A question on modification of a list via a function invocation

[Steve D'Aprano]

On Mon, 4 Sep 2017 08:12 pm, Rustom Mody wrote:

> Its because reference (or pointer or …) is central to python's semantics
> that we need to use them to talk/explain/understand.

References are central to understanding the implementation of Python
interpreters. (Perhaps not *all* interpreters, but using references of some
sort is the most obvious and easy way to implement Python's semantics.)

Dropping down into the implementation level and talking about references is
often useful. But it is not essential -- one can explain a lot of code without
any reference to "reference" ( pun intended *wink* ) at all. If we're willing
to accept a conceptual model where Python objects are capable of being in two
or more locations at once, we can explain ALL Python code without needing
references.

I accept that many people dislike, or do not understand, conceptual models where
objects can be in more than one location at once. For many people, dropping
into the implementation and talking about references is easier to understand.
But that doesn't make it essential.

The semantics of Python is that we assign objects to names, not references to
objects to names. There's no "get reference" or "address of" operation in
Python. We write:

x = y

not 

x = &y


The interpreter may (or may not) use references or pointers in order to
implement Python's object model, but strictly outside of the scope of Python
the language.



> Its because pointers have been de-first-classed (from C say, as a starting
> point) that the disagreements arise: - One bunch feel that since they've been
> de-first-classed they've been removed

Pointers are not merely second-class values, like functions and procedures in
Pascal, or strings in C. They're not values *at all*.

Its not just that Python doesn't allow you to return a pointer from a function,
or pass a pointer to a function as argument. You cannot dereference a pointer
either, or get a pointer to an object at all. (Although you can emulate
pointers in Python using objects.)


For more about first-class values, see this Stackoverflow thread:

https://stackoverflow.com/questions/2578872/about-first-second-and-third-class-value

We can quibble whether Pascal functions are first-, second- or first-and-a-half
class values, or whether "third-class" even makes sense, but pointers, and
references, are not values of *any* class in Python.




-- 
Steve
“Cheer up,” they said, “things could be worse.” So I cheered up, and sure
enough, things got worse.