On 1 March 2018 at 06:00, Chris Angelico <rosuav@gmail.com> wrote:
On Thu, Mar 1, 2018 at 6:35 AM, Brendan Barnwell <brenbarn@brenbarn.net> wrote:
> On 2018-02-28 07:18, Chris Angelico wrote:
>> Except that assignment is evaluated RHS before LHS as part of a single
>> statement. When Python goes to look up the name "a" to store it (as
>> the final step of the assignment), the SLNB is still active (it's
>> still the same statement - note that this is NOT expression-local), so
>> it uses the temporary.
>         Wait, so you're saying that if I do
> a = (2 as a)
>         The "a = " assignment assigns to the SLNB, and so is then discarded
> after the statement finishes?
>         That seems very bad to me.  If there are SLNBs with this special
> "as" syntax, I think the ONLY way to assign to an SLNB should be with the
> "as" syntax.  You shouldn't be able to assign to an SLNB with regular
> assignment syntax, even if you created an SNLB with the same name as the LHS
> within the RHS.

That seems a reasonable requirement on the face of it, but what about
these variants?

a = (x as a)
a[b] = (x as a)
b[a] = (x as a)
a[b].c = (x as a)
b[a].c = (x as a)

Which of these should use the SLNB, which should be errors, which
should use the previously-visible binding of 'a'?

This is the kind of ambiguity of intent that goes away if statement locals are made syntactically distinct in addition to being semantically distinct:

  .a = (2 as .a) # Syntax error (persistent bindings can't target statement locals)
  a = (2 as .a) # Binds both ".a" (ephemerally) and "a" (persistently) to "2"
  .a[b] = (x as .a) # Syntax error (persistent bindings can't target statement locals)
  b[.a] = (x as .a) # LHS references .a
  .a[b].c = (x as .a) # Syntax error (persistent bindings can't target statement locals)
  b[.a].c = (x as .a) # LHS references .a

We may still decide that even the syntactically distinct variant poses a net loss to overall readability, but I do think it avoids many of the confusability problems that arise when statement locals use the same reference syntax as regular variable names.


Nick Coghlan   |   ncoghlan@gmail.com   |   Brisbane, Australia