sum() requires number, not simply __add__

Duncan Booth duncan.booth at invalid.invalid
Fri Feb 24 12:40:10 CET 2012

Stefan Behnel <stefan_ml at> wrote:

> I know that you just meant this as an example, but it's worth
> mentioning in this context that it's not exactly efficient to "sum up"
> lists this way because there is a lot of copying involved. Each adding
> of two lists creates a third one and copies all elements into it. So
> it eats a lot of time and space.

If you search back through this group far enough you can find an 
alternative implementation of sum that I suggested which doesn't have the 
same performance problem with lists or strings and also improves the 
accuracy of the result with floats.

In effect what it does is instead of:
  (((((a + b) + c) + d) + e) + f)
it calculates the sum as:
  ((a + b) + (c + d)) + (e + f)

i.e. in as balanced a manner as it can given that it still has to work from 
left to right.

Of course that could still change the final result for some user defined 
types and never having converted my code to C I have no idea whether or not 
the performance for the intended case would be competitive with the builtin 
sum though I don't see why it wouldn't be.

Duncan Booth

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