Augument assignment versus regular assignment

[Piet van Oostrum]

>>>>> Antoon Pardon <apardon at forel.vub.ac.be> (AP) wrote:

>AP> As I read the language reference the x stands for a target expression.
>AP> Now what does it mean to evaluate a target expression like col[key].
>AP> IMO it means finding the location of the item in the collection: the
>AP> bucket in the directory, the node in the tree ... grosso mode it
>AP> boils down to the call to __setitem__ or __getitem__ depending
>AP> on where the col[key] was located in the line (or if you prefer
>AP> the view from the interpreter it boils down to the BINARY_SUBSCR
>AP> and STORE_SUBSCR opcodes).

>AP> So if the language reference seems to implies that col[key] will
>AP> only be evaluated once in a line like: "col[key] += 1" I expect
>AP> only one call from __setitem__ or __getitem__ (or only one
>AP> from BINARY_SUBSCR or STORE_SUBSCR)

You need both the __setitem__ and the __getitem__ otherwise it won't work.
I think the "evaluated once" clause is for cases like:

f(a)[g(b)] += 1

where f and/or g have side effects. These side effects should then take
place only once. Side effects in __setitem__ and __getitem__ is not what it
is talking about.
-- 
Piet van Oostrum <piet at cs.uu.nl>
URL: http://www.cs.uu.nl/~piet [PGP 8DAE142BE17999C4]
Private email: piet at vanoostrum.org