something to the docs on the web).

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Comment By: Gary H. Loechelt (loechelt)
Date: 2001-11-26 10:59

Message:
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After further looking into this, I am not sure that it is a
bug "per se".  It may have been the intentionally designed
behavior.  It is certainly possible for a user to rewrite
arithmetic methods (i.e. '__add__') to explicitly call a
coerce method.  However, in the very least, changing from
the "old-style" to the "new-style" class does break existing
code that relies upon the '__coerce__' method, and this fact
is not very well documented.

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