On Oct 27, 2008, at 11:28 PM, Gabriel Genellina wrote:
En Tue, 28 Oct 2008 00:58:10 -0200, greg
<[EMAIL PROTECTED]> escribió:
Let's look at the definitions of the terms:
(1) Call by value: The actual parameter is an expression. It is
evaluated and the result is assigned to the formal parameter.
Subsequent assignments to the formal parameter do not affect
the actual parameter.
(2) Call by reference: The actual parameter is an lvalue. The
formal parameter becomes an alias for the actual parameter,
so that assigning to the formal parameter has the same
effect as assigning to the actual parameter.
Seems to me that (1) describes exactly how parameter passing
works in Python. So why insist that it's *not* call by value?
Greg is right on the money here. It really is as simple as that.
Those definitions are only applicable to unstructured, primitive
types, where the only relevant operations are "get value" and
"assign value".
Nonsense. They apply to any type. See here for an introduction to
the topic in VB.NET:
http://www.homeandlearn.co.uk/net/nets9p4.html
The example shown uses an Integer, but guess what -- you can pass ANY
type either ByRef or ByVal, including object references (which are, of
course, quite common in VB.NET). The default is ByVal, which means
that (as in Python) the actual parameter is an expression, and no
assignments to it can affect the actual parameter. It DOES NOT MATTER
that both the formal parameter and the actual parameter may refer to
the same object, and if that object is mutable, you can mutate that
object.
But ByRef is another option, and if you pass an object reference
ByRef, then the formal parameter is an alias for the actual parameter,
and assignments to it change the actual parameter. Python doesn't
have this feature (nor much need for it, given its other features,
like tuple packing/unpacking). So, parameter passing in ByRef is
clearly exactly the same as the default "ByVal" parameter passing in
VB.NET... as well as Java, RB, C++ (when you remember that an object
reference in modern languages is like a pointer in C++), and every
other OOP language I've looked into.
Some of those languages, like Python, don't have different modes, and
so the language designers had no call to give their one mode a name.
So let's look at those that did have such a need, like VB.NET and
REALbasic. What's the default mode called? Why, it's "ByVal". Even
when that value is an object reference. This is short for "by value"
-- it's not short for "by sharing" or "by object" or any other such
silliness. No such terms are needed.
There are only the two cases, which Greg quite succinctly and
accurately described above. One is by value, the other is by
reference. Python quite clearly uses by value. Parameters are
expressions that are evaluated, and the resulting value copied into
the formal parameter, pure and simple. The continued attempts to
obfuscate this is pointless and wrong.
Best,
- Joe
--
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