sum() requires number, not simply __add__

Buck Golemon buck at
Thu Feb 23 16:23:45 EST 2012

On Feb 23, 1:19 pm, Buck Golemon <b... at> 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'


def sum(values,
      values =

          result =
      except StopIteration:
          return base

      for value in values:
          result += value
      return result

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