The “does Python have variables?” debate

[Terry Reedy]

On 5/8/2014 4:14 AM, Chris Angelico wrote:
> On Thu, May 8, 2014 at 6:07 PM, Joseph Martinot-Lagarde
> <joseph.martinot-lagarde at m4x.org> wrote:
>> For me, names bound to values is the same concept as pointer pointing to
>> memory.

You are missing a level of indirection. In CPython, a name (always in 
some namespace) represents a pointer to a pointer. The first pointer is 
calculated as an offset from the base pointer of the namespace. For a C 
array namespace (used for functions), the offset is constant (for that 
function and namespace). For a python dict namespace, the offset is 
calculated from the hash of the name, the size of the dict, and the 
previous contents of the hash table. This complication allows the 
namespace to grow and shrink. In either case, the second pointer if 
overwritten to 'rebind' the name.

>> bar = foo copies the pointer and not the underlying memory.

It copies the second pointer of foo, in the underlying memory pointed at 
by the foo's first pointer, to the memory pointed at by bar's first pointer.

 >> This is not a foreign concept to C programmers.

Once one gets the indirection correct.

> That is how it's implemented in CPython, after all... modulo the whole
> refcounting thing, of course. But that's not strictly a part of the
> definition of Python; it's just implementation. You could implement
> Python on pencil-and-paper (aka a dry run), and as long as you have a
> concept of names that reference objects, it's going to be fine.

For paper-aided mental execution, most people would not bother to 
implement version and run dependent features, such as 'id()' and most of 
the possible uses of 'is' (other than 'is None', etc.). For code that 
also avoids such features, such shortcuts would indeed be fine.

> "reference" might be done with a physical piece of string connecting a
> Post-It note with a name to a box with the object's state, meaning
> there's no way of copying any pointer, and it's still Python.

-- 
Terry Jan Reedy