Ben Finney wrote: > Chris Carlen <[EMAIL PROTECTED]> writes: > > def change(some_list): > some_list[1] = 4 > > x = [1,2,3] > change(x) > print x # Prints out [1,4,3] > --- > def nochange(x): > x = 0 > > y = 1 > nochange(y) > print y # Prints out 1 > >>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? > > I've had some success with the following way of thinking about it. > > Some languages have "variables", which act like boxes that have names > etched on the side. Once created, the box can contain an object, and > it can be inspected while in the box; to change the variable, you > throw out the object and put a different object in the same box.
Yes, so y = x takes a copy of the stuff in the x box and puts it in the y box. Which is what really happens in the hardware. > That's not how Python works. Every value is an object; the assignment > operator binds a name to an object. This is more like writing the name > on a sticky-note, and sticking it onto the object. > > * The object itself doesn't change or "move". > > * The object can be referred to by that name, but isn't "inside" the > name in any way. > > * Assigning multiple names to the same object just means you can > refer to that same object by all those names. > > * When a name goes away, the object still exists -- but it can't be > referred to if there are no longer any names left on it. > > * Assigning a different object to an existing name just means that > the same sticky-note has moved from the original object to the new > one. Referring to the same name now references a different object, > while the existing object keeps all the other names it had. Excellent description. This understandable to me since I can envision doing this with pointers. But I have no idea how Python actually implements this. It also appears that I am being guided away from thinking about it in terms of internal implementation. > When you pass an object as a parameter to a function, the object > receives a new sticky-label: the parameter name under which it was > received into the function scope. Assignment is an act of binding a > name to an object; no new object is created, and it still has all the > other names it had before. Ok, so I can understand the code above now. In the first case I pass the reference to the list to change(). In the function, some_list is another name referring to the actual object [1,2,3]. Then the function changes the object referred to by the second element of the list to be a 4 instead of a 2. (Oh, the concept applies here too!) Out of the function, the name x refers to the list which has been changed. In the second case, y refers to a '1' object and when the function is called the object 1 now gets a new reference (name) x inside the function. But then a new object '0' is assigned to the x name. But the y name still refers to a '1'. I get it. But I don't like it. Yet. Not sure how this will grow on me. > When the function ends, all the names that were created inside that > function's scope disappear; but the objects still exist under any > names they had previously, and if you use those names you'll be > looking at the same object as was manipulated inside the function. > > When the object has lost all its names -- for example, they've > disappeared because the scope they were in has closed, or they've been > re-bound to other objects -- they can no longer be referenced. At some > point after that, the automatic garbage collection will clean that > object out of memory. > > This sticky-note analogy, and the behaviour described, is what is > meant by "references". A name refers to an object; changing the object > means that by referring to that same object under its different names, > you will see the same, modified, object. > > In Python, all names are references to objects. The assignment > operator '=' doesn't create or change a "variable"; instead, it binds > a name as reference to an object. All functions receive their > parameters as the existing object with a new name -- a reference to > that object, just like any other name. > > Hope that helps. A great deal of help, thanks. Excellent explanation. Wow. This is strange. A part of me wants to run and hide under the nearest 8-bit microcontroller. But I will continue learning Python. -- Good day! ________________________________________ Christopher R. Carlen Principal Laser&Electronics Technologist Sandia National Laboratories CA USA [EMAIL PROTECTED] NOTE, delete texts: "RemoveThis" and "BOGUS" from email address to reply. -- http://mail.python.org/mailman/listinfo/python-list
