D
Derek Martin
It's trivial to show that this is not necessarily the case -- you just
haven't thought of all the ways in which a difference might matter to
you..
Suppose that one of the 10-dollar bills had a serial number which was
never printed by the government (i.e. it is counterfiet). Are they
still the same in every way that you care about?
What is the value of these two objects? Is it the same?
The answer, in both cases, is, "it depends." The value of both is
context-specific, just as I argue is the value of a python object.
First let's briefly examine the value of the counterfiet:
The "intrinsic" value (or rather, the value generally accepted by the
authorities) of the counterfeit is zero. However, if you have one in
your hand, and you pass it to a merchant who is not able to detect
that it is a counterfeit, then, isn't its value $10 in that context?
If you walk away from the merchant with $10 worth of goods (and/or
change in non-counterfeit currency), then I would say it would have to
be, *in that context*.
Now suppose that the merchant is actually a U.S. Secret Service agent
investigating counterfeit money. He notices the bill and arrests you.
What is the value of the bill now? I doubt you could really measure
it, except perhaps in terms of the amount of money it would cost you
to defend yourself in court (if your case went to court), the amount
of hastle to deal with the authorities, etc. Suffice it to say that
the value is rather negative, possibly with a very high magnitude,
depending on what happens next.
What about the genuine $10 bill?
If you take it to your local mall, or your local gas station, or your
corner grocer, it's clearly worth $10... or is it? Prices of gasoline
vary from station to station, city to city, state to state, even if
the gas is sold by the same company. A can of peas might be $0.89 at
a market in New Hampshire, or the same can of peas might be $1.59 at a
market in down town Manhattan. And if you take your $10 bill to pay
for your favorite cup of coffee at Starbucks in your city, it might
cost you $4 with change back of $6... But if you bring that same $10
bill to a Starbucks in a small city in Germany, good luck getting any
coffee...
Just as in the real world, the value of a Python object depends on the
context in which it is evaluated.
Philosophically, no, not irrelevant. You haven't explicitly defined "the
same" in enough detail to make that call. Perhaps because the objects are
in different locations (different IDs) is enough to make them different,
even if everything else about them is identical.
For example, consider the two electrons around a helium nucleus. They
have the same mass, the same speed, the same spin, the same electric
charge, the same magnetic moment, they even have the same location in
space (technically, the same wave function). They are identical in every
possible way. Are they the same electron, or two different electrons?
What does the question even mean?
My point is that as a philosophical position, identity may or may not
count as "sameness". It depends on the circumstances.
Philosophically, sure, but practically, perhaps not. The foo attribute
may be entirely irrelevant to your purposes, the equivalent of a random
speck of dirt or a slight tear on a ten dollar note.
Luckily, Python doesn't try to guess whether differences are significant
or not. You can override the __eq__ method on classes and choose for
yourself what properties of a class are differences that make a
difference and which are mere markers of no particular concern.
That's because you haven't asked the objects themselves if they are the
same. Not every aspect of objects are implemented as attributes.
Of course you do. Just look at the object and you will see it is a 2.
Why should there be a value attribute? Python attributes are just a
particular interface for accessing data. It's not the only possible
interface.
You forgot to look at the object as a whole. Consider a pair of trousers.
You're rifling through the pockets of one looking for a coin or a wallet
or some other object to distinguish it from another pair of trousers,
without noticing that one is made of blue cotton and the other is green
spandex.
The objects themselves tell us that they are 2 or 3. They don't need an
extra attribute, and in fact the implementation of int objects don't
allow for extra attributes because they have no __dict__ to store them in.
I don't think this is a valuable distinction to make. What's the
intrinsic value of a dict?
Let's not.
The type of an object is a label that tells Python where to look in order
to interpret the object. It is certainly *not* the attributes on the
object:
<type 'int'>
What do you mean by "local attributes"?
Do you mean instance attributes? That's well defined: an attribute is an
instance attribute if it is in obj.__dict__. It is a class attribute if
it is in obj.__class__.__dict__.
(Slots make this a little more complicated in practice, but we can ignore
them for now.)
I don't think we gain anything from distinguishing value and intrinsic
value, except confusion.
How do you deal with compound objects. What's the value of [1, 2, 3] if
it isn't "the list consisting of ints 1, 2 and 3"?
And recursive objects?
class Parrot:
def __init__(self):
self.me = self
p = Parrot()
a = []
a.append(a)
I don't understand Python well enough to defend any of these
descriptions (I now realize, despite my previous postings to the
contrary.
But what I do defend is the concept of intrinsic value which I have not
ever seen explicitly stated anywhere and which clarifies a lot of things
for me.
Think about lists. The "intrinsic" value of a list is the items in the
list itself, if it is anything, but you have an interface for looking at
them individually:
alist
Here's another problem with your idea of "intrinsic value". Consider a
list. The list object in CPython is an array of cells, containing data.
Some of those cells are in use, some of them are free, but regardless of
whether they are free or in use they still have data in them. (The data
may be obsolete in the case of the free cells. The list object also
includes a cell which records the number of cells in the array, and
another cell recording private information of use to the Python garbage
collector. These cells are not necessarily the same size.
Are they part of the "intrinsic value"? You can't access them via
attributes.
What if we use another implementation of list, say, a linked list? What
if we use PyPy, an implementation of Python in Python, and (for the sake
of the argument) lists are implemented as follows:
class list(object): # untested
def __init__(self, values):
for i, x in enumerate(values):
setattr(self, i, x)
(This would make a lousy implementation. Don't try it!) Does that mean
that the value of [1, 2, 3] depends on the implementation of Python you
create it under?
To me, that seems just as silly as arguing that zero is not a number, or
that white pixels are "nothing" and black pixels are "something". Or
maybe they should be the other way around?
None is None. It is what it is, and that is it's value. What *meaning*
you put to that is up to your program. It is a convention, a useful
convention but still merely a convention, that None is used to represent
values which otherwise would be missing, if there could be actual holes
in Python code.
Not true.
... print "x has the value two"
... elif x == 3:
... print "x has the value three"
...
x has the value three
At no stage did I call str() or repr() on x.
Expressions always evaluate to objects (unless they don't return at all).
The value of an expression is the object it evaluates to. What is wrong
with that claim? Why do you believe that we need a more complicated,
convoluted understanding of value? What problem are you trying to solve?
--
Derek D. Martin
http://www.pizzashack.org/
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