Dave Kuhlman wrote: > On Thu, Jul 27, 2006 at 05:34:13PM +0100, Alan Gauld wrote: > >> Hi Dave, >> >> You are causing yourself some confusion by still treating variables >> as something other than a name. Your first paragraph says: >> >> (Actually, functions and classes are just variables that hold >> references to function and class objects.) >> >> Which is wrong. variables are simply names that refer to objects, >> which includes functions and classes(and instances of classes) >> Thus a function is never a variable. variables refer to functions. >> >> In Computer Science terms a function is a lamda expression >> and a def in Python should be a shorthand way of doing >> >> var = lambda params... : expression >> >> Unfortunately, in practice, in Python it's the other way around. >> The lambda is really an alternative for >> >> def var(params...): >> return expression >> >> But it is important to realize that names in Python - all names - are >> simply references to objects of some kind. and that classes, >> instances, >> functions, numbers, lists, characters etc are all objects in this >> sense. >> >> > > Rats. I wish I'd said it that way. Can I steal that quote? > > Thanks, Alan and Kent, for trying to straighten me out on this. > And, by the way, since I'll be trying to explain this in the next > class I teach, you are doing more than just clear the darkness of > confusion from *my* mind; you may also be helping to prevent me > from confusing students in my next class. > > Let me try to summarize a few points that Alan has tried to > explain about variables, names, namespaces, values, and objects. > Let's see if I can get it right this time: > > A variable is a name bound to a value in a namespace. > > A namespace is a dictionary in which Python can look up a name > (possibly) to obtain its value. > > Names refer to objects. Objects can be integers, tuples, lists, > dictionaries, strings, instances of classes, functions, classes > (themselves), other Python built-in types, and instances of > classes. > > In Python, (and here I'm trying to go a bit beyond Alan) since the > use of objects and references to them are so pervasive and > consistent, we sometimes conflate a variable and the object it > refers to. So, for example, if we have the following code:: > > total = 25 > items = [11, 22, 33] > def func1(): > pass > class Class1: > pass > > we sometimes say: > > - ``total`` is an integer. > > - ``items`` is an list. > > - ``func1`` is a function. > > - ``Class1`` is a class. > > But, if we were more careful, we might say: > > - ``total`` is a variable that refers to an integer object. > > - ``items`` is a variable that refers to a list object. > > - ``func1`` is a variable that refers to a function object. > > - ``Class1`` is a variable that refers to a class object. > > Or, even: > > - ``total`` is a name bound to an integer object in the current > namespace. > > - ``items`` is a name bound to a list object in the current namespace. > > - ``func1`` is a name bound to an function object in the current > namespace. > > - ``Class1`` is a name bound to an class object in the current > namespace. > > And, it is important to remember: > > 1. Names are references to objects and > > 2. Objects are first class, which means that we can: > Right up to here, I'm standing on my chair and cheering. > - Store them in variables. > Ouch! No! Variables don't store values, they refer to values. Thinking of variables as containers doesn't work in Python. > - Store them in data structures. > That's a little better, but really you store a reference to a value in a data structure. It's references all the way down ;) > - Pass them to functions and methods. > > - Return them from functions and methods. > > There, am I any closer? > Lots closer. > The point that ``def func1: ...`` creates a variable "func1" is > one that I believe is not clear to someone new to programming or > even to someone familiar only with languages like C, C++, or Java. > Better to say "def func1:" creates a function object and binds it to the name "func1". And your point is a good one, that this takes a little getting used to. By the way "class k2:" creates a class object and binds it to a name also. Namespaces and name binding are pervasive in Python.
Kent _______________________________________________ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
