Doc/ref/ref4.tex

   1 \chapter{Execution model \label{execmodel}}
   2 \index{execution model}
   3
   4
   5 \section{Naming and binding \label{naming}}
   6 \indexii{code}{block}
   7 \index{namespace}
   8 \index{scope}
   9
  10 \dfn{Names}\index{name} refer to objects.  Names are introduced by
  11 name binding operations.  Each occurrence of a name in the program
  12 text refers to the \dfn{binding}\indexii{binding}{name} of that name
  13 established in the innermost function block containing the use.
  14
  15 A \dfn{block}\index{block} is a piece of Python program text that is
  16 executed as a unit.  The following are blocks: a module, a function
  17 body, and a class definition.  Each command typed interactively is a
  18 block.  A script file (a file given as standard input to the
  19 interpreter or specified on the interpreter command line the first
  20 argument) is a code block.  A script command (a command specified on
  21 the interpreter command line with the `\strong{-c}' option) is a code
  22 block.  The file read by the built-in function \function{execfile()}
  23 is a code block.  The string argument passed to the built-in function
  24 \function{eval()} and to the \keyword{exec} statement is a code block.
  25 The expression read and evaluated by the built-in function
  26 \function{input()} is a code block.
  27
  28 A code block is executed in an \dfn{execution
  29 frame}\indexii{execution}{frame}.  A frame contains some
  30 administrative information (used for debugging) and determines where
  31 and how execution continues after the code block's execution has
  32 completed.
  33
  34 A \dfn{scope}\index{scope} defines the visibility of a name within a
  35 block.  If a local variable is defined in a block, its scope includes
  36 that block.  If the definition occurs in a function block, the scope
  37 extends to any blocks contained within the defining one, unless a
  38 contained block introduces a different binding for the name.  The
  39 scope of names defined in a class block is limited to the class block;
  40 it does not extend to the code blocks of methods.
  41
  42 When a name is used in a code block, it is resolved using the nearest
  43 enclosing scope.  The set of all such scopes visible to a code block
  44 is called the block's \dfn{environment}\index{environment}.
  45
  46 If a name is bound in a block, it is a local variable of that block.
  47 If a name is bound at the module level, it is a global variable.  (The
  48 variables of the module code block are local and global.)  If a
  49 variable is used in a code block but not defined there, it is a
  50 \dfn{free variable}\indexii{free}{variable}.
  51
  52 When a name is not found at all, a
  53 \exception{NameError}\withsubitem{(built-in
  54 exception)}{\ttindex{NameError}} exception is raised.  If the name
  55 refers to a local variable that has not been bound, a
  56 \exception{UnboundLocalError}\ttindex{UnboundLocalError} exception is
  57 raised.  \exception{UnboundLocalError} is a subclass of
  58 \exception{NameError}.
  59
  60 The following constructs bind names: formal parameters to functions,
  61 \keyword{import} statements, class and function definitions (these
  62 bind the class or function name in the defining block), and targets
  63 that are identifiers if occurring in an assignment, \keyword{for} loop
  64 header, or in the second position of an \keyword{except} clause
  65 header.  The \keyword{import} statement of the form ``\samp{from
  66 \ldots import *}''\stindex{from} binds all names defined in the
  67 imported module, except those beginning with an underscore.  This form
  68 may only be used at the module level.
  69
  70 A target occurring in a \keyword{del} statement is also considered bound
  71 for this purpose (though the actual semantics are to unbind the
  72 name).  It is illegal to unbind a name that is referenced by an
  73 enclosing scope; the compiler will report a \exception{SyntaxError}.
  74
  75 Each assignment or import statement occurs within a block defined by a
  76 class or function definition or at the module level (the top-level
  77 code block).
  78
  79 If a name binding operation occurs anywhere within a code block, all
  80 uses of the name within the block are treated as references to the
  81 current block.  This can lead to errors when a name is used within a
  82 block before it is bound.
  83
  84 The previous rule is a subtle.  Python lacks declarations and allows
  85 name binding operations to occur anywhere within a code block.  The
  86 local variables of a code block can be determined by scanning the
  87 entire text of the block for name binding operations.
  88
  89 If the global statement occurs within a block, all uses of the name
  90 specified in the statement refer to the binding of that name in the
  91 top-level namespace.  Names are resolved in the top-level namespace by
  92 searching the global namespace, i.e. the namespace of the module
  93 containing the code block, and the builtin namespace, the namespace of
  94 the module \module{__builtin__}.  The global namespace is searched
  95 first.  If the name is not found there, the builtin namespace is
  96 searched.  The global statement must precede all uses of the name.
  97
  98 The built-in namespace associated with the execution of a code block
  99 is actually found by looking up the name \code{__builtins__} in its
 100 global namespace; this should be a dictionary or a module (in the
 101 latter case the module's dictionary is used).  Normally, the
 102 \code{__builtins__} namespace is the dictionary of the built-in module
 103 \module{__builtin__} (note: no `s').  If it isn't, restricted
 104 execution\indexii{restricted}{execution} mode is in effect.
 105
 106 The namespace for a module is automatically created the first time a
 107 module is imported.  The main module for a script is always called
 108 \module{__main__}\refbimodindex{__main__}.
 109
 110 The global statement has the same scope as a name binding operation
 111 in the same block.  If the nearest enclosing scope for a free variable
 112 contains a global statement, the free variable is treated as a global.
 113
 114 A class definition is an executable statement that may use and define
 115 names.  These references follow the normal rules for name resolution.
 116 The namespace of the class definition becomes the attribute dictionary
 117 of the class.  Names defined at the class scope are not visible in
 118 methods.
 119
 120 \subsection{Interaction with dynamic features \label{dynamic-features}}
 121
 122 There are several cases where Python statements are illegal when
 123 used in conjunction with nested scopes that contain free
 124 variables.
 125
 126 If a variable is referenced in an enclosing scope, it is illegal
 127 to delete the name.  An error will be reported at compile time.
 128
 129 If the wild card form of import --- \samp{import *} --- is used in a
 130 function and the function contains or is a nested block with free
 131 variables, the compiler will raise a SyntaxError.
 132
 133 If \keyword{exec} is used in a function and the function contains or
 134 is a nested block with free variables, the compiler will raise a
 135 \exception{SyntaxError} unless the exec explicitly specifies the local
 136 namespace for the \keyword{exec}.  (In other words, \samp{exec obj}
 137 would be illegal, but \samp{exec obj in ns} would be legal.)
 138
 139 The \function{eval()}, \function{execfile()}, and \function{input()}
 140 functions and the \keyword{exec} statement do not have access to the
 141 full environment for resolving names.  Names may be resolved in the
 142 local and global namespaces of the caller.  Free variables are not
 143 resolved in the nearest enclosing namespace, but in the global
 144 namespace.\footnote{This limitation occurs because the code that is
 145     executed by these operations is not available at the time the
 146     module is compiled.}
 147 The \keyword{exec} statement and the \function{eval()} and
 148 \function{execfile()} functions have optional arguments to override
 149 the global and local namespace.  If only one namespace is specified,
 150 it is used for both.
 151
 152 \section{Exceptions \label{exceptions}}
 153 \index{exception}
 154
 155 Exceptions are a means of breaking out of the normal flow of control
 156 of a code block in order to handle errors or other exceptional
 157 conditions.  An exception is
 158 \emph{raised}\index{raise an exception} at the point where the error
 159 is detected; it may be \emph{handled}\index{handle an exception} by
 160 the surrounding code block or by any code block that directly or
 161 indirectly invoked the code block where the error occurred.
 162 \index{exception handler}
 163 \index{errors}
 164 \index{error handling}
 165
 166 The Python interpreter raises an exception when it detects a run-time
 167 error (such as division by zero).  A Python program can also
 168 explicitly raise an exception with the \keyword{raise} statement.
 169 Exception handlers are specified with the \keyword{try} ... \keyword{except}
 170 statement.  The \keyword{try} ... \keyword{finally} statement
 171 specifies cleanup code which does not handle the exception, but is
 172 executed whether an exception occurred or not in the preceding code.
 173
 174 Python uses the ``termination''\index{termination model} model of
 175 error handling: an exception handler can find out what happened and
 176 continue execution at an outer level, but it cannot repair the cause
 177 of the error and retry the failing operation (except by re-entering
 178 the offending piece of code from the top).
 179
 180 When an exception is not handled at all, the interpreter terminates
 181 execution of the program, or returns to its interactive main loop.  In
 182 either case, it prints a stack backtrace, except when the exception is
 183 \exception{SystemExit}\withsubitem{(built-in
 184 exception)}{\ttindex{SystemExit}}.
 185
 186 Exceptions are identified by string objects or class instances.
 187 Selection of a matching except clause is based on object identity
 188 (i.e., two different string objects with the same value represent
 189 different exceptions!)  For string exceptions, the \keyword{except}
 190 clause must reference the same string object.  For class exceptions,
 191 the \keyword{except} clause must reference the same class or a base
 192 class of it.
 193
 194 When an exception is raised, an object (maybe \code{None}) is passed
 195 as the exception's \emph{value}; this object does not affect the
 196 selection of an exception handler, but is passed to the selected
 197 exception handler as additional information.  For class exceptions,
 198 this object must be an instance of the exception class being raised.
 199
 200 \begin{notice}[warning]
 201 Messages to exceptions are not part of the Python API.  Their contents may
 202 change from one version of Python to the next without warning and should not
 203 be relied on by code which will run under multiple versions of the
 204 interpreter.
 205 \end{notice}
 206
 207 See also the description of the \keyword{try} statement in
 208 section~\ref{try} and \keyword{raise} statement in
 209 section~\ref{raise}.