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update section 4.1 to describe nested scopes

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Jeremy Hylton 2001-02-01 03:50:59 +00:00
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@ -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.
most importantly) defines the environment in which names are resolved.
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
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!
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__}.
\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 \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