[Tutor] When is = a copy and when is it an alias

[spir]

On 01/27/2014 06:04 PM, Denis Heidtmann wrote:
> Apparently a[0]=b[0] does not qualify as "symbolic assignment" in this
> case. a[0] is not a reference to b[0].  I think I see the essential
> distinction.  Experience will complete the picture for me.

"symolic assignment" is my term, so whatever I mean with it qualifies as 
symbolic assignment ;-); and tes, your example a[0]=b[0] is indeed a symbolic 
assignment (because the right side "b[0]" denotes a value, an object, and could 
be on the left side of an assignment)
the distinction between "replacement" & "modification" also uses my terms; 
better you know that because if you talk with programmers using these terms as 
key words, others will be surprised

The real point is: maybe read again my previous post. I insisted on the fact 
that in python _symbols_ are not corelated, never ever. Your remark here seems 
to show that you expect a[0] and b[0] to be corelated, in such a way that if we 
change one of them the second should follow. No. Values are made unique by 
symbolic assignment; but this can only show if you modify them partly (not 
replace globally), thus can only show if those are complex values.

>>> a = [1, [1,2]]
>>> b = a
>>> b
[1, [1, 2]]
>>> b is a
True

a's and b's values are a single, unique object... as long as I only modifie them 
(the values) partly:

>>> a = [1,[2,3]]
>>> a[0] = 0
>>> b
[0, [1, 2]]
>>> a[1] = [0,0]
>>> b
[0, [0, 0]]
>>> a is b
True

>>> a[0] = b[0]
>>> a[0] is b[0]
True

Here i make their first elements the same unique value. But I'm blocked to show 
it (other than using 'is') because I cannot modify such objects (they are simple 
numbers). Since it is the _values_ which are related, not the symbols, if I 
replace one of them I break the relation.

>>> a[0] = 9
>>> a
[9, [2, 3]]
>>> b
[1, [0, 0]]

Right? On the other, if I create a relatoin between their second elements, then 
I can have something more interesting:

>>> a[1] = b[1]
>>> a[1] is b[1]
True
>>> a, b
([9, [0, 0]], [1, [0, 0]])
>>> a[1][1] = 9
>>> b[1][1]
9
>>> a, b
([9, [0, 9]], [1, [0, 9]])

You will get used to it... Also, it just works most of the time, except for 
corner cases where you will be trapped 2-3 times until you get it. (The reason 
is that unconsciously, when we want several symbols to have their proper values, 
we just do it right by assigning them apart, even if they (initially) have the 
same values. When instead we want a symbol to refer to the same value as 
another, we correctly use a symbolic assignment; instead of incorrectly 
assigning an equal, but distinct, value.]

denis