Call by binding [was Re: [Tutor] beginning to code]

[bartc]

On 24/09/2017 15:49, Steve D'Aprano wrote:
> On Mon, 25 Sep 2017 12:35 am, Stefan Ram wrote:
> 
>>    WRT to assertions about Python, I try to base them on the
>>    "The Python Language Reference, Release 3.6.0" (PRL).
>>
>>    So, WRT to parameter passing, I would use this part of the PRL:
>>
>>        »The following constructs bind names: formal parameters
>>        to functions,« PRL 4.2.1
>>
>>    . Therefore, what Python does, I'd call »call by binding«.
> 
> I am not aware of "call by binding" being a well-known or recognised term. Did
> you make it up?
> 
> Also, the problem is that *any* form of function call binds *something* to the
> function parameters. If we consider the Pascal declaration:
> 
> procedure proc(x: integer; var y: integer);
> 
> 
> and then we call it:
> 
> var
>    a, b: integer;
> begin
>    a := 1;
>    b := 2;
>    proc(a, b);
> end.
> 
> 
> then 1 is bound to x and 2 is bound to y. They happen to exist in different
> scopes (x is local to proc, the *name* y is local to proc, but the variable is
> bound to y is global) but that's not important.
> 
> The point I am making is that we could describe just about any and all languages
> with functions "call by binding", whether they are call by value like C, call
> by reference like Fortran, call by need like Haskell, or call by sharing like
> Python.
Then 'binding' is either ill-defined or used wrongly.

Imagine that each name (a,b,x,y) is a little box.

In Pascal, the boxes for a and b contain 1 and 2 respectively. For x and 
y, they contain pointers (dereferenced behind the scenes). Fortran is 
similar (or Fortran IV was; I don't know modern ones). C works the same 
way: every variable has a box, and that box directly contains a value.

In Python, those boxes don't contain anything, except one end of a piece 
of string. The other end is attached to an object.

When you pass a variable to a function like this:

  def fn(X):

  A=1
  fn(A)

Then X gets set up with a string which has the same object at the other 
end as A. (And the object might have a reference count to reflect that 
extra string.)

This is why you can't change A in the caller as there is no way to get 
from X's box to A's box. And the strings to the object are one-way.

In the case of fn(A+2), a new object is created (with value '3', or an 
existing '3' might be used), a new string is attached to it, and the 
other is attached to X.

-- 
bartc