nonlocal fails ?

[Terry Reedy]

On 11/14/2019 12:53 PM, Richard Damon wrote:
>>
>> On Nov 14, 2019, at 12:20 PM, R.Wieser <address at not.available> wrote:
>>
>> MRAB,
>>
>>> 'nonlocal' is used where the function is nested in another function
>>
>> The problem is that that was not clear to me from the description - nor is
>> it logical to me why it exludes the main context from its use.
>>
>> Regards,
>> Rudy Wieser
> 
> Module ‘main’ content is already available via ‘global’, nonlocal was likely added later to get to enclosing function scope, which isn’t global, nor is it local.

Correct, and the addition was in two stages.  At first, automatically 
*accessing* non-locals was added.  Note that no declaration is needed, 
just as no declaration is needed to access globals.

 >>> >>> f(3)
3

The 'nonlocal' keyward to *rebind* nonlocals, analogous to the 'global' 
keyword to rebind module globals, was added later, for 3.0, after 
considerable bikeshedding  on the new keyword.

The access mechanism is quite different.  Function f accesses globals 
through f.__globals__, which points to the global dict.  In CPython, at 
least, it accesses nonlocals through f.__closure__, a tuple of 'cells'.

 >>> def f(a):
	b = 'b outer'
	def g():
		nonlocal b
		print(a, b)
		b = 'b inner'
		print(a, b)
	print(g.__closure__)
	g()

	
 >>> f(0)
(<cell at 0x00000277AAA90910: int object at 0x00007FFD9ACC9680>, <cell 
at 0x00000277AAAF7880: str object at 0x00000277AAB0DAB0>)
0 b outer
0 b inner
 >>> print(f.__closure__)
None

Closures are used in functional programs, instead of classes, to create 
'groups' of functions than share 'group' variables.

-- 
Terry Jan Reedy