Integer coercion and 2.2 Type/Class changes

[Guido van Rossum]

com-nospam at ccraig.org (Christopher A. Craig) writes:

> Consider the following Python 2.2 code:
> 
> >>> class foo(int):
> ...   def __init__(self, n=0):
> ...     self=n

This __init__ is a total no-op.  The assignment to self is a local
variable rebinding that has no effect on the object that was initially
represented by self.  In addition, since int is an immutable type, you
can't affect the value in __init__; you must use __new__ instead.

Your class works despite all that, because the default __new__
constructor does the right thing. :-)

> ...   def bar(self): return self+6
> ... 
> >>> t=foo(4)
> >>> t.bar()
> 10
> >>> type(t)
> <class '__main__.foo'>
> >>> type(t+6)
> <type 'int'>
> >>> type(t+t)
> <type 'int'>
> 
> This is the expected result, but probably often not the desired one.
> So my question is, is there some way, without redefining all of
> the numeric operations, to define a subclass that recasts all numeric
> operations to it's own type?  

No, you'll have to redefine all operators.

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