Finding the instance reference of an object

[Dale Roberts]

On Oct 30, 11:03 am, Joe Strout <j... at strout.net> wrote:
> ...
>> Are you saying that C++ is capable of using the Call By Reference idiom,
>> but C is not, because C does not have a reference designation for formal
>> function parameters?
>
> It's been a LONG time since I did anything in C, but yes, I believe  
> that reference parameters were an addition that came with C++.

Okay, I completely understand you, and I think we will just have to
agree to disagree about the best term to use for Python's parameter
passing mechanism, and this will likely be my concluding post on this
topic (although I have enjoyed it very much and have solidified my own
understanding).

I even found a few web sites that very strongly support your viewpoint
(as it relates to Java):

  http://www.ibm.com/developerworks/library/j-praxis/pr1.html
  http://javadude.com/articles/passbyvalue.htm
  http://www.yoda.arachsys.com/java/passing.html

The difference is that I would say that C supports the Pass By
Reference idiom using this syntax:

  myfunc(int *val){}   /*CALL:*/ myfunc(&i);

which actually passes an address expression (not a variable) by value,
but "looks and feels" like a reference to the "i" variable, which
contains the real value that we care about - and allows modification
of that value.

C++ adds a syntactic change for this very commonly used C idiom, but
does not add any new capability - the results are absolutely
indistinguishable.

Python, likewise, in relation to the values we care about (the values
contained only in objects, never in variables) behaves like Call by
Object Reference.

If I tell someone that Python uses only Call By Value (and that is all
I tell them), they may come away with the impression that variables
contain the values they care about, and/or that the contents of
objects are copied, neither of which is the case, even for so-called
"simple", immutable objects (indeed, at the start of this thread, you
said you believed that, like Java, simple values were contained within
Python variables).

But Python, unlike Java or most other commonly used languages, can
ONLY EVER pass an object reference, and never an actual value I care
about, and I think that idiom deserves a different name which
distinguishes it from the commonly accepted notion of Pass By Value.

Thanks for a thoughtful discussion,
dale