On Wed, 21 Jun 2006, Guido van Rossum wrote: > I worry (a bit) about this case: > > y = 12 > def foo(x, y): > switch x: > case y: print "something" > > which to the untrained observer (I care about untrained readers much > more than about untrained writers!) looks like it would print > something if x equals y, the argument, while in fact it prints > something if x equals 12.
I am quite concerned about this case too. I think if Python were to behave this way, it would be a new pitfall for people learning the language -- like other pitfalls such as using unbound locals, mutable default arguments, or the historical non-nested scopes. I'm not saying the other pitfalls don't have good reasons -- some are outweighed by other design advantages (unbound locals are a consequence of having no variable declarations) and some have since been fixed (like nested scopes). But i'd be wary of adding a new pitfall to that list without a very substantial win. > Me too. I guess I was just pointing out that "just" evaluating it in > the global scope would not give an error, just like this is valid (but > confusing): > > y = 12 > def foo(y=y): > print y > y = 13 > foo() # prints 12 I see how frozen-cases and default-arguments could have comparable semantics, but i do think frozen-cases are more confusing. In this default-arguments example, there is at least a hint from the syntax that we're introducing a new local variable, so there is a landmark where the reader can hang the mental note that a new thing is being introduced. Also, it is easier to see that default arguments are being fixed at function-definition time because their value expressions are localized in the source code in the "def" line, a line that makes sense to be evaluating at definition time. For frozen-cases, you don't have this kind of landmark, and the bits that are evaluated at function-definition time are scattered and mixed with the rest of the function evaluated at function-call time. That's pretty subtle; i can't think of any other Python construct off the top of my head that mixes evaluation times like that. (Yes, the compiler does optimize literals, but it's done in a way that doesn't affect semantics.) -- ?!ng _______________________________________________ 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
