IsString

[Chris Mellon]

<snip a bunch of stuff>
> Terminology is important, because we understand the world through
> language. If your language is confused or misleading, your understanding
> will also be confused or incomplete.

I think the issue is pretty simple, myself:

With mutable objects, Python has the semantics normally associated
with pass-by-reference.

With immutable objects, Python has the semantics normally associated
with pass-by-value.

So you could say that Python has both pass-by reference and
pass-by-value (modulo the fact that pass-by-reference doesn't exist at
some level, and is a semantic layer on top of pass-by-value. But
semantics are what we're talking about here).

However, there is *no way* in Python to get pass-by-value semantics
with a mutable object, or pass-by-reference semantics with an
immutable one (you can build your own scaffolding to mimic it, but the
language won't change it's semantics for you).

Therefore, it's more correct to say that Python has neither
pass-by-reference semantics or pass-by-value semantics, but some third
thing. And thus the name pass-by-object, where the calling semantics
are that you always get a reference to an object, but what (if any)
other names are bound to that object, or if they can be bound to that
object, depends on the object.

When you can pass a mutable int to a function (not pass a
namespace/name pair, but a real live mutable int), then Python will
have pass-by-reference.

When you can pass a list that isn't a shared reference without
explicitly copying it or subclassing it to be immutable, then Python
will have pass-by-value.

In the meantime, it has pass-by-object.

>
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> Steven.
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