Finding the instance reference of an object

Steven D'Aprano steve at
Fri Nov 7 11:58:33 CET 2008

On Fri, 07 Nov 2008 00:44:21 -0500, Steve Holden wrote:

> Steven D'Aprano wrote:
>> On Thu, 06 Nov 2008 09:59:37 -0700, Joe Strout wrote:
> [...]
>> And by definition, "call by value" means that the parameter is a copy.
>> So if you pass a ten megabyte data structure to a function using
>> call-by- value semantics, the entire ten megabyte structure is copied.
>> Since this does not happen in Python, Python is not a call-by-value
>> language. End of story.
>>> Without knowing that, you don't know what assignments to the formal
>>> parameter will do, or even what sort of arguments are valid. Answer:
>>> it's a copy of it.
>> Lies, all lies. Python doesn't copy variables unless you explicitly ask
>> for a copy. That some implementations of Python choose to copy pointers
>> rather than move around arbitrarily large blocks of memory instead is
>> an implementation detail. It's an optimization and irrelevant to the
>> semantics of argument passing in Python.
> [...]
> Are you sure you meant to write this?

I'm not sure I understand what you're getting at. The only thing I can 
imagine is that you think there's some sort of contradiction between my 
two statements. I don't see why you think so. In principle one could 
create an implementation of Python with no stack at all, where everything 
lives in the heap, and all argument passing is via moving the actual 
objects ("large blocks of memory") rather than references to the objects.

I don't mean to imply that this is a practical way to go about 
implementing Python, particularly given current computer hardware where 
registers are expensive and the call stack is ubiquitous. But there are 
other computing models, such as the "register machine" model which uses a 
hypothetically-infinite number of registers, no stack and no heap. With 
sufficient effort, one could implement a Python compiler using only a 
Turing Machine. Turing Machines don't have call-by-anything semantics. 
What would that say about Python?

Alternatively, you're commenting about my comment about copying pointers. 
I thought I had sufficiently distinguished between what takes place at 
the level of the Python VM (nothing is copied without an explicit request 
to copy it) and what takes place in the VM's implementation (the 
implementation is free to copy whatever pointers it wants as an 
optimization). If I failed to make that clear, I hope I have done so now.

Pointers do not exist at the level of the Python VM, but they clearly 
exist in the C implementation. We agree that C is call-by-value. CPython 
is (obviously) built on top of C's call-by-value semantics, by passing 
pointers (or references if you prefer) to objects. But that doesn't imply 
that Python's calling semantics are the same as C's.

Algol is call-by-name. Does anyone doubt that we could develop an Algol 
implementation of Python that used nothing but call-by-name semantics at 
the implementation level? Would that mean that Python was call-by-name?

The point is that Joe's argument is based on a confusion between the C 
implementation (calling a function with argument x makes a copy of a 
pointer to x and puts it into the stack for the function) and what 
happens at the level of the Python language.


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