On Fri, Dec 11, 2009 at 7:59 PM, Nick Coghlan <ncogh...@gmail.com> wrote: > It follows the standard left-to-right evaluation order within an expression: > > <subexpr1>(<subexpr2>) > > (i.e. a function call always determines which function is going to be > called before determining any arguments to be passed) > > Splitting it into two lines then clearly changes the evaluation order: > > temp = <subexpr2> > <subexpr1>(temp) > > I'm not sure what behaviour could be suggested as being more intuitive - > the problem in this case arose due to both referencing and mutating the > same object in a single statement, which is always going to be > problematic from an ordering point of view, since it depends on subtle > details of statement definitions that people often won't know. Better to > split the mutation and the reference into separate statements so the > intended order is clear regardless of how well the reader knows the > subtleties of Python's evaluation order. > > Cheers, > Nick. >
Thanks for the explanation, Nick. I understand what is happening now. y[1].update resolves to the update() method of the old set referenced by y[1], but this set is then popped so that the update() manages to change the old set, but not the new one that is returned by any subsequent reference to y[1]. I suppose I will have to keep this in two statements. A similar thing happened with the SWIG extensions for GDAL, in which one had to separate statements so that SWIG objects were not destroyed prematurely, i.e. no more one-liners. _______________________________________________ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
