[Python-Dev] assignment expressions: an alternative proposal

[Chris Angelico]

On Wed, Apr 25, 2018 at 2:28 AM, Steven D'Aprano <steve at pearwood.info> wrote:
> On Tue, Apr 24, 2018 at 10:58:24AM -0400, Yury Selivanov wrote:
>
>> Since 'diff' and 'g' must be new names according to rule (3), those
>> who read the code will notice that both were not previously bound.
>
> How am I supposed to notice that they've never been bound without
> carefully reading through the rest of the function in detail, checking
> every single expression and statement?
>
> And besides, you have already established that there are exceptions to
> the rule "names must be new names". For example, in loops.
>
> What other exceptions are there?
>

Yuri is talking about "new" in the syntactic sense. A new name is one
which, reading lexically through the code, has not yet been assigned
to in the function. (I don't know what happens with global/nonlocal
declarations.) Loops have a single point at which the name is assigned
to. This has a single point where the name is assigned, too, even
though you'll never hit it:

def f(x):
    if x is not x:
        y = 1
    print(y) # UnboundLocalError

While I disagree with the proposal, it is at least sane from the
compiler's POV. I don't think it makes sense from a human's POV, but
it's internally consistent.

ChrisA