Mutability of function arguments?

[Mike Meyer]

bonono at gmail.com writes:
> Mike Meyer wrote:
>> bonono at gmail.com writes:
>> > Mike Meyer wrote:
>> >> "ex_ottoyuhr" <ex_ottoyuhr at hotmail.com> writes:
>> >> > I'm trying to create a function that can take arguments, say, foo and
>> >> > bar, and modify the original copies of foo and bar as well as its local
>> >> > versions -- the equivalent of C++ funct(&foo, &bar).
>> >> C++'s '&' causes an argument to be passed by reference. Python does
>> >> that with all arguments. Any changes you make to the argument in the
>> >> function will be seen in the caller unless you explicitly make a copy
>> >> to pass.
>> > except when foo and bar are bound to immutable objects.
>> Wrong.
>> > In C:
>> >
>> > int foo=1;
>> > int bar=2;
>> >
>> > void update(int *a, int *b) { *a=3; *b=4}
>> >
>> > update(&foo, &bar);
>>
>> Note that this update is using an assignment statement, and thus
>> changing the arguments.
> void update(int a, int b) { a=3; b=4}
> Is this also an assignment statement ?

Yes. But C calls by value, and passes copies of the original a and b
in. So in this case, you change the copies.

>> > In Python:
>> >
>> > foo=1
>> > bar=2
>> >
>> > def update(a,b): a=3; b=4
>> >
>> > update(foo,bar)
>> This update isn't changing the objects, it's rebinding the names in
>> the local name space. Since you didn't change the objects, there's no
>> change to see in the calling environment.
>> > Many people from C/C++ background would be tricked for this situation.
>> That's because they don't understand binding. Any language that has
>> bindings instead of has assignments will "trick" them this way.
> Sure, any language that behave like this would trick them, I am not
> saying python is the only one or that there is anything wrong with this
> behaviour. I was just saying that it is a situation that they get
> tricked, because of their "perception" about "=", especially when they
> are told that any change to the arguments are seen by the caller.

Except "trick" is a poor word choice. Nobody is playing a trick on
them - they just don't understand what is going on.

     <mike
-- 
Mike Meyer <mwm at mired.org>			http://www.mired.org/home/mwm/
Independent WWW/Perforce/FreeBSD/Unix consultant, email for more information.