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gh-56374: Clarify documentation of nonlocal (#116942)

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Define 'nonlocal scopes' in a way that excludes class scopes.
Rearrange the rest of the doc.  Add "Programmer's note".
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Terry Jan Reedy 2024-03-19 13:55:21 -04:00 committed by GitHub
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Doc/reference/simple_stmts.rst
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@ -1006,25 +1006,29 @@ The :keyword:`!nonlocal` statement
.. productionlist:: python-grammar
nonlocal_stmt: "nonlocal" `identifier` ("," `identifier`)*
The :keyword:`nonlocal` statement causes the listed identifiers to refer to
previously bound variables in the nearest enclosing scope excluding globals.
This is important because the default behavior for binding is to search the
local namespace first. The statement allows encapsulated code to rebind
variables outside of the local scope besides the global (module) scope.
When the definition of a function or class is nested (enclosed) within
the definitions of other functions, its nonlocal scopes are the local
scopes of the enclosing functions. The :keyword:`nonlocal` statement
causes the listed identifiers to refer to names previously bound in
nonlocal scopes. It allows encapsulated code to rebind such nonlocal
identifiers. If a name is bound in more than one nonlocal scope, the
nearest binding is used. If a name is not bound in any nonlocal scope,
or if there is no nonlocal scope, a :exc:`SyntaxError` is raised.
Names listed in a :keyword:`nonlocal` statement, unlike those listed in a
:keyword:`global` statement, must refer to pre-existing bindings in an
enclosing scope (the scope in which a new binding should be created cannot
be determined unambiguously).
Names listed in a :keyword:`nonlocal` statement must not collide with
pre-existing bindings in the local scope.
The nonlocal statement applies to the entire scope of a function or
class body. A :exc:`SyntaxError` is raised if a variable is used or
assigned to prior to its nonlocal declaration in the scope.
.. seealso::
:pep:`3104` - Access to Names in Outer Scopes
The specification for the :keyword:`nonlocal` statement.
**Programmer's note:** :keyword:`nonlocal` is a directive to the parser
and applies only to code parsed along with it. See the note for the
:keyword:`global` statement.
.. _type:
The :keyword:`!type` statement