Is 'everything' a refrence or isn't it?
steve at REMOVEMEcyber.com.au
Wed Jan 4 21:00:54 EST 2006
Mike Meyer wrote:
> Steven D'Aprano <steve at REMOVETHIScyber.com.au> writes:
>>On Wed, 04 Jan 2006 10:54:17 -0800, KraftDiner wrote:
>>>I was under the assumption that everything in python was a refrence...
>>>so if I code this:
>>>lst = [1,2,3]
>>>for i in lst:
>>> if i==2:
>>> i = 4
>>>I though the contents of lst would be modified.. (After reading that
>>>'everything' is a refrence.)
>>See, this confusion is precisely why I get the urge to slap people who
>>describe Python as "call by reference". It isn't.
> Except this doesn't have *anything at all* to do with python being (or
> not being) call by reference. This is a confusion about name binding
> vs. assignment to a variable. The proper people to slap around for
> this case are the ones who talk about assignment to a variable.
Mike, you are wrong. Not about talk about "assigning to
variables" being harmful, I agree with that. But this
confusion is *exactly* about the call by reference
misunderstanding. How do I know this?
Because the Original Poster said so! He said, to
paraphrase, "Hey, I thought Python was call by
reference, but I tried this, and it didn't work, what
This is not an isolated case either.
You can't get any clearer than that. Talk about call by
reference leads to confusion and incorrect assumptions
about Python's behaviour. End of story.
>>It is "call by object" -- you pass around *objects*. Internally, this is
>>quite fast, because the entire object doesn't need to be moved, only
>>pointers to objects, but you don't get the behaviour of either call by
>>reference or call by value.
> No, you get *exactly* that behavior from call by reference when you
> start passing objects around by reference.
Who cares what Python does internally? It is strictly
irrelevant whether Python internally passes around
pointers, or copies values, or sometimes one or the other.
> If I declare a C object as
> "struct foo bar" and do the C "call-by-reference" hack of passing
> &bar, I get the exact same behavior I get when I pass an object
> referenced by bar to a Python subroutine.
It is not a question of whether Python's behaviour is
*sometimes* the same as call by reference. The question
whether Python's behaviour is *always* the same as CBR,
and it is not. Regardless of the reasons, Python does
not behave like CBR in other languages, and it is
harmful to imply that it does.
Likewise, you shouldn't argue that Python is obviously
call by value just because *sometimes* it has the same
behaviour as CBV.
I mean, come on! A whole bunch of people argue black
and blue that Python "obviously" is call by reference,
and another vocal bunch argue that it is "obviously"
call by value. Isn't that the tiniest hint that Python
-- not the underlying C/Java/Python/whatever
implementation, but Python the language --- does not
fall into either camp, but is doing something that
sometimes looks like one and sometimes like the other?
Just google on "Python is call by value" and "Python is
call by reference" -- including quotes -- to see this
argument come up time and time again, year after year
after year. And all because of some false dichotomy
that CBR and CBV are the only two valid descriptions of
a computer language.
More information about the Python-list