On Thu, Mar 1, 2018 at 3:31 PM, Nick Coghlan email@example.com wrote:
On 28 February 2018 at 08:27, Chris Angelico firstname.lastname@example.org wrote:
- The current implementation  implements statement-local names using a special (and mostly-invisible) name mangling. This works perfectly inside functions (including list comprehensions), but not at top level. Is this a serious limitation? Is it confusing?
It isn't clear to me from the current PEP what the intended lifecycle of the bound names actually is, especially for compound statements.
I think you're looking at an old version of the PEP, but that's kinda gonna happen on the first day of a hot topic :) But to remove all confusion, I've now added a section clarifying execution order, using several of your examples.
x = (expr as y) assert x == y # Does this pass? Or raise NameError for 'y'?
NameError. The SLNB is gone at end of line.
if (condition as c): assert c # Does this pass? Or raise NameError for 'c'? else: assert not c # Does this pass? Or raise NameError for 'c'? assert c or not c # Does this pass? Or raise NameError for 'c'?
c is available in the indented blocks, and is gone once the entire 'if/else' block is done.
class C: x = (True as y) assert C.y # Does this pass? Or raise AttributeError for 'y'?
That'll raise. (At least, according to the specification and intent. The reference implementation may be lagging a bit.)
I think it would also be worth explicitly considering a syntactic variant that requires statement local references to be explicitly disambiguated from regular variable names by way of a leading dot:
result = [[(f(x) as .y), .y] for x in range(5)]
Since ".NAME" is illegal for both variable and attribute names, this makes the fact statement locals are a distinct namespace visible to readers as well as to the compiler, and also reduces the syntactic ambiguity in with statements and exception handlers.
I've mentioned this in the alternate syntaxes, but I don't like having to state a variable's scope in its name. Python doesn't force us to adorn all global names with a character, and the difference between function-local and statement-local is generally less important than the difference between global and function-local. But it IS a viable syntax, and I'm saying so in the PEP.
Thanks for the feedback! Much appreciated.