On Fri, 31 Oct 2008 20:22:23 +1300, greg wrote: > Steven D'Aprano wrote: > >> Which definition of call-by-value? Certainly not the one that most >> people are familiar with, where c-b-v implies that calling a function >> with a large array will copy the entire array. > > But that's only true in languages where *assigning* a large array will > also copy the entire array.
Maybe, maybe not. To be consistent, it should be true. But how do you know if a language you are unfamiliar with is consistent? In any case, to somebody only familiar with the classical value-vs- reference model, Python seems rather inconsistent: x = 1 y = x # does assignment make copies? y += 1 assert x == 1 => succeeds, which implies that Python makes a copy when assigning x = [1] y = x # does assignment make copies? y += [1] assert x == [1] => fails, which implies that Python uses references when assigning Hence you get people who argue that Python does call-by-value for numbers and call-by-reference for lists -- and yes, I've seen people make that exact argument. The reality is that Python isn't inconsistent, it merely seems to be if you assume it is some other language. Python actually treats numbers and lists identically. When people discover that Python's behaviour doesn't fall neatly into their preconceived notions of c-b-v and c-b-r, there are two ways of dealing with that: (1) Accept that perhaps there's at least one more way of doing assignment and parameter passing. (2) Or jump through hoops trying to force how Python works to somehow match your preconceptions. > This does not happen in Python, therefore > there is no reason to suppose that it will happen when passed as a > parameter. Of course there is a reason: we've been told that Python is call by value, and call by value implies that a copy is made when you pass it to a function. The fact that many programmers already have an idea of what they think c-b-v implies is what makes calling Python c-b-v such a pernicious mistake. If you want to call Python "call by ginger" I won't object, because most people have no preconceived ideas of what call by ginger means and therefore won't be lured into incorrect assumptions about Python. > Before you can understand parameter passing, whether by-value or > by-reference, you first have to understand how assignment works *in the > language concerned*, not some other language you happen to know > previously. It's certainly true that the baggage people bring from their previous languages can sometimes be a serious barrier, but that especially happens when folk insist on using the same terminology to describe different things. For example, to somebody coming to Python from Lisp or Scheme, the word "list" carries particular connotations. It's unfortunate that Python lists are not like Lisp lists, but we're stuck with that now, and besides there's only so many good names for a list-like array, and whatever name was chosen would trip up somebody. Another example, augmented assignment in Python trips up C programmers, because x += y looks the same but behaves differently than x += y in C. There's enough difficulty with learning a new programming language without people adding to it by misusing terms like "call by value" to describe what Python does. -- Steven -- http://mail.python.org/mailman/listinfo/python-list
