Official definition of call-by-value (Re: Finding the instance reference...)

[Derek Martin]

On Thu, Nov 13, 2008 at 11:58:18AM -0800, rurpy at yahoo.com wrote:
> I have yet to see any reasonable definition of a Python value in the
> Python docs or elsewhere, despite the fact that a value is one of
> the three defining characteristics of an object, a central concept
> in Python.

Why does it need to be defined in the Python docs?  Is this really
even an important question to answer?  Are you unable to write correct
functional programs in Python without having it answered?  I suspect
it's not an issue...

Let's assume for the moment that it is, though.  The term "value"
already has a meaning... the one ascribed to it by its use in natural
language.  One on-line dictionary includes this among its definitions:

  magnitude; quantity; number represented by a figure, symbol, or the
  like: the value of an angle; the value of x; the value of a sum.

It seems clear that this, or something extremely close to this, is
what is meant in the Python docs by the unqualified use of the term.

So, then, what is the value of a Python object?  As has been alluded by
others, it is not possible to formally define or enumerate what such a
value is, in general terms, because the term "object" refers to 
to a thing with neither a deterministic nor static identity; in the
abstract an object has no inherent or intrinsic value.  

The best you can hope to do is define it in context.  To illustrate:
In natural language and the physical world, an object has any number
of values; for example a book has a title, a topic, a weight, a
height, a length, a width, a page count, a word count, a purchase
price, a printing cost, a number of copies sold, a profit per unit
sold, etc. to the limits of your imagination regarding ways to
describe books.  Which of these is its "value" depends upon the
context in which you are discussing the book.  To the reader, the
value is the price he pays for the book, or perhaps some measure of
the enjoyment he derives from reading it (possibly the amount he would
be willing to pay to buy it).  To the seller, the value is perhaps
best represented by the profit per unit sold.  To the publisher's
shipper (think FedEx), the value might best be described in terms of
its weight...

Just as in the physical world, in Python an object can be defined such
that it evaluates to different values in different contexts, even
though its state may not have changed between those different
contexts.  Therefore the value of an object is dependent upon its data
attributes AND its behaviors defined as methods, as well as the
context in which it is accessed, and is the value to which the object
evaluates in a given expression or context.

If you like, you could think of the value of an object as the set of
all possible values to which the object may evaluate in every possible
context, given a particular state of the object.

-- 
Derek D. Martin
http://www.pizzashack.org/
GPG Key ID: 0x81CFE75D

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