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From: Peter Otten <__peter__@web.de>
Subject: Re: sum() requires number, not simply __add__
Date: Fri, 24 Feb 2012 09:29:20 +0100
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Xref: x330-a1.tempe.blueboxinc.net comp.lang.python:20788

Buck Golemon wrote:

> I feel like the design of sum() is inconsistent with other language
> features of python. Often python doesn't require a specific type, only
> that the type implement certain methods.
> 
> Given a class that implements __add__ why should sum() not be able to
> operate on that class?
> 
> We can fix this in a backward-compatible way, I believe.
> 
> Demonstration:
>     I'd expect these two error messages to be identical, but they are
> not.
> 
>      >>> class C(object): pass
>      >>> c = C()
>      >>> sum((c,c))
>     TypeError: unsupported operand type(s) for +: 'int' and 'C'
>     >>> c + c
>     TypeError: unsupported operand type(s) for +: 'C' and 'C'

You could explicitly provide a null object:

>>> class Null(object):
...     def __add__(self, other):
...             return other
...
>>> null = Null()
>>> class A(object):
...     def __init__(self, v):
...             self.v = v
...     def __add__(self, other):
...             return A("%s+%s" % (self, other))
...     def __str__(self):
...             return self.v
...     def __repr__(self):
...             return "A(%r)" % self. v
...
>>> sum(map(A, "abc"))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for +: 'int' and 'A'
>>> sum(map(A, "abc"), null)
A('a+b+c')