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Re: sum() requires number, not simply __add__

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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')

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sum() requires number, not simply __add__ Buck Golemon <buck@yelp.com> - 2012-02-23 13:19 -0800
  Re: sum() requires number, not simply __add__ Buck Golemon <buck@yelp.com> - 2012-02-23 13:23 -0800
    Re: sum() requires number, not simply __add__ Arnaud Delobelle <arnodel@gmail.com> - 2012-02-23 21:41 +0000
    Re: sum() requires number, not simply __add__ Chris Angelico <rosuav@gmail.com> - 2012-02-24 08:53 +1100
      Re: sum() requires number, not simply __add__ Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-02-23 23:33 +0000
        Re: sum() requires number, not simply __add__ Chris Angelico <rosuav@gmail.com> - 2012-02-24 10:39 +1100
        Re: sum() requires number, not simply __add__ Antoon Pardon <antoon.pardon@rece.vub.ac.be> - 2012-02-24 12:41 +0100
          Re: sum() requires number, not simply __add__ Roy Smith <roy@panix.com> - 2012-02-24 08:23 -0500
            Re: sum() requires number, not simply __add__ Terry Reedy <tjreedy@udel.edu> - 2012-02-24 16:58 -0500
    Re: sum() requires number, not simply __add__ Arnaud Delobelle <arnodel@gmail.com> - 2012-02-23 21:59 +0000
    Re: sum() requires number, not simply __add__ Ian Kelly <ian.g.kelly@gmail.com> - 2012-02-23 15:00 -0700
    Re: sum() requires number, not simply __add__ Chris Angelico <rosuav@gmail.com> - 2012-02-24 09:04 +1100
    Re: sum() requires number, not simply __add__ Arnaud Delobelle <arnodel@gmail.com> - 2012-02-23 22:09 +0000
  Re: sum() requires number, not simply __add__ Arnaud Delobelle <arnodel@gmail.com> - 2012-02-23 21:32 +0000
  Re: sum() requires number, not simply __add__ Chris Rebert <clp2@rebertia.com> - 2012-02-23 13:32 -0800
    Re: sum() requires number, not simply __add__ Buck Golemon <buck@yelp.com> - 2012-02-23 13:38 -0800
      Re: sum() requires number, not simply __add__ Ian Kelly <ian.g.kelly@gmail.com> - 2012-02-23 14:54 -0700
  Re: sum() requires number, not simply __add__ Stefan Behnel <stefan_ml@behnel.de> - 2012-02-23 22:42 +0100
    Re: sum() requires number, not simply __add__ Duncan Booth <duncan.booth@invalid.invalid> - 2012-02-24 11:40 +0000
  Re: sum() requires number, not simply __add__ Peter Otten <__peter__@web.de> - 2012-02-24 09:29 +0100

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