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

[Nick Coghlan]

On 2 March 2018 at 16:39, Ethan Furman <ethan at stoneleaf.us> 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).


> No grammatical grit on anyone's monitor, no confusion about which variable
> is being accessed, and no confusion about the lifetime of that variable
> (okay, no /extra/ confusion ;) .
>

Unfortunately, it would mean a lot more "All I did was name a repeated
subexpression and now my function is behaving weirdly".


> 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.

Cheers,
Nick.

-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia
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