On 10/09/2013 20:08, Paul Moore wrote:

On 10 September 2013 19:31, Antoine Pitrou <solipsis@pitrou.net> wrote:

I think it would be a flaw to have this detail implementation-defined.
This would be like saying that it is implementation-defined which
of A,B,C is returned from "A and B and C" if all are true.

Ok, it seems everyone (except me :-)) agrees that it should return the
first key value, so that's how it will be.

If you retain the first key value, it's easy enough for the
application to implement "retain the last" semantics:

try:
     del d[k]
finally:
     d[k] = v

That would raise a KeyError is the key was missing. A better way is:

d.pop(k, None)

d[k] = v

If you provide "retain the last", I can't see any obvious way of
implementing "retain the first" in application code without in effect
reimplementing the class.

"Retain the first" does feel more natural to me.

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