nonlocal fails ?

[Chris Angelico]

On Sat, Nov 16, 2019 at 5:41 PM Gregory Ewing
<greg.ewing at canterbury.ac.nz> wrote:
>
> On 16/11/19 8:22 am, Chris Angelico wrote:
> > That's the typical sort of description you get from someone who mostly
> > understands Python's semantics, but is hung up on the idea that
> > everything is either call-by-value or call-by-reference, and is trying
> > to figure out which box Python fits into.
>
> Or they may have read the definition of "call by value" in the
> Algol60 report, which says that "The actual parameter expression
> is evaluated and the result is assigned to the formal parameter."
> Which is exactly what Python does...
>

This is 100% true, but actually just punts on the question of
"call-by-X". All the subtleties are bound up in this little bit:

> the result is **assigned** to the formal parameter

Which is actually a really REALLY good way to start peeling things
back. In Python - and in many many other languages - the semantics of
function calls can be summarized as assignment. You evaluate an
expression in the caller's context, and assign the result to a local
name in the callee's context. So far, C and Python both have the exact
same semantics.

So the real question is: What is assignment? And that's where
everything about name binding comes in. In C, that assignment operates
on a bit-copying level, cloning some value (which might be a pointer).
In Python, that same assignment operates as a name binding, giving the
target a reference to some particular object.

ChrisA