How to convert list of tuples into a single list of unique values ?

[Hendrik Reichel]

Skip Montanaro wrote:
> I was originally thinking of using a single for
> loop and flattening the tuples:
> 
>     P = []
>     [P.extend(list(t)) for t in L]

  from operator import add
  P = list(reduce(add, L))

seems like a cleaner and faster way to achieve this goal,
avoiding building up an unused list of [None, None, ...].
(Just in case someone's copy&pasting the code ;-)