Doc/libexcs.tex

   1 \section{Built-in Exceptions}
   2
   3 Exceptions are string objects.  Two distinct string objects with the
   4 same value are different exceptions.  This is done to force programmers
   5 to use exception names rather than their string value when specifying
   6 exception handlers.  The string value of all built-in exceptions is
   7 their name, but this is not a requirement for user-defined exceptions
   8 or exceptions defined by library modules.
   9
  10 The following exceptions can be generated by the interpreter or
  11 built-in functions.  Except where mentioned, they have an `associated
  12 value' indicating the detailed cause of the error.  This may be a
  13 string or a tuple containing several items of information (e.g., an
  14 error code and a string explaining the code).
  15
  16 User code can raise built-in exceptions.  This can be used to test an
  17 exception handler or to report an error condition `just like' the
  18 situation in which the interpreter raises the same exception; but
  19 beware that there is nothing to prevent user code from raising an
  20 inappropriate error.
  21
  22 \renewcommand{\indexsubitem}{(built-in exception)}
  23
  24 \begin{excdesc}{AttributeError}
  25 % xref to attribute reference?
  26   Raised when an attribute reference or assignment fails.  (When an
  27   object does not support attributes references or attribute assignments
  28   at all, \code{TypeError} is raised.)
  29 \end{excdesc}
  30
  31 \begin{excdesc}{EOFError}
  32 % XXXJH xrefs here
  33   Raised when one of the built-in functions (\code{input()} or
  34   \code{raw_input()}) hits an end-of-file condition (\EOF{}) without
  35   reading any data.
  36 % XXXJH xrefs here
  37   (N.B.: the \code{read()} and \code{readline()} methods of file
  38   objects return an empty string when they hit \EOF{}.)  No associated value.
  39 \end{excdesc}
  40
  41 \begin{excdesc}{IOError}
  42 % XXXJH xrefs here
  43   Raised when an I/O operation (such as a \code{print} statement, the
  44   built-in \code{open()} function or a method of a file object) fails
  45   for an I/O-related reason, e.g., `file not found', `disk full'.
  46 \end{excdesc}
  47
  48 \begin{excdesc}{ImportError}
  49 % XXXJH xref to import statement?
  50   Raised when an \code{import} statement fails to find the module
  51   definition or when a \code{from {\rm \ldots} import} fails to find a
  52   name that is to be imported.
  53 \end{excdesc}
  54
  55 \begin{excdesc}{IndexError}
  56 % XXXJH xref to sequences
  57   Raised when a sequence subscript is out of range.  (Slice indices are
  58   silently truncated to fall in the allowed range; if an index is not a
  59   plain integer, \code{TypeError} is raised.)
  60 \end{excdesc}
  61
  62 \begin{excdesc}{KeyError}
  63 % XXXJH xref to mapping objects?
  64   Raised when a mapping (dictionary) key is not found in the set of
  65   existing keys.
  66 \end{excdesc}
  67
  68 \begin{excdesc}{KeyboardInterrupt}
  69   Raised when the user hits the interrupt key (normally
  70   \kbd{Control-C} or
  71 \key{DEL}).  During execution, a check for interrupts is made regularly.
  72 % XXXJH xrefs here
  73   Interrupts typed when a built-in function \code{input()} or
  74   \code{raw_input()}) is waiting for input also raise this exception.  No
  75   associated value.
  76 \end{excdesc}
  77
  78 \begin{excdesc}{MemoryError}
  79   Raised when an operation runs out of memory but the situation may
  80   still be rescued (by deleting some objects).  The associated value is
  81   a string indicating what kind of (internal) operation ran out of memory.
  82   Note that because of the underlying memory management architecture
  83   (\C{}'s \code{malloc()} function), the interpreter may not always be able
  84   to completely recover from this situation; it nevertheless raises an
  85   exception so that a stack traceback can be printed, in case a run-away
  86   program was the cause.
  87 \end{excdesc}
  88
  89 \begin{excdesc}{NameError}
  90   Raised when a local or global name is not found.  This applies only
  91   to unqualified names.  The associated value is the name that could
  92   not be found.
  93 \end{excdesc}
  94
  95 \begin{excdesc}{OverflowError}
  96 % XXXJH reference to long's and/or int's?
  97   Raised when the result of an arithmetic operation is too large to be
  98   represented.  This cannot occur for long integers (which would rather
  99   raise \code{MemoryError} than give up).  Because of the lack of
 100   standardization of floating point exception handling in \C{}, most
 101   floating point operations also aren't checked.  For plain integers,
 102   all operations that can overflow are checked except left shift, where
 103   typical applications prefer to drop bits than raise an exception.
 104 \end{excdesc}
 105
 106 \begin{excdesc}{RuntimeError}
 107   Raised when an error is detected that doesn't fall in any of the
 108   other categories.  The associated value is a string indicating what
 109   precisely went wrong.  (This exception is a relic from a previous
 110   version of the interpreter; it is not used any more except by some
 111   extension modules that haven't been converted to define their own
 112   exceptions yet.)
 113 \end{excdesc}
 114
 115 \begin{excdesc}{SyntaxError}
 116 % XXXJH xref to these functions?
 117   Raised when the parser encounters a syntax error.  This may occur in
 118   an \code{import} statement, in an \code{exec} statement, in a call
 119   to the built-in function \code{eval()} or \code{input()}, or
 120   when reading the initial script or standard input (also
 121   interactively).
 122 \end{excdesc}
 123
 124 \begin{excdesc}{SystemError}
 125   Raised when the interpreter finds an internal error, but the
 126   situation does not look so serious to cause it to abandon all hope.
 127   The associated value is a string indicating what went wrong (in
 128   low-level terms).
 129
 130   You should report this to the author or maintainer of your Python
 131   interpreter.  Be sure to report the version string of the Python
 132   interpreter (\code{sys.version}; it is also printed at the start of an
 133   interactive Python session), the exact error message (the exception's
 134   associated value) and if possible the source of the program that
 135   triggered the error.
 136 \end{excdesc}
 137
 138 \begin{excdesc}{SystemExit}
 139 % XXXJH xref to module sys?
 140   This exception is raised by the \code{sys.exit()} function.  When it
 141   is not handled, the Python interpreter exits; no stack traceback is
 142   printed.  If the associated value is a plain integer, it specifies the
 143   system exit status (passed to \C{}'s \code{exit()} function); if it is
 144   \code{None}, the exit status is zero; if it has another type (such as
 145   a string), the object's value is printed and the exit status is one.
 146
 147   A call to \code{sys.exit} is translated into an exception so that
 148   clean-up handlers (\code{finally} clauses of \code{try} statements)
 149   can be executed, and so that a debugger can execute a script without
 150   running the risk of losing control.  The \code{posix._exit()} function
 151   can be used if it is absolutely positively necessary to exit
 152   immediately (e.g., after a \code{fork()} in the child process).
 153 \end{excdesc}
 154
 155 \begin{excdesc}{TypeError}
 156   Raised when a built-in operation or function is applied to an object
 157   of inappropriate type.  The associated value is a string giving
 158   details about the type mismatch.
 159 \end{excdesc}
 160
 161 \begin{excdesc}{ValueError}
 162   Raised when a built-in operation or function receives an argument
 163   that has the right type but an inappropriate value, and the
 164   situation is not described by a more precise exception such as
 165   \code{IndexError}.
 166 \end{excdesc}
 167
 168 \begin{excdesc}{ZeroDivisionError}
 169   Raised when the second argument of a division or modulo operation is
 170   zero.  The associated value is a string indicating the type of the
 171   operands and the operation.
 172 \end{excdesc}