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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 |