How to Teach Python "Variables"

hdante hdante at gmail.com
Tue Nov 27 19:21:36 CET 2007


On Nov 26, 7:49 am, Hrvoje Niksic <hnik... at xemacs.org> wrote:
> greg <g... at cosc.canterbury.ac.nz> writes:
> > none wrote:
> >>     IIRC, I once saw an explanation how Python doesn't have
> >> "variables" in the sense that, say, C does, and instead has bindings
> >> from names to objects.
>
> > If you're talking to C programmers, just tell them that Python
> > variables always contain pointers. That should give them the right
> > mental model to build on.
>
> That is a convenient shortcut when it works, but in my experience it
> tends to confuse the issue.  The reason is that one of the main uses
> of pointers in C is implementing pass-by-reference.  A C programmer
> told that Python variables internally hold pointers expects this code:
>
> def func(a):
>   a = 10
> ...
> func(x)
>
> to change the value of x.

 This shouldn't confuse a C programmer if he understands that
assignment changes the pointer address, instead of copying the value:

 void func(int *a) {
   int *tmp = malloc(sizeof(int));
   *tmp = 10;
   a = tmp;
   free(tmp);
 }

 int *x = some_address;
 func(x);
 assert(x == some_address);

 The confusion is that assignment does not copy the object. Python
variables are pointers and that's it.



More information about the Python-list mailing list