update section 4.1 to describe nested scopes

1 files changed, 83 insertions, 94 deletions

diff --git a/Doc/ref/ref4.tex b/Doc/ref/ref4.tex
index 636bc1d..16a1a8d 100644
--- a/Doc/ref/ref4.tex
+++ b/Doc/ref/ref4.tex
@@ -32,128 +32,117 @@ A code block is executed in an execution frame.  An \dfn{execution
 frame}\indexii{execution}{frame} contains some administrative
 information (used for debugging), determines where and how execution
 continues after the code block's execution has completed, and (perhaps
-most importantly) defines two namespaces, the local and the global
-namespace, that affect execution of the code block.
-
-A \dfn{namespace}\index{namespace} is a mapping from names
-(identifiers) to objects.  A particular namespace may be referenced by
-more than one execution frame, and from other places as well.  Adding
-a name to a namespace is called \dfn{binding}\indexii{binding}{name} a
-name (to an object); changing the mapping of a name is called
-\dfn{rebinding}\indexii{rebinding}{name}; removing a name is
+most importantly) defines the environment in which names are resolved.
+
+A \dfn{namespace}\indexii{namespace} is a mapping from names
+(identifiers) to objects.  An \dfn{environment}\index{environment} is
+a hierarchical collection of the namespaces that are visible to a
+particular code block.  Python namespaces are statically scoped in the
+tradition of Algol, but also has \keyword{global} statement that can
+be used to access the top-level namespace on the environment.
+
+Names refers to objects.  Names are introduced by name
+\dfn{binding}\indexii{binding}{name} operations.  Each occurrence of a name
+in the program text refers to the binding of that name established in
+the innermost function namespace containing the use.  Changing the
+mapping of a name to an object is called
+\dfn{rebinding}\indexii{rebinding}{name}; removing a name is 
 \dfn{unbinding}\indexii{unbinding}{name}.  Namespaces are functionally
 equivalent to dictionaries (and often implemented as dictionaries).
 
-The \dfn{local namespace}\indexii{local}{namespace} of an execution
-frame determines the default place where names are defined and
-searched.  The
-\dfn{global namespace}\indexii{global}{namespace} determines the place
-where names listed in \keyword{global}\stindex{global} statements are
-defined and searched, and where names that are not bound anywhere in
-the current code block are searched.
+When a name is bound, a mapping is created in the \dfn{local
+namespace}\indexii{local}{namespace} of the execution frame unless the
+name is declared global.  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 local namespace.  (Note: This can lead to errors
+when a name is used within a block before it is bound.)
+
+The \dfn{global namespace}\indexii{global}{namespace} determines the
+place where names listed in \keyword{global}\stindex{global}
+statements are defined and searched.  The global namespace of a block
+is the namespace of the module in which the block was defined.
+
+If a name is used within a code block, but it is not bound there and
+is not declared global, it is a \dfn{free variable}
+\indexii{free}{variable}.  A free variable is resolved using the
+nearest enclosing function block that has a binding for the name.  If
+no such block exists, the name is resolved in the global namespace.
+
+When a name is not found at all, a
+\exception{NameError}\withsubitem{(built-in
+exception)}{\ttindex{NameError}} exception is raised.
+
+The local namespace of a class definition becomes the attribute
+dictionary of the class. If a block is contained within a class
+definition, the name bindings that occur in the containing class block
+are not visible to enclosed blocks.
+
+The following constructs bind names: formal parameters to functions,
+\keyword{import} statements, class and function definitions (these bind
+the class or function name in the defining block), and identifiers
+occurring as the target of an assignment, in a \keyword{for} loop header
+(including list comprehensions), or in the second position of an
+\keyword{except} clause.
 
 Whether a name is local or global in a code block is determined by
 static inspection of the source text for the code block: in the
 absence of \keyword{global} statements, a name that is bound anywhere
 in the code block is local in the entire code block; all other names
 are considered global.  The \keyword{global} statement forces global
-interpretation of selected names throughout the code block.  The
-following constructs bind names: formal parameters to functions,
+interpretation of selected names throughout the code block.  
+
+The following constructs bind names: formal parameters to functions,
 \keyword{import} statements, class and function definitions (these
 bind the class or function name in the defining block), and targets
 that are identifiers if occurring in an assignment, \keyword{for} loop
 header, or in the second position of an \keyword{except} clause
-header.  Local names are searched only on the local namespace; global
-names are searched only in the global and built-in
-namespace.\footnote{
-  If the code block contains \keyword{exec} statements or the
-  construct ``\samp{from \ldots import *}'', the semantics of local
-  names change: local name lookup first searches the local namespace,
-  then the global namespace and the built-in namespace.}
+header.  The \keyword{import} statement of the form ``\samp{from
+\ldots import *}'' binds all names defined in the imported module,
+except those beginning with an underscore.  This form may only be used
+at the module level.
 
 A target occurring in a \keyword{del} statement is also considered bound
-for this purpose (though the actual semantics are to ``unbind'' the
-name).
+for this purpose (though the actual semantics are to unbind the
+name).  It is illegal to unbind a name that is referenced by an
+enclosing scope; the compiler will report a \exception{SyntaxError}.
 
 When a global name is not found in the global namespace, it is
 searched in the built-in namespace (which is actually the global
-namespace of the module
-\module{__builtin__}\refbimodindex{__builtin__}).  The built-in
-namespace associated with the execution of a code block is actually
-found by looking up the name \code{__builtins__} is its global
-namespace; this should be a dictionary or a module (in the latter case
-its dictionary is used).  Normally, the \code{__builtins__} namespace
-is the dictionary of the built-in module \module{__builtin__} (note:
-no `s'); if it isn't, restricted
-execution\indexii{restricted}{execution} mode is in effect.  When a 
-name is not found at all, a
-\exception{NameError}\withsubitem{(built-in
-exception)}{\ttindex{NameError}} exception is raised.
+namespace of the module \module{__builtin__}\refbimodindex{__builtin__}).  
+The built-in namespace associated with the execution of a code block
+is actually found by looking up the name \code{__builtins__} is its
+global namespace; this should be a dictionary or a module (in the
+latter case its dictionary is used).  Normally, the
+\code{__builtins__} namespace is the dictionary of the built-in module
+\module{__builtin__} (note: no `s').  If it isn't, restricted
+execution\indexii{restricted}{execution} mode is in effect.
 \stindex{from}
 \stindex{exec}
 \stindex{global}
 
-The following table lists the meaning of the local and global
-namespace for various types of code blocks.  The namespace for a
-particular module is automatically created when the module is first
-imported (i.e., when it is loaded).  Note that in almost all cases,
-the global namespace is the namespace of the containing module ---
-scopes in Python do not nest!
-
-\begin{tableiv}{l|l|l|l}{textrm}
-  {Code block type}{Global namespace}{Local namespace}{Notes}
-  \lineiv{Module}
-         {n.s. for this module}
-         {same as global}{}
-  \lineiv{Script (file or command)}
-         {n.s. for \module{__main__}\refbimodindex{__main__}}
-         {same as global}{(1)}
-  \lineiv{Interactive command}
-         {n.s. for \module{__main__}\refbimodindex{__main__}}
-         {same as global}{}
-  \lineiv{Class definition}
-         {global n.s. of containing block}
-         {new n.s.}{}
-  \lineiv{Function body}
-         {global n.s. of containing block}
-         {new n.s.}{(2)}
-  \lineiv{String passed to \keyword{exec} statement}
-         {global n.s. of containing block}
-         {local n.s. of containing block}{(2), (3)}
-  \lineiv{String passed to \function{eval()}}
-         {global n.s. of caller}
-         {local n.s. of caller}{(2), (3)}
-  \lineiv{File read by \function{execfile()}}
-         {global n.s. of caller}
-         {local n.s. of caller}{(2), (3)}
-  \lineiv{Expression read by \function{input()}}
-         {global n.s. of caller}
-         {local n.s. of caller}{}
-\end{tableiv}
-
-Notes:
-
-\begin{description}
-
-\item[n.s.] means \emph{namespace}
-
-\item[(1)] The main module for a script is always called
-\module{__main__}; ``the filename don't enter into it.''
-
-\item[(2)] The global and local namespace for these can be
-overridden with optional extra arguments.
-
-\item[(3)] The \keyword{exec} statement and the \function{eval()} and
+The namespace for a module is automatically created the first time a
+module is imported.  The main module for a script is always called
+\module{__main__}\refbimodindex{__main__}.
+
+The \function{eval()}, \function{execfile()}, and \function{input()}
+functions and the \keyword{exec} statement do not have access to the
+full environment for resolving names.  Names may be resolved in the
+local and global namespaces of the caller.  Free variables are not
+resolved in the nearest enclosing namespaces, but in the global
+namespace.\footnote{This limitation occurs because the code that is
+    executed by these operations is not available at the time the
+    module is compiled.}
+The \keyword{exec} statement and the \function{eval()} and
 \function{execfile()} functions have optional arguments to override
 the global and local namespace.  If only one namespace is specified,
 it is used for both.
 
 \end{description}
 
-The built-in functions \function{globals()} and \function{locals()} returns a
-dictionary representing the current global and local namespace,
-respectively.  The effect of modifications to this dictionary on the
-namespace are undefined.\footnote{
+The built-in functions \function{globals()} and \function{locals()}
+each return a dictionary, representing the current global and local
+namespace respectively.  The effect of modifications to these
+dictionaries on the namespace are undefined.\footnote{
   The current implementations return the dictionary actually used to
   implement the namespace, \emph{except} for functions, where the
   optimizer may cause the local namespace to be implemented