On 05/06/2016 07:37, Steven D'Aprano wrote:
On Sun, 5 Jun 2016 01:17 pm, Lawrence D’Oliveiro wrote:
On Saturday, June 4, 2016 at 3:55:12 AM UTC+12, Matt Wheeler wrote:
It's best to think of them as names, rather than variables, as names in
python don't behave quite how you'll expect variables to if you're coming
from some other languages.
I have made heavy use of Python as well as many other languages, and I
don’t know what you mean. What “other languages” are you thinking of?
Are variables like little boxes? Yes they are.
That's *exactly* what variables in Python are not like.
People can imagine a box around a variable name if they like.
The confusion might arise when they think the value attached to the name
is in the same box. But often, for variables holding or referring to
simple scalar values, it doesn't matter.
But they are all the same
size, while objects may be large (even very large) or small. That is why
variables hold, not objects, but references to objects.
No they don't. You are confusing the implementation with the programming
model.
Following the assignment:
x = 99
if you print(x), do you see something like "reference 0x12345"? No.
Nearly every high level language will perform an automatic dereference
in such cases, between the written name of the variable (that sometimes
corresponds to its /address/), and its value (for example, the contents
of that address).
In Python there might well be a double dereference, from name to
location, which contains an object reference, and from the object
reference to the value that is printed.
While Python can give you access to the object reference rather than the
object value, AFAIK you can't get the location where that object
reference is stored for that variable. All you have is the name.
So for Python it makes more sense to have two boxes, one for the name,
and another for the object value. With an arrow between the two. (Among
other things, this allows same object to be shared between several
variables: multiple boxed variable names all pointing to the same object
box.)
But again, in the case of simple, immutable types, and where the
variable always keeps the same type as is usually the case, there is no
real problem in showing "x" and "99" in the same box, or "99" in the box
and "x" immediately outside:
x:[99]
instead of:
x:[0x12345] -> 0x12345:[99]
--
Bartc
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