On 2/15/2014 1:07 AM, Rustom Mody wrote:
On Saturday, February 15, 2014 10:50:35 AM UTC+5:30, Ian wrote:
On Fri, Feb 14, 2014 at 9:24 PM, Rustom Mody wrote:
In the case of physical objects like dice there is a fairly
unquestionable framing that makes identity straightforward --
4-dimensional space-time coordiantes. If the space-time coordinates of
2 objects are all equal then the objects are identical, else not.
Now we analogize the space-time identity of physical objects to
computer identity of computer objects (so-called) and all sorts of
problems ensue.
To start with we say two objects are identical if they have the same
memory address.
This is false. It happens to hold for CPython, but that's an
implementation detail. The definition of object identity does not
depend on memory address. It also doesn't have anything to do with
space-time coordinates. The concept of object identity is an
abstraction, not an analogy from physics.
The language reference states, "Every object has an identity, a type
and a value. An object's identity never changes once it has been
created; you may think of it as the object's address in memory."
Okay, so that quote does bring up memory address, but in my
interpretation that's just an analogy to introduce the concept. The
more important part of that sentence is the first part, which ties an
object's identity to its creation. If two objects share the same
creation, then they're the same object.
Whats the notion of object identity is the question.
Ok so you reject the memory addr as an 'implementation detail'
Then you are obliged to provide some other way of understanding object-identity
'Identity' is an abstraction. For immutable objects, it is essentially
irrelevant. For mutable objects, it is essential in the following sense.
A and B are the same object if mutating A necessarily mutates B and
different if they can be independently mutated.
The id() of an object is a fixed integer that is temporally unique. An
implementation might use the id to access objects. When one executes
Python mentally, one might skip implementing id(). Its main use is to
test an implementation. Most beginners should ignore it.
--
Terry Jan Reedy
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