On Fri, Dec 10, 2010 at 2:07 AM, Alan Gauld <alan.ga...@btinternet.com> wrote: > > "Alex Hall" <mehg...@gmail.com> wrote >> >> val=val or 1 > >> I am guessing that val is an int. If val==0, the 'or' kicks in and >> val=1, else the or is not needed and val=val. Am I close? > > Yes this is a combination of what is known as short circuit evaluation of > boolean expressions and a quirk of Python that returns the actual value of > something that is being treated as a boolean. >
Doesn't short-circuit evaluation refer specifically to the behavior where arguments are only evaluated if they need to be? It's a very useful feature, but not technically required for the "val = val or 1" behavior to work. Also, returning on of its operands rather than a boolean is hardly a quirk, since basically all dynamic languages do it ever since perl made "val = val or 1" an idiom (at least, I think it was perl). There is more to the innocent little or. See http://docs.python.org/library/stdtypes.html#truth-value-testing It works on any type, not just numbers, and operates by truth value testing, which is more complicated than it might first seem. Hugo _______________________________________________ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
