Chris Mellon wrote: > On 7/13/07, Chris Carlen <[EMAIL PROTECTED]> wrote: >> Ben Finney wrote: >>> Chris Carlen <[EMAIL PROTECTED]> writes: > >>> 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. >> > > I assume that you're familiar with the general concept of hash tables. > Scopes in Python are hash tables mapping strings to Python objects. > Names are keys into the hash table. > > a = 10 > > is the same as > currentScope["a"] = 10 > > print a > > is the same as > print currentScope["a"] > > As you can see, there's no way that assignment can result in any sort > of sharing. The only way that changes can be seen between shared > objects is if they are mutated via mutating methods. > > Python objects (the values in the hashtable) are refcounted and can be > shared between any scope. > >>> 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. > > Each scope is it's own hashtable. The values can be shared, but not > the keys. When you call a function, the objects (retrieved from the > callers local scope by name) are inserted into the functions local > scope, bound to the names in the function signature. This is what > Guido calls "call by object reference". > This is, of course, something of an oversimplification. First, the global statement allows you to modify a name of module scope from inside another scope such as a functions.
> > >> 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. >> > > You need to learn about Pythons scoping now, not it's object passing > semantics. When you're used to thinking in terms of pointers and > memory addresses it can take some work to divorce yourself from those > habits. Then, of course, there are the nested scoping rule for functions defined inside other functions. regards Steve -- Steve Holden +1 571 484 6266 +1 800 494 3119 Holden Web LLC/Ltd http://www.holdenweb.com Skype: holdenweb http://del.icio.us/steve.holden --------------- Asciimercial ------------------ Get on the web: Blog, lens and tag the Internet Many services currently offer free registration ----------- Thank You for Reading ------------- -- http://mail.python.org/mailman/listinfo/python-list
