On Tue, Apr 24, 2018 at 11:15 AM, Steven D'Aprano <st...@pearwood.info> wrote:
[..]
>> >> 3. Most importantly: it is *not* allowed to mask names in the current
>> >> local scope.
>
> That means you can't rebind existing variables. That means you can't
> rebind to the same variable in a loop.
No, it doesn't. The check is performed during compile phase, and
Python does not unroll loops. Anyways, read below.
> I believe that one of the most important use-cases for binding-
> expression syntax is while loops, like this modified example taken from
> PEP 572 version 3:
>
> while (data = sock.read()):
> print("Received data:", data)
>
> If you prohibit re-binding data, that prohibits cases like this, or even
> using it inside a loop:
>
> for value in sequence:
> process(arg, (item = expression), item+1)
No it doesn't. symtable in Python works differently. I encourage you
to test my reference implementation:
py> for val in [1, 2, 3]:
... print((item=val), item+1)
...
1 2
2 3
3 4
> Why is this allowed?
>
> x = 1 # both are statement forms
> x = 2
>
> but this is prohibited?
>
> x = 1
> (x = 2) # no rebinding is allowed
>
> and even more confusing, this is allowed!
>
> (x = 1) # x doesn't exist yet, so it is allowed
> x = 2 # statement assignment is allowed to rebind
These all are very limited code snippets that you're unlikely to see
in real code. I can write (and I did in this thread) a bunch of
examples of where PEP 572 is also inconsistent.
Yury
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