Pass by reference or by value?

[Steve Holden]

Robert Dailey wrote:
[but he top-posted, so he should consider himself smacked on the wrist]
> On 8/16/07, *Steve Holden* <steve at holdenweb.com 
> <mailto:steve at holdenweb.com>> wrote:
> 
>     Robert Dailey wrote:
>      > So immutable objects cannot be modified directly? I guess this means
>      > integers are immutable and the act of assigning to one is a
>     completely
>      > new definition?
> 
>     Correct. A new value is bound to the name or item on the left-hand side
>     - remember, all variables are pointers to values, there's no way to
>     replace the value of a variable because of the automatic dereferencing.
>     And yes, immutable objects can't be modified. At all, period. Hence the
>     name.
> 
>      >                 So if I were to create a class called Integer and
>     give
>      > it a .set() method, this would allow me to create mutable
>     integers, and
>      > thus passing in an object of type class Integer would allow me to
>     modify
>      > the value from inside the function?
>      >
>     Well, the .set() method wouldn't be called automatically on an
>     assignment statement but yes, if you passed an Integer object into your
>     function and called its .set() method from inside then it should work.
> 
>     As long as you get your mutable integer implementation correct, of
>     course ;-). I'd suggest delegating everything except the .set() method
>     to the underlying integer value.

 > Thanks Steve for your explanation. It was very helpful. I think I
 > understand it now. By the way, by the .set method I meant:
 >
 > class Integer:
 >     def __init__( self, number=0 ):
 >        self._int = number
 >
 >     def set( self, number ):
 >        self._int = number
 >
 >
 > # later on....
 >
 > mutableInt = Integer( 5 )
 > def change_me( var ):
 >     var.set( 6 )
 >
 >
 > Of course, I'd probably use overloaded operators in a more realized
 > example.
 >
 >
Fair enough, but I was trying to point out that assignment isn't an 
operator so you won't be able to achieve what you want by assignment to 
a name local to the function.

You could, of course, assign to a name local to the instance you passed 
in, but then there wouldn't be much point in using anything but a simple 
subclass of object (normally referred to as Bunch).

regards
  Steve
-- 
Steve Holden        +1 571 484 6266   +1 800 494 3119
Holden Web LLC/Ltd           http://www.holdenweb.com
Skype: holdenweb      http://del.icio.us/steve.holden
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