Doc/ref/ref4.tex

   1 \chapter{Execution model\label{execmodel}}
   2 \index{execution model}
   3
   4 \section{Code blocks, execution frames, and namespaces\label{execframes}}
   5 \index{code block}
   6 \indexii{execution}{frame}
   7 \index{namespace}
   8
   9 A \dfn{code block} is a piece of Python program text that can be
  10 executed as a unit, such as a module, a class definition or a function
  11 body.  Some code blocks (like modules) are normally executed only once, others
  12 (like function bodies) may be executed many times.  Code blocks may
  13 textually contain other code blocks.  Code blocks may invoke other
  14 code blocks (that may or may not be textually contained in them) as
  15 part of their execution, e.g., by invoking (calling) a function.
  16 \index{code block}
  17 \indexii{code}{block}
  18
  19 The following are code blocks: A module is a code block.  A function
  20 body is a code block.  A class definition is a code block.  Each
  21 command typed interactively is a separate code block; a script file (a
  22 file given as standard input to the interpreter or specified on the
  23 interpreter command line the first argument) is a code block; a script
  24 command (a command specified on the interpreter command line with the
  25 `\code{-c}' option) is a code block.  The file read by the built-in
  26 function \function{execfile()} is a code block.  The string argument
  27 passed to the built-in function \function{eval()} and to the
  28 \keyword{exec} statement is a code block.  And finally, the expression
  29 read and evaluated by the built-in function \function{input()} is a
  30 code block.
  31
  32 A code block is executed in an execution frame.  An \dfn{execution
  33 frame} contains some administrative information (used for debugging),
  34 determines where and how execution continues after the code block's
  35 execution has completed, and (perhaps most importantly) defines two
  36 namespaces, the local and the global namespace, that affect
  37 execution of the code block.
  38 \indexii{execution}{frame}
  39
  40 A \dfn{namespace} is a mapping from names (identifiers) to objects.
  41 A particular namespace may be referenced by more than one execution
  42 frame, and from other places as well.  Adding a name to a namespace
  43 is called \dfn{binding} a name (to an object); changing the mapping of
  44 a name is called \dfn{rebinding}; removing a name is \dfn{unbinding}.
  45 Namespaces are functionally equivalent to dictionaries (and often
  46 implemented as dictionaries).
  47 \index{namespace}
  48 \indexii{binding}{name}
  49 \indexii{rebinding}{name}
  50 \indexii{unbinding}{name}
  51
  52 The \dfn{local namespace} of an execution frame determines the default
  53 place where names are defined and searched.  The \dfn{global
  54 namespace} determines the place where names listed in \keyword{global}
  55 statements are defined and searched, and where names that are not
  56 bound anywhere in the current code block are searched.
  57 \indexii{local}{namespace}
  58 \indexii{global}{namespace}
  59 \stindex{global}
  60
  61 Whether a name is local or global in a code block is determined by
  62 static inspection of the source text for the code block: in the
  63 absence of \keyword{global} statements, a name that is bound anywhere
  64 in the code block is local in the entire code block; all other names
  65 are considered global.  The \keyword{global} statement forces global
  66 interpretation of selected names throughout the code block.  The
  67 following constructs bind names: formal parameters to functions,
  68 \keyword{import} statements, class and function definitions (these
  69 bind the class or function name in the defining block), and targets
  70 that are identifiers if occurring in an assignment, \keyword{for} loop
  71 header, or in the second position of an \keyword{except} clause
  72 header.  Local names are searched only on the local namespace; global
  73 names are searched only in the global and built-in
  74 namespace.\footnote{
  75 If the code block contains \keyword{exec} statements or the construct
  76 ``\samp{from \ldots import *}'', the semantics of local names change:
  77 local name lookup first searches the local namespace, then the global
  78 namespace and the built-in namespace.}
  79
  80 A target occurring in a \keyword{del} statement is also considered bound
  81 for this purpose (though the actual semantics are to ``unbind'' the
  82 name).
  83
  84 When a global name is not found in the global namespace, it is
  85 searched in the built-in namespace (which is actually the global
  86 namespace of the module \module{__builtin__}).  The built-in namespace
  87 associated with the execution of a code block is actually found by
  88 looking up the name \code{__builtins__} is its global namespace; this
  89 should be a dictionary or a module (in the latter case its dictionary
  90 is used).  Normally, the \code{__builtins__} namespace is the
  91 dictionary of the built-in module \module{__builtin__} (note: no `s');
  92 if it isn't, restricted execution mode is in effect.  When a name is
  93 not found at all, a \exception{NameError} exception is raised.%
  94 \refbimodindex{__builtin__}
  95 \stindex{from}
  96 \stindex{exec}
  97 \stindex{global}
  98 \indexii{restricted}{execution}
  99 \withsubitem{(built-in exception)}{\ttindex{NameError}}
 100
 101 The following table lists the meaning of the local and global
 102 namespace for various types of code blocks.  The namespace for a
 103 particular module is automatically created when the module is first
 104 imported (i.e., when it is loaded).  Note that in almost all cases,
 105 the global namespace is the namespace of the containing module ---
 106 scopes in Python do not nest!
 107
 108 \begin{tableiv}{l|l|l|l}{textrm}
 109   {Code block type}{Global namespace}{Local namespace}{Notes}
 110   \lineiv{Module}
 111          {n.s. for this module}
 112          {same as global}{}
 113   \lineiv{Script (file or command)}
 114          {n.s. for \module{__main__}}
 115          {same as global}{(1)}
 116   \lineiv{Interactive command}
 117          {n.s. for \module{__main__}}
 118          {same as global}{}
 119   \lineiv{Class definition}
 120          {global n.s. of containing block}
 121          {new n.s.}{}
 122   \lineiv{Function body}
 123          {global n.s. of containing block}
 124          {new n.s.}{(2)}
 125   \lineiv{String passed to \keyword{exec} statement}
 126          {global n.s. of containing block}
 127          {local n.s. of containing block}{(2), (3)}
 128   \lineiv{String passed to \function{eval()}}
 129          {global n.s. of caller}
 130          {local n.s. of caller}{(2), (3)}
 131   \lineiv{File read by \function{execfile()}}
 132          {global n.s. of caller}
 133          {local n.s. of caller}{(2), (3)}
 134   \lineiv{Expression read by \function{input()}}
 135          {global n.s. of caller}
 136          {local n.s. of caller}{}
 137 \end{tableiv}
 138 \refbimodindex{__main__}
 139
 140 Notes:
 141
 142 \begin{description}
 143
 144 \item[n.s.] means \emph{namespace}
 145
 146 \item[(1)] The main module for a script is always called
 147 \module{__main__}; ``the filename don't enter into it.''
 148
 149 \item[(2)] The global and local namespace for these can be
 150 overridden with optional extra arguments.
 151
 152 \item[(3)] The \keyword{exec} statement and the \function{eval()} and
 153 \function{execfile()} functions have optional arguments to override
 154 the global and local namespace.  If only one namespace is specified,
 155 it is used for both.
 156
 157 \end{description}
 158
 159 The built-in functions \function{globals()} and \function{locals()} returns a
 160 dictionary representing the current global and local namespace,
 161 respectively.  The effect of modifications to this dictionary on the
 162 namespace are undefined.\footnote{
 163 The current implementations return the dictionary actually used to
 164 implement the namespace, \emph{except} for functions, where the
 165 optimizer may cause the local namespace to be implemented differently,
 166 and \function{locals()} returns a read-only dictionary.}
 167
 168 \section{Exceptions\label{exceptions}}
 169
 170 Exceptions are a means of breaking out of the normal flow of control
 171 of a code block in order to handle errors or other exceptional
 172 conditions.  An exception is \emph{raised} at the point where the error
 173 is detected; it may be \emph{handled} by the surrounding code block or
 174 by any code block that directly or indirectly invoked the code block
 175 where the error occurred.
 176 \index{exception}
 177 \index{raise an exception}
 178 \index{handle an exception}
 179 \index{exception handler}
 180 \index{errors}
 181 \index{error handling}
 182
 183 The Python interpreter raises an exception when it detects a run-time
 184 error (such as division by zero).  A Python program can also
 185 explicitly raise an exception with the \keyword{raise} statement.
 186 Exception handlers are specified with the \keyword{try} ... \keyword{except}
 187 statement.  The \keyword{try} ... \keyword{finally} statement
 188 specifies cleanup code which does not handle the exception, but is
 189 executed whether an exception occurred or not in the preceding code.
 190
 191 Python uses the ``termination'' model of error handling: an exception
 192 handler can find out what happened and continue execution at an outer
 193 level, but it cannot repair the cause of the error and retry the
 194 failing operation (except by re-entering the offending piece of
 195 code from the top).
 196
 197 When an exception is not handled at all, the interpreter terminates
 198 execution of the program, or returns to its interactive main loop.  In
 199 either case, it prints a stack backtrace, except when the exception is
 200 \exception{SystemExit}.\ttindex{SystemExit}
 201
 202 Exceptions are identified by string objects or class instances.
 203 Selection of a matching except clause is based on object identity
 204 (i.e., two different string objects with the same value represent
 205 different exceptions!)  For string exceptions, the \keyword{except}
 206 clause must reference the same string object.  For class exceptions,
 207 the \keyword{except} clause must reference the same class or a base
 208 class of it.
 209
 210 When an exception is raised, an object (maybe \code{None}) is passed
 211 as the exception's ``parameter'' or ``value''; this object does not
 212 affect the selection of an exception handler, but is passed to the
 213 selected exception handler as additional information.  For class
 214 exceptions, this object must be an instance of the exception class
 215 being raised.
 216
 217 See also the description of the \keyword{try} and \keyword{raise}
 218 statements in chapter 7.