Statement local namespaces summary (was Re: python3: 'where' keyword)

[Andrey Tatarinov]

Nick Coghlan wrote:
> Nick Coghlan wrote:
> 
>> Semantics
>> ---------
>> The code::
>>
>> <statement> with:
>>    <suite>
>>
>> translates to::
>>
>> def unique_name():
>>     <suite>
>>     <statement>
>> unique_name()
> I've come to the conclusion that these semantics aren't what I would 
> expect from the construct. Exactly what I would expect can't really be 
> expressed in current Python due to the way local name bindings work. The 
> main thing to consider is what one would expect the following to print:
> 
> def f():
>     a = 1
>     b = 2
>     print 1, locals()
>     print 3, locals() using:
>         a = 2
>         c = 3
>         print 2, locals()
>     print 4, locals()
> 
> I think the least suprising result would be:
> 
> 1 {'a': 1, 'b': 2}         # Outer scope
> 2 {'a': 2, 'c': 3}         # Inner scope
> 3 {'a': 2, 'b': 2, 'c': 3} # Bridging scope
> 4 {'a': 1, 'b': 2}         # Outer scope

as for me, I would expect following:

1 {'a': 1, 'b': 2}
2 {'a': 2, 'b': 2, 'c': 3'}
3 {'a': 2, 'b': 2, 'c': 3'}
4 {'a': 1, 'b': 2}

otherwise that would be impossible to do calculations based on scope 
variables and "using:" would be useless =), consider example of usage:

current_position = 1
current_environment # = ...
current_a_lot_of_other_parameters # = ...
scores = [count_score(move) for move in aviable_moves] using:
	def count_score(move):
		#walking through current_environment
		return score