[Python-ideas] PEP 572 version 2: Statement-Local Name Bindings

[Rhodri James]

On 02/03/18 11:43, Chris Angelico wrote:
> After dozens of posts and a wide variety of useful opinions and
> concerns being raised, here is the newest version of PEP 572 for your
> debating pleasure.

I haven't said this yet, so thanks Chris for putting this all together. 
Even if the result is a rejected PEP, at least we have everything in one 
place.

[snip]

>      # Compound statements usually enclose everything...
>      if (re.match(...) as m):
>          print(m.groups(0))
>      print(m) # NameError

This (and the equivalent in while loops) is the big win in the PEP, in 
my opinion.  The number of ugly loops I've had to write in Python 
because I can't write "while (something_to_do() as event):"...  +1 on this.

>      # Using a statement-local name
>      stuff = [[(f(x) as y), x/y] for x in range(5)]

As Paul said, the asymmetry of this bothers me a lot.  It doesn't read 
naturally to me. -1 on this.

> 1. What happens if the name has already been used? ``(x, (1 as x), x)``
>     Currently, prior usage functions as if the named expression did not
>     exist (following the usual lookup rules); the new name binding will
>     shadow the other name from the point where it is evaluated until the
>     end of the statement.  Is this acceptable?  Should it raise a syntax
>     error or warning?

I wouldn't worry too much about this case.  Anyone gratuitously reusing 
names like that deserves all that will be coming to them.

> Alternative proposals
> =====================
> 3. ``with... as``::
> 
>         stuff = [(y, x/y) with f(x) as y for x in range(5)]
> 
>     As per option 2, but using ``as`` in place of the equals sign. Aligns
>     syntactically with other uses of ``as`` for name binding, but a simple
>     transformation to for-loop longhand would create drastically different
>     semantics; the meaning of ``with`` inside a comprehension would be
>     completely different from the meaning as a stand-alone statement.

Honestly I prefer this syntax for comprehensions.  It doesn't read 
perfectly but it's good enough (though I am a mathematician by original 
training, so set notation works for me anyway), and the syntax is clear 
and limited.

I'm not sure the case for fully general statement-local variables has 
been made.

So, counter-proposal(s):

1. Allow "(f() as a)" in the conditions of "if" and "while" statements, 
after some arguing as to whether "a" is a special snowflake or just a 
normal local variable.

2. Add a "with" clause to comprehensions to make comprehension-local 
variables (presumably the same class of thing as the iteration variables).

-- 
Rhodri James *-* Kynesim Ltd