Issue #24129: Clarify reference docs for name resolution.

This includes removing the assumption that readers will be familiar with the
name resolution scheme Python used prior to the introduction of lexical
scoping for function namespaces.

Patch by Ivan Levkivskyi.

1 files changed, 101 insertions, 62 deletions

diff --git a/Doc/reference/executionmodel.rst b/Doc/reference/executionmodel.rst
index 6309e59..5dfa0d2 100644
--- a/Doc/reference/executionmodel.rst
+++ b/Doc/reference/executionmodel.rst
@@ -5,29 +5,18 @@
 Execution model
 ***************
 
-.. index:: single: execution model
-
-
-.. _naming:
-
-Naming and binding
-==================
-
 .. index::
+   single: execution model
    pair: code; block
-   single: namespace
-   single: scope
 
-.. index::
-   single: name
-   pair: binding; name
+.. _prog_structure:
 
-:dfn:`Names` refer to objects.  Names are introduced by name binding operations.
-Each occurrence of a name in the program text refers to the :dfn:`binding` of
-that name established in the innermost function block containing the use.
+Structure of a programm
+=======================
 
 .. index:: block
 
+A Python program is constructed from code blocks.
 A :dfn:`block` is a piece of Python program text that is executed as a unit.
 The following are blocks: a module, a function body, and a class definition.
 Each command typed interactively is a block.  A script file (a file given as
@@ -43,43 +32,25 @@ A code block is executed in an :dfn:`execution frame`.  A frame contains some
 administrative information (used for debugging) and determines where and how
 execution continues after the code block's execution has completed.
 
-.. index:: scope
-
-A :dfn:`scope` defines the visibility of a name within a block.  If a local
-variable is defined in a block, its scope includes that block.  If the
-definition occurs in a function block, the scope extends to any blocks contained
-within the defining one, unless a contained block introduces a different binding
-for the name.  The scope of names defined in a class block is limited to the
-class block; it does not extend to the code blocks of methods -- this includes
-comprehensions and generator expressions since they are implemented using a
-function scope.  This means that the following will fail::
-
-   class A:
-       a = 42
-       b = list(a + i for i in range(10))
+.. _naming:
 
-.. index:: single: environment
+Naming and binding
+==================
 
-When a name is used in a code block, it is resolved using the nearest enclosing
-scope.  The set of all such scopes visible to a code block is called the block's
-:dfn:`environment`.
+.. index::
+   single: namespace
+   single: scope
 
-.. index:: pair: free; variable
+.. _bind_names:
 
-If a name is bound in a block, it is a local variable of that block, unless
-declared as :keyword:`nonlocal`.  If a name is bound at the module level, it is
-a global variable.  (The variables of the module code block are local and
-global.)  If a variable is used in a code block but not defined there, it is a
-:dfn:`free variable`.
+Binding of names
+----------------
 
 .. index::
-   single: NameError (built-in exception)
-   single: UnboundLocalError
+   single: name
+   pair: binding; name
 
-When a name is not found at all, a :exc:`NameError` exception is raised.  If the
-name refers to a local variable that has not been bound, an
-:exc:`UnboundLocalError` exception is raised.  :exc:`UnboundLocalError` is a
-subclass of :exc:`NameError`.
+:dfn:`Names` refer to objects.  Names are introduced by name binding operations.
 
 .. index:: statement: from
 
@@ -99,6 +70,46 @@ this purpose (though the actual semantics are to unbind the name).
 Each assignment or import statement occurs within a block defined by a class or
 function definition or at the module level (the top-level code block).
 
+.. index:: pair: free; variable
+
+If a name is bound in a block, it is a local variable of that block, unless
+declared as :keyword:`nonlocal` or :keyword:`global`.  If a name is bound at
+the module level, it is a global variable.  (The variables of the module code
+block are local and global.)  If a variable is used in a code block but not
+defined there, it is a :dfn:`free variable`.
+
+Each occurrence of a name in the program text refers to the :dfn:`binding` of
+that name established by the following name resolution rules.
+
+.. _resolve_names:
+
+Resolution of names
+-------------------
+
+.. index:: scope
+
+A :dfn:`scope` defines the visibility of a name within a block.  If a local
+variable is defined in a block, its scope includes that block.  If the
+definition occurs in a function block, the scope extends to any blocks contained
+within the defining one, unless a contained block introduces a different binding
+for the name.
+
+.. index:: single: environment
+
+When a name is used in a code block, it is resolved using the nearest enclosing
+scope.  The set of all such scopes visible to a code block is called the block's
+:dfn:`environment`.
+
+.. index::
+   single: NameError (built-in exception)
+   single: UnboundLocalError
+
+When a name is not found at all, a :exc:`NameError` exception is raised.
+If the current scope is a function scope, and the name refers to a local
+variable that has not yet been bound to a value at the point where the name is
+used, an :exc:`UnboundLocalError` exception is raised.
+:exc:`UnboundLocalError` is a subclass of :exc:`NameError`.
+
 If a name binding operation occurs anywhere within a code block, all uses of the
 name within the block are treated as references to the current block.  This can
 lead to errors when a name is used within a block before it is bound.  This rule
@@ -115,7 +126,41 @@ global namespace is searched first.  If the name is not found there, the
 builtins namespace is searched.  The :keyword:`global` statement must precede
 all uses of the name.
 
-.. XXX document "nonlocal" semantics here
+The :keyword:`global` statement has the same scope as a name binding operation
+in the same block.  If the nearest enclosing scope for a free variable contains
+a global statement, the free variable is treated as a global.
+
+.. XXX say more about "nonlocal" semantics here
+
+The :keyword:`nonlocal` statement causes corresponding names to refer
+to previously bound variables in the nearest enclosing function scope.
+:exc:`SyntaxError` is raised at compile time if the given name does not
+exist in any enclosing function scope.
+
+.. index:: module: __main__
+
+The namespace for a module is automatically created the first time a module is
+imported.  The main module for a script is always called :mod:`__main__`.
+
+Class definition blocks and arguments to :func:`exec` and :func:`eval` are
+special in the context of name resolution.
+A class definition is an executable statement that may use and define names.
+These references follow the normal rules for name resolution with an exception
+that unbound local variables are looked up in the global namespace.
+The namespace of the class definition becomes the attribute dictionary of
+the class. The scope of names defined in a class block is limited to the
+class block; it does not extend to the code blocks of methods -- this includes
+comprehensions and generator expressions since they are implemented using a
+function scope.  This means that the following will fail::
+
+   class A:
+       a = 42
+       b = list(a + i for i in range(10))
+
+.. _restrict_exec:
+
+Builtins and restricted execution
+---------------------------------
 
 .. index:: pair: restricted; execution
 
@@ -135,26 +180,20 @@ weak form of restricted execution.
    :keyword:`import` the :mod:`builtins` module and modify its
    attributes appropriately.
 
-.. index:: module: __main__
-
-The namespace for a module is automatically created the first time a module is
-imported.  The main module for a script is always called :mod:`__main__`.
-
-The :keyword:`global` statement has the same scope as a name binding operation
-in the same block.  If the nearest enclosing scope for a free variable contains
-a global statement, the free variable is treated as a global.
-
-A class definition is an executable statement that may use and define names.
-These references follow the normal rules for name resolution.  The namespace of
-the class definition becomes the attribute dictionary of the class.  Names
-defined at the class scope are not visible in methods.
-
-
 .. _dynamic-features:
 
 Interaction with dynamic features
 ---------------------------------
 
+Name resolution of free variables occurs at runtime, not at compile time.
+This means that the following code will print 42::
+
+   i = 10
+   def f():
+       print(i)
+   i = 42
+   f()
+
 There are several cases where Python statements are illegal when used in
 conjunction with nested scopes that contain free variables.