Antoon Pardon <antoon.par...@rece.vub.ac.be>:
> Your challenge, shows that you don't fully understand what reference
> variables are. The behaviour you see in Pascal, doesn't depend (alone)
> on the parameter being a reference parameter. It also depends on the
> fact that the assignment in pascal mutates the variable that is
> assigned to. Variables are references if they are aliases, so that if
> you mutate through one alias, the mutation is visible through other
> aliases. So your challenge comes down to expecting me to mutate
> something by means that in python don't allow mutation.
I think bringing Pascal in this discussion is only confusing matters.
Let me repeat the abstract Python data model I gave a couple of days
back:
- there are labeled *pegs* ("variables")
- there are *puppies* ("objects")
- each peg has one *leash* hanging from it
- each leash is tied to a puppy
- each puppy can have zero one or more leashes tied to it
- some puppies can hold leashes in their *mouths*
- some puppies can take hold of new leashes and let go of leashes
I'm not joking. Everybody is arguing about preconceived notions tied to
terminology. The peg-leash-puppy model is accurate and extensive.
We can now give semantics to Python's execution model. For example,
- every rvalue expression evaluates to a leash
- the lvalue expression identifies a peg or a mouth
- the assignment statement hangs a leash on a peg or in a mouth
Marko
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