What is a function parameter =[] for?

[Ned Batchelder]

On Tuesday, November 24, 2015 at 12:25:54 PM UTC-5, Oscar Benjamin wrote:
> On 24 November 2015 at 15:27, Ned Batchelder <ned at nedbatchelder.com> wrote:
> > On Tuesday, November 24, 2015 at 10:10:51 AM UTC-5, Antoon Pardon wrote:
> >> Op 24-11-15 om 15:18 schreef Ned Batchelder:
> >>
> >> > 2) In Python, "value" means, what object does a name refer to, or what
> >> > object did an evaluation produce.
> >>
> >> I don't think this is correct because that would imply that objects don't
> >> change values (since the value would be the object).
> >
> > Sorry, I should have been more precise.  Python documentation suffers from
> > this two-meaning world also.  There are places where "value" is used in
> > this second sense, perhaps more places than you realize.  I know when I
> > wrote "Python Names and Values", I meant it in this second sense.
> 
> Hi Ned, I read your talk on this subject which was linked to further
> up in the thread and it is excellent so thanks for that. I forwarded
> it to a colleague for inspiration in our own introductory programming
> (with Python) teaching unit.
> 
> However when I read it and imagined explaining it to our students I
> found myself wanting to adjust the terminology. I would not use the
> word value in the way that you suggest. Rather I would want to
> contrast "names" (or "references"), "objects" and "values". I think
> that many things can be expressed more cleanly with these terms and I
> think it corresponds more to the way in which I see Python programmers
> use these terms.
> 
> So I would say that an object has a value. Immutable objects have
> unchanging values but the value of mutable objects can change. The
> expression "a is b" determines whether or not a and b are bound to the
> same object whereas "a == b" determines if the objects bound to a and
> b have the same value. To copy an object is to create a new object
> having the same value as the copied object. To mutate an object is to
> change its value etc.
> 
> Earlier in this thread you said:
> """
> In Python, a default value expression
> for a function argument is evaluated only once, when the function is defined.
> That value (in this case, the actual list) is stored with the function. The
> expression is not stored, the value of the expression is. That value (the
> actual list) is supplied as the value of the argument if no other value is
> supplied.  If you modify that value in the function, the value is modified,
> and used again at the next function call.
> """
> I think that's a good example of where it would be clearer to
> distinguish between objects and values, rather than using the word
> value for both. In particular what is stored with the function is the
> *object* that results from the default value expression. The *value*
> of that object may change if it is mutated but it will still be the
> same object each time the function is called (without a corresponding
> argument being passed).
> 
> --
> Oscar

Oscar, thanks for the thoughtful comments. I agree that using "object" for
the result of an expression, and for the referent of a name, would go some
ways to clarifying things.

Perhaps the Python world uses "value" less to mean "object" than I am thinking.
But we do talk about "the value of an expression", and "what value does X
have," and so on.

--Ned.