passing by refference

[Joshua Marshall]

Tim Peters <tim.one at comcast.net> wrote:
...
> Sorry, that didn't make sense.  It's true that, in CPython, pointers to
> objects are passed under the covers, in C's call-by-value fashion, but
> that's an implementation detail, and has no visible effect on the Python
> language's semantics.  Call-by-object really is the best description of
> Python's semantics.  If you do a literature search, you'll also see that
> described as "call by object reference" and "call by sharing".
> Call-by-value it ain't -- unless you view the implementation reference as
> "the value", but that simply isn't helpful to anyone except Python's
> implementors (of which Jeremy and I are two, BTW).

>>>> def f(x):
> ...     x[:] = [-1] * 3
>>>> y = [1, 2, 3]
>>>> f(y)
>>>> y
> [-1, -1, -1]
>>>>

> There's no rational sense in which that can be called call-by-value.  A
> similar program using C structs shows what call-by-value does mean:

Let me add a couple lines to your program:

  >>> def f(x):
  ...     x[:] = [-1] * 3
  >>> y = [1, 2, 3]
  >>> id(y)
  135466204
  >>> f(y)
  >>> y
  [-1, -1, -1]
  >>> id(y)
  135466204

The variable y holds the same value before and after the call to f;
only the list to which y holds a reference has been altered.
Scheme is also a call-by-value language, yet:

  => (define (f x)
       (set-car! x -1)
       (set-cdr! x '(-1 -1)))
  F
  => (define y '(1 2 3))
  Y
  => (f y)
  #unspecified
  => y
  (-1 -1 -1)