On Oct 30, 2008, at 7:56 AM, Dale Roberts wrote:
That's the misconception that is leading some folks around here into
tangled nots of twisty mislogic, ultimately causing them to make up
new terms for what every other modern language is perfectly happy
calling Call-By-Value.
Doesn't this logic also apply to Call By Reference? Isn't that term
redundant too? (see my 3 C++ examples above). If not, why not?
It's not. Wikipedia explains the difference pretty well.
Are you saying that C++ is capable of using the Call By Reference
idiom,
but C is not, because C does not have a reference designation for
formal
function parameters?
It's been a LONG time since I did anything in C, but yes, I believe
that reference parameters were an addition that came with C++.
"Call By Object Reference" is an idiom, just like Call By Reference.
It is not a physical description of what is going on internally at the
register/stack level (which is always just shuffling values around -
or flipping bits, as Steven points out), it is a higher level concept
that helps people understand the *intention* (not necessarily the
implementation) of the mechanism.
Right. And you can easily tell the difference, by whether an
assignment to the formal parameter causes any change to the actual
parameter that was passed in. If it does, that's call-by-reference.
If it doesn't, it's call-by-value. In Python, it doesn't. In VB.NET
(and relatives), it doesn't if the parameter is declared (explicitly
or implicitly) "ByVal", and does if it's declared "ByRef". (Whether
the parameter is a reference type or a simple value type makes no
difference.)
Python's behavior is exactly and always equivalent to the "ByVal"
behavior of languages that have both behaviors. It also matches the
definition of call-by-value. I quite agree that it's not helpful to
delve into the physical flipping of transistor states. We're talking
about the behavior, and the behavior, very clearly, is call-by-value.
To call it something else only clouds the issue and results in
confusion. (Perhaps explaining why there appears to be far more
confusion about call semantics in the Python community than in the
community of other languages where the default semantics are exactly
the same.)
Best,
- Joe
--
http://mail.python.org/mailman/listinfo/python-list
