Subclassing complex with computed arguments

Nick Coghlan ncoghlan at email.com
Sat Nov 27 13:43:09 CET 2004


<resend, since my newsreader had an argument with the newsserver. Kent's latest 
post already addressed some of my points below>

Kent Johnson wrote:
 > I don't know if there is an easy way around this. A brute force approach
 > might be to override __add__(), etc to return point objects instead of
 > complex -

Yup, this is the only way I know to prevent ordinary complex() objects leaking
through the interface.

 > though I haven't been able to figure out how to do this. I tried:
 >     def __add__(self, *args, **kwds):
 >         val = complex.__add__(self, *args, **kwds)
 >         return point(val)

Try something like:

  def __add__(self, other):
	c_self, c_other = coerce(self, other)
         val = c_self.__add__(c_other)
         return point(val)

Since:

 >>> x = 1
 >>> c = 1j
 >>> c.__add__(x)
NotImplemented
 >>> c2, x2 = coerce(c, x)
 >>> c2.__add__(x2)
(1+1j)

Don't forget to override the __radd__ operators and friends, too.

 > class point(object):
 >
 > I tried adding a delegating __coerce__() to class point but it is never
 > called...

new-style classes do not invoke coercion unless you explicitly call coerce() on
them. If you don't inherit from object, the __coerce__ method should get called
if it's needed.

Here's some essential reading if you want to play with numerical operators:
http://www.python.org/doc/2.3.4/ref/numeric-types.html
http://www.python.org/doc/2.3.4/ref/coercion-rules.html

(I found these two pages of the Language Reference Manual essential while
helping out with the decimal implementation for Python 2.4 - much that had been
mysterious suddenly became clear after I read these docs)

Cheers,
Nick.



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