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
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