En Thu, 12 Jul 2007 21:51:08 -0300, Chris Carlen <[EMAIL PROTECTED]> escribió:
> Hi: > > I have begun learning Python by experimenting with the code snippets > here: > > http://hetland.org/writing/instant-python.html > > In the section on functions, Magnus Lie Hetland writes: > > -------------------------------------------------------------------- > For those of you who understand it: When you pass a parameter to a > function, you bind the parameter to the value, thus creating a new > reference. If you change the “contents” of this parameter name (i.e. > rebind it) that won’t affect the original. This works just like in Java, > for instance. Let’s take a look at an example: > > def change(some_list): > some_list[1] = 4 > > x = [1,2,3] > change(x) > print x # Prints out [1,4,3] > > As you can see, it is the original list that is passed in, and if the > function changes it, these changes carry over to the place where the > function was called. Note, however the behaviour in the following > example: > > def nochange(x): > x = 0 > > y = 1 > nochange(y) > print y # Prints out 1 > > Why is there no change now? Because we don’t change the value! The value > that is passed in is the number 1 — we can’t change a number in the same > way that we change a list. The number 1 is (and will always be) the > number 1. What we did do is change the contents of the local variable > (parameter) x, and this does not carry over to the environment. > -------------------------------------------------------------------- > > What this looks like to me is what would happen if in C I passed a > pointer to the list x to the function change(), as in: > > change(&x); > > Thus the function could change the original list. > > I don't understand Hetland's terminology though, when he is speaking of > "binding" and "reference." Actually, Hetland's entire first paragraph > is unclear. > > Can anyone reword this in a way that is understandable? First, see this short article http://effbot.org/zone/python-objects.htm Now, forget about C "variables" and "pointers" because you won't get much far with those concepts. Objects exist - and we usually use names to refer to them. This line: a = 1 means "make the name 'a' refer to the object 1", or, "bind the name 'a' to the instance of type int with value 1", or "let 'a' be a reference to the object 1" This line: some_list[1] = 4 means "make the second element of some_list refer to the object 4", or "alter some_list so its element [1] is a reference to the object 4" bind the name 'a' to the instance of type int with value 1", or "let 'a' be a reference to the object 1" Note that some objects are immutable - like the number 1, which will always be the number 1 (*not* an integer "variable" that can hold any integer value). Other objects -like lists and dictionaries, by example, or most user defined classes- are mutable, and you can change its contents and properties. Modifying an object is not the same as rebinding its name: x = [1,2,3] y = x x[1] = 4 print x # [1, 4, 3] print y # [1, 4, 3] x = [1,2,3] y = x x = [1,4,3] print x # [1, 4, 3] print y # [1, 2, 3] You can test is two names refer to the same object using the is operator: x is y. You will get True in the first case and False in the second case. -- Gabriel Genellina -- http://mail.python.org/mailman/listinfo/python-list
