surprising += for lists

[Terry Reedy]

On 11/4/2012 7:45 AM, Dave Angel wrote:

> What I wonder about is why list's __add__  is so fussy.

Guido's reason is that it is not clear what the types of [1,2] + (3,4), 
(1,2) + [3,4], [] + range(4), range(2) + [3,4], etcetera should be. Such 
mixtures may be bugs. Seq.__add__ exists to implement '+'.

[].extend() will clearly be a list, and accepts any iterable.

I believe the same logic is used for set operations.

-- 
Terry Jan Reedy