On 24/11/2015 14:18, Ned Batchelder wrote:
On Tuesday, November 24, 2015 at 7:57:54 AM UTC-5, Marko Rauhamaa wrote:
Antoon Pardon <antoon.par...@rece.vub.ac.be>:
You then switching to talking about objects, just gives the impression
that object is a synonym for value.
It isn't?
1) In English, "value" means something like, what is this equal to? There
isn't another good word to use in place of "value" here. We might awkwardly
use the word "equivalence" as a synonym. I'll use the word "evalue" (short
for "equal value" or "equivalence value" or "English value"). The ==
operator is useful for testing whether two values are "the same evalue".
In this sense, the expression "[]" always evaluates to the same value,
an empty list:
>>> a = []
>>> b = []
>>> a == b
True
2) In Python, "value" means, what object does a name refer to, or what
object did an evaluation produce. We could use "referent" as a
synonym for this meaning. The Python operator "is" tests whether two
values are the same referent, or, the same object. In this sense, "[]"
always evaluates to a different value, because you get a new empty list
each time:
>>> a = []
>>> b = []
>>> a is b
False
And [] is [] gives False too!
For my own benefit, I use the term Handle when shared values are involved.
It comes from the use of file handles (what you get in some languages
when you open or create a file) which everyone can understand:
* If you copy handle F to G like in an assignment, it obviously doesn't
copy the contents of the file, just the handle.
* If I compare the contents of files F and H, they might be the same,
but they are not necessarily the same file.
* If F and G are the same handle, then if I update a file via F, I will
see the same change via handle G.
The main difference is that when F and G go out of scope and the handles
disappear, the file hopefully still exists! (And ideally the file is
first closed via one of the handles.)
From your example, the syntax '[]' is roughly equivalent to
'create_empty_file()' in my analogy.
(I don't really want to get back to default arguments, but that would
extend to:
def fn(a = create_empty_file()):
which makes it easier to appreciate the implications of 'early' and
'late' binding. But I still think that reads as though you get a fresh,
empty file each time.)
--
Bartc
--
bartc
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