Doc/ref/ref4.tex

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