Finding the instance reference of an object

Steven D'Aprano steve at
Fri Oct 31 02:56:11 CET 2008

On Thu, 30 Oct 2008 09:03:42 -0600, Joe Strout wrote:

> Python's behavior is exactly and always equivalent to the "ByVal"
> behavior of languages that have both behaviors.

Pascal has both ByVal and By Ref, and Python's behaviour is absolutely 
not the same as Pascal's ByVal.

> It also matches the definition of call-by-value.

Which definition of call-by-value? Certainly not the one that most people 
are familiar with, where c-b-v implies that calling a function with a 
large array will copy the entire array.

> I quite agree that it's not helpful to
> delve into the physical flipping of transistor states.

Nor is it helpful to delve into the implementation details of pointers 
and references. There are no such things in Python code, and the Python 
virtual machine doesn't see them. Only the underlying implementation sees 
them, and as Fuzzyman has already explained, the .Net implementation of 
IronPython differs from the C implementation of CPython, and this has 
real consequences for Python code exposed to .Net objects.

> We're talking
> about the behavior, and the behavior, very clearly, is call-by-value.

Only if you redefine c-b-v to something that it unrecognizable to anyone 

Joe, you've got a cheek sneering at us for inventing new terms (that we 
didn't in fact invent, that have a long history in the IT field, one 
involving such giants as Barbara Liskov) when your entire argument rests 
on the hijacking of the word "value" to mean "some arbitrary, 
implementation-dependent reference to the thing which is the actual 

It is an abuse of language and common sense to claim that after executing 
x=42, the "value" of x is 149605700 (or whatever arbitrary memory 
location you happen to get). That nonsensical idea is the bedrock of your 
entire argument.


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