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operator overloading

Emile van Sebille emile at fenx.com
Mon Jun 5 19:38:36 EDT 2000 
Ken,

You missed my point, while your point is taken.

My point, if I understood the question, was how could
one overload the less than operator.  In absence of an
example, I took that to mean how could one change
the behavior of the less than symbol.  Which needs
to happen in a class definition, which then allows
the class to decide what less than means.  So I 
provided an example that showed changed behavior
for less than.  ;-)  Of course, only the class
can know how two instances should compare, and it
should then return an appropriate value.

That the example should have fully shown that 
return values can be negative, zero, or positive,
indicating less than, equals, and greater than
respectively, is of course my fault.

What-if-a-six-turned-out-to-be-nine-ly y'rs

Emile van Sebille
emile at fenx.com
-------------------


----- Original Message ----- 
From: Ken Seehof 
Newsgroups: comp.lang.python
To: python-list at python.org 
Sent: Monday, June 05, 2000 3:02 PM
Subject: Re: operator overloading


Emile van Sebille wrote: 
  Here's a quick example of using __cmp__ 
  >>> class Test: 
   def __init__(self,a): 
    self.a = a 
   def __cmp__(self,other): 
    return self.a < other.a 

  >>> a = Test(3) 
  >>> b = Test(4) 
  >>> print a<b 
  0 
  >>> print a>b 
  1 

  HTH, 

  Emile van Sebille 
  emile at fenx.com 
  -------------------

Um.  No.  Not exactly.  Usually 1 designates "true" and 0 designates "false" unless I'm mistaken.  Also IMHO, > means "greater than" and < means "less than".  BTW there is yet another comparison operator == than means "equal to".  :-) 
The return value of __cmp__ should be -1, 0, 1 for <, ==, >, respectively. 

The following more closely conforms to these conventions: 

>>> class Test: 
...  def __init__(self,a): 
...   self.a = a 
...  def __cmp__(self,other): 
...   if self.a == other.a: 
...    return 0 
...   elif self.a < other.a: 
...    return -1 
...   else: 
...    return 1 
... 
>>> a = Test(1) 
>>> b = Test(2) 
>>> c = Test(2) 
>>> a<b, a>b, a==b, a!=b 
(1, 0, 0, 1) 
>>> b==c, b<c, b!=c 
(1, 0, 0) 

-- Ken Seehof 
kens at sightreader.com 

  ----- Original Message ----- 
  From: <kazan at kazan.fenx.com> 
  Newsgroups: comp.lang.python 
  To: <python-list at python.org> 
  Sent: Monday, June 05, 2000 1:01 PM 
  Subject: operator overloading 
  > Hello 
  > 
  > How can I overload the less than operator? 
  > learning python did mention something about __cmp__ but 
  did 
  > not provide any examples. 
  > 
  > 
  > Thanks in advance 
  > 
  > Erling 
  > --

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