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

[spir]

On 01/27/2014 07:16 AM, Denis Heidtmann wrote:
> Running python 2.7 in linux
>
> Below are two extremes.  Can I get some guidance on this?
>
> Thanks,
> -Denis H
>
>>>> a=zeros((2,3),dtype=int)
>>>> b=a
>>>> a[:,0]=[1,2]
>>>> a
> array([[1, 0, 0],
>         [2, 0, 0]])
>>>> b
> array([[1, 0, 0],
>         [2, 0, 0]])
>>>> a=2
>>>> a
> 2
>>>> b
> array([[1, 0, 0],
>         [2, 0, 0]])

Note: your example is strongly obscured by using weird and rare features that 
don't bring any helpful point to the actual problematic concepts you apparently 
want to deal with.

It seems you are confusing 2 issues: relation (reference) between values 
(objects) and relations between symbols (variables).

The last part of your example implies that you expect that, maybe, symbol 'b' 
may now magically point to 2 just because it were corelated with 'a', which was 
set to point to 2. Correct? If so, you are wrong: there is no relation between 
symbols in python (nore in any other language I know, for the matter). Symbols 
'a' and 'b' are independant, whatever the possible relations between their 
values. If you *replace* a's value, this has no effect on b, even if they 
previously held the very same, unique, value.

Python 3.3.2+ (default, Oct  9 2013, 14:50:09)
[GCC 4.8.1] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> a = [1,2,3]
>>> b = a
>>> a is b
True
>>> a = (1,2)	# replacement
>>> b
[1, 2, 3]

Now, there are 2 ways to change a symbol's value: to *replace* it globally as 
above, or to *modify* it partly

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

A misleading point is exemplified by "a is b": it does not mean that both 
symbols actually are the same one (unique), but that _their value objects_ are 
the same one (unique). This is the role of symbols: once defined, they are used 
for whatever they represent. Here symbols a & b just play their normal role of 
symbols, right?

The above example of modification is only possible if the value is complex (and 
mutable). Simple values like numbers or strings obviously cannot be modified, 
only replaced. Thus, such simple values just behave like "plain old values" in 
static or historical languages (in which there aren't symbols & values are 
runtime, instead plain adresses & raw data). In such languages there is 
systematic copy on assignment, unless one explicitely uses pointers. Maybe you 
are used to that, and expect such behaviour in python.

Quite the opposite, in python "symbolic assignment" (where the right side also 
is a symbol) never copies, in fact never creates a new value, but bind the left 
symbol to the same, unique value, as the right symbol.

Note: such are the semantics (the meaning) of the language. But since as said 
above this does not make any difference in practice for simple values, the 
language implementation is in fact free to copy under the hood, if this is 
simpler or more efficient to do so: the language's semantics are preserved 
nevertheless. However, python's standard implementation does not appear to do so:

>>> a = -12.345
>>> b = a
>>> a is b
True		# a & b are bound to the same value, there's only one -12.345

d