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diff --git a/Doc/ref/ref4.tex b/Doc/ref/ref4.tex deleted file mode 100644 index f38b948..0000000 --- a/Doc/ref/ref4.tex +++ /dev/null @@ -1,215 +0,0 @@ -\chapter{Execution model \label{execmodel}} -\index{execution model} - - -\section{Naming and binding \label{naming}} -\indexii{code}{block} -\index{namespace} -\index{scope} - -\dfn{Names}\index{name} refer to objects. Names are introduced by -name binding operations. Each occurrence of a name in the program -text refers to the \dfn{binding}\indexii{binding}{name} of that name -established in the innermost function block containing the use. - -A \dfn{block}\index{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 standard input to the -interpreter or specified on the interpreter command line the first -argument) is a code block. A script command (a command specified on -the interpreter command line with the `\strong{-c}' option) is a code -block. The file read by the built-in function \function{execfile()} -is a code block. The string argument passed to the built-in function -\function{eval()} and to the \keyword{exec} statement is a code block. -The expression read and evaluated by the built-in function -\function{input()} is a code block. - -A code block is executed in an \dfn{execution -frame}\indexii{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. - -A \dfn{scope}\index{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. - -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{environment}. - -If a name is bound in a block, it is a local variable of that block. -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}\indexii{free}{variable}. - -When a name is not found at all, a -\exception{NameError}\withsubitem{(built-in -exception)}{\ttindex{NameError}} exception is raised. If the name -refers to a local variable that has not been bound, a -\exception{UnboundLocalError}\ttindex{UnboundLocalError} exception is -raised. \exception{UnboundLocalError} is a subclass of -\exception{NameError}. - -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. The \keyword{import} statement of the form ``\samp{from -\ldots import *}''\stindex{from} 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). It is illegal to unbind a name that is referenced by an -enclosing scope; the compiler will report a \exception{SyntaxError}. - -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). - -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 is subtle. Python lacks declarations and allows -name binding operations to occur anywhere within a code block. The -local variables of a code block can be determined by scanning the -entire text of the block for name binding operations. - -If the global statement occurs within a block, all uses of the name -specified in the statement refer to the binding of that name in the -top-level namespace. Names are resolved in the top-level namespace by -searching the global namespace, i.e. the namespace of the module -containing the code block, and the builtin namespace, the namespace of -the module \module{__builtin__}. The global namespace is searched -first. If the name is not found there, the builtin namespace is -searched. The global statement must precede all uses of the name. - -The built-in namespace associated with the execution of a code block -is actually found by looking up the name \code{__builtins__} in its -global namespace; this should be a dictionary or a module (in the -latter case the module's dictionary is used). By default, when in the -\module{__main__} module, \code{__builtins__} is the built-in module -\module{__builtin__} (note: no `s'); when in any other module, -\code{__builtins__} is an alias for the dictionary of the -\module{__builtin__} module itself. \code{__builtins__} can be set -to a user-created dictionary to create a weak form of restricted -execution\indexii{restricted}{execution}. - -\begin{notice} - Users should not touch \code{__builtins__}; it is strictly an - implementation detail. Users wanting to override values in the - built-in namespace should \keyword{import} the \module{__builtin__} - (no `s') module and modify its attributes appropriately. -\end{notice} - -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 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. - -\subsection{Interaction with dynamic features \label{dynamic-features}} - -There are several cases where Python statements are illegal when -used in conjunction with nested scopes that contain free -variables. - -If a variable is referenced in an enclosing scope, it is illegal -to delete the name. An error will be reported at compile time. - -If the wild card form of import --- \samp{import *} --- is used in a -function and the function contains or is a nested block with free -variables, the compiler will raise a \exception{SyntaxError}. - -If \keyword{exec} is used in a function and the function contains or -is a nested block with free variables, the compiler will raise a -\exception{SyntaxError} unless the exec explicitly specifies the local -namespace for the \keyword{exec}. (In other words, \samp{exec obj} -would be illegal, but \samp{exec obj in ns} would be legal.) - -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 namespace, 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. - -\section{Exceptions \label{exceptions}} -\index{exception} - -Exceptions are a means of breaking out of the normal flow of control -of a code block in order to handle errors or other exceptional -conditions. An exception is -\emph{raised}\index{raise an exception} at the point where the error -is detected; it may be \emph{handled}\index{handle an exception} by -the surrounding code block or by any code block that directly or -indirectly invoked the code block where the error occurred. -\index{exception handler} -\index{errors} -\index{error handling} - -The Python interpreter raises an exception when it detects a run-time -error (such as division by zero). A Python program can also -explicitly raise an exception with the \keyword{raise} statement. -Exception handlers are specified with the \keyword{try} ... \keyword{except} -statement. The \keyword{try} ... \keyword{finally} statement -specifies cleanup code which does not handle the exception, but is -executed whether an exception occurred or not in the preceding code. - -Python uses the ``termination''\index{termination model} model of -error handling: an exception handler can find out what happened and -continue execution at an outer level, but it cannot repair the cause -of the error and retry the failing operation (except by re-entering -the offending piece of code from the top). - -When an exception is not handled at all, the interpreter terminates -execution of the program, or returns to its interactive main loop. In -either case, it prints a stack backtrace, except when the exception is -\exception{SystemExit}\withsubitem{(built-in -exception)}{\ttindex{SystemExit}}. - -Exceptions are identified by class instances. The \keyword{except} -clause is selected depending on the class of the instance: it must -reference the class of the instance or a base class thereof. The -instance can be received by the handler and can carry additional -information about the exceptional condition. - -Exceptions can also be identified by strings, in which case the -\keyword{except} clause is selected by object identity. An arbitrary -value can be raised along with the identifying string which can be -passed to the handler. - -\begin{notice}[warning] -Messages to exceptions are not part of the Python API. Their contents may -change from one version of Python to the next without warning and should not -be relied on by code which will run under multiple versions of the -interpreter. -\end{notice} - -See also the description of the \keyword{try} statement in -section~\ref{try} and \keyword{raise} statement in -section~\ref{raise}. |