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Wed Mar 16 17:28:22 CET 2005 · https://mail.python.org/pipermail/python-dev/2005-March/052227.html

> > Thinking ahead to generic types, I'd like the full signature to be:
> >
> >   def sum(seq: sequence[T], initial: T = 0) -> T.
> 
> Would this _syntax_ work with generic types:
> 
>   def sum(seq: sequence[T], initial: T = T()) -> T.

Maybe, but it would preclude union types; continuing with the (bad)
example of strings, what should one choose for T when seq == ['a',
u'b']? The general case is a sequence of objects of different types
that are mutually addable. This can be made to work with the
(hypothetical!!!!) type system by using unions, but you can't
instantiate an instance of a union without being more specific.

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

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[Python-Dev] Rationale for sum()'s design?