On 03/02/2018 02:47 AM, Nick Coghlan wrote:
On 2 March 2018 at 16:39, Ethan Furman wrote:
On 03/01/2018 09:08 PM, Nick Coghlan wrote:
Adding statement local variables into that mix *without* some form of syntactic marker would mean taking an already complicated system, and making it even harder to reason about correctly (especially if statement locals interact with nested scopes differently from the way other locals in the same scope do).
Seems like it would far easier and (IMHO) more useful to scale the proposal back from a statement scope to simple expression assignment, and the variable is whatever scope it would have been if assigned to outside the expression (default being local, but non-local or global if already declared as such).
Because that would put us back in the exact same problematic situation we had when "[x*x for x in sequence]" leaked the iteration variable (only worse): no function locals would be safe, since arbitrary expressions could clobber them, not just name binding operations (assignment, import statements, for loops, with statements, exception handlers, class and function definitions).
Ah, right. Since the PEP primarily covers comprehensions, but then went on to discuss multi-line statements, I had forgotten the comprehension part. The answer is easy: assignment expressions in comprehensions stay inside comprehensions, just like other inner comprehension variables already do (function sub-scope, after all). Thank you for pointing that out.
Maybe somebody could explain why a statement-local limited scope variable is better than an ordinary well-understood local-scope variable? Particularly why it's better enough to justify more line-noise in the syntax. I'm willing to be convinced (not happy to, just willing ;) .
It breaks the expectation that only a well defined subset of statement can make changes to local name bindings.
We already use the keyword "as" to assign names, so there is nothing extra there -- the only change is that we can now use it in one more place.
Are those valid counter-arguments, or is there still something I am missing?