Doc/lib/libexcs.tex

   1 \section{Built-in Exceptions}
   2
   3 \declaremodule{standard}{exceptions}
   4 \modulesynopsis{Standard exceptions classes.}
   5
   6
   7 Exceptions can be class objects or string objects.  Though most
   8 exceptions have been string objects in past versions of Python, in
   9 Python 1.5 and newer versions, all standard exceptions have been
  10 converted to class objects, and users are encouraged to do the same.
  11 The exceptions are defined in the module \module{exceptions}.  This
  12 module never needs to be imported explicitly: the exceptions are
  13 provided in the built-in namespace.
  14
  15 Two distinct string objects with the same value are considered different
  16 exceptions.  This is done to force programmers to use exception names
  17 rather than their string value when specifying exception handlers.
  18 The string value of all built-in exceptions is their name, but this is
  19 not a requirement for user-defined exceptions or exceptions defined by
  20 library modules.
  21
  22 For class exceptions, in a \keyword{try}\stindex{try} statement with
  23 an \keyword{except}\stindex{except} clause that mentions a particular
  24 class, that clause also handles any exception classes derived from
  25 that class (but not exception classes from which \emph{it} is
  26 derived).  Two exception classes that are not related via subclassing
  27 are never equivalent, even if they have the same name.
  28
  29 The built-in exceptions listed below can be generated by the
  30 interpreter or built-in functions.  Except where mentioned, they have
  31 an ``associated value'' indicating the detailed cause of the error.
  32 This may be a string or a tuple containing several items of
  33 information (e.g., an error code and a string explaining the code).
  34 The associated value is the second argument to the
  35 \keyword{raise}\stindex{raise} statement.  For string exceptions, the
  36 associated value itself will be stored in the variable named as the
  37 second argument of the \keyword{except} clause (if any).  For class
  38 exceptions, that variable receives the exception instance.  If the
  39 exception class is derived from the standard root class
  40 \exception{Exception}, the associated value is present as the
  41 exception instance's \member{args} attribute, and possibly on other
  42 attributes as well.
  43
  44 User code can raise built-in exceptions.  This can be used to test an
  45 exception handler or to report an error condition ``just like'' the
  46 situation in which the interpreter raises the same exception; but
  47 beware that there is nothing to prevent user code from raising an
  48 inappropriate error.
  49
  50 \setindexsubitem{(built-in exception base class)}
  51
  52 The following exceptions are only used as base classes for other
  53 exceptions.
  54
  55 \begin{excdesc}{Exception}
  56 The root class for exceptions.  All built-in exceptions are derived
  57 from this class.  All user-defined exceptions should also be derived
  58 from this class, but this is not (yet) enforced.  The \function{str()}
  59 function, when applied to an instance of this class (or most derived
  60 classes) returns the string value of the argument or arguments, or an
  61 empty string if no arguments were given to the constructor.  When used
  62 as a sequence, this accesses the arguments given to the constructor
  63 (handy for backward compatibility with old code).  The arguments are
  64 also available on the instance's \member{args} attribute, as a tuple.
  65 \end{excdesc}
  66
  67 \begin{excdesc}{StandardError}
  68 The base class for all built-in exceptions except
  69 \exception{SystemExit}.  \exception{StandardError} itself is derived
  70 from the root class
  71 \exception{Exception}.
  72 \end{excdesc}
  73
  74 \begin{excdesc}{ArithmeticError}
  75 The base class for those built-in exceptions that are raised for
  76 various arithmetic errors: \exception{OverflowError},
  77 \exception{ZeroDivisionError}, \exception{FloatingPointError}.
  78 \end{excdesc}
  79
  80 \begin{excdesc}{LookupError}
  81 The base class for the exceptions that are raised when a key or
  82 index used on a mapping or sequence is invalid: \exception{IndexError},
  83 \exception{KeyError}.  This can be raised directly by
  84 \function{sys.setdefaultencoding()}.
  85 \end{excdesc}
  86
  87 \begin{excdesc}{EnvironmentError}
  88 The base class for exceptions that
  89 can occur outside the Python system: \exception{IOError},
  90 \exception{OSError}.  When exceptions of this type are created with a
  91 2-tuple, the first item is available on the instance's \member{errno}
  92 attribute (it is assumed to be an error number), and the second item
  93 is available on the \member{strerror} attribute (it is usually the
  94 associated error message).  The tuple itself is also available on the
  95 \member{args} attribute.
  96 \versionadded{1.5.2}
  97
  98 When an \exception{EnvironmentError} exception is instantiated with a
  99 3-tuple, the first two items are available as above, while the third
 100 item is available on the \member{filename} attribute.  However, for
 101 backwards compatibility, the \member{args} attribute contains only a
 102 2-tuple of the first two constructor arguments.
 103
 104 The \member{filename} attribute is \code{None} when this exception is
 105 created with other than 3 arguments.  The \member{errno} and
 106 \member{strerror} attributes are also \code{None} when the instance was
 107 created with other than 2 or 3 arguments.  In this last case,
 108 \member{args} contains the verbatim constructor arguments as a tuple.
 109 \end{excdesc}
 110
 111
 112 \setindexsubitem{(built-in exception)}
 113
 114 The following exceptions are the exceptions that are actually raised.
 115
 116 \begin{excdesc}{AssertionError}
 117 \stindex{assert}
 118 Raised when an \keyword{assert} statement fails.
 119 \end{excdesc}
 120
 121 \begin{excdesc}{AttributeError}
 122 % xref to attribute reference?
 123   Raised when an attribute reference or assignment fails.  (When an
 124   object does not support attribute references or attribute assignments
 125   at all, \exception{TypeError} is raised.)
 126 \end{excdesc}
 127
 128 \begin{excdesc}{EOFError}
 129 % XXXJH xrefs here
 130   Raised when one of the built-in functions (\function{input()} or
 131   \function{raw_input()}) hits an end-of-file condition (\EOF{}) without
 132   reading any data.
 133 % XXXJH xrefs here
 134   (N.B.: the \method{read()} and \method{readline()} methods of file
 135   objects return an empty string when they hit \EOF{}.)
 136 \end{excdesc}
 137
 138 \begin{excdesc}{FloatingPointError}
 139   Raised when a floating point operation fails.  This exception is
 140   always defined, but can only be raised when Python is configured
 141   with the \longprogramopt{with-fpectl} option, or the
 142   \constant{WANT_SIGFPE_HANDLER} symbol is defined in the
 143   \file{config.h} file.
 144 \end{excdesc}
 145
 146 \begin{excdesc}{IOError}
 147 % XXXJH xrefs here
 148   Raised when an I/O operation (such as a \keyword{print} statement,
 149   the built-in \function{open()} function or a method of a file
 150   object) fails for an I/O-related reason, e.g., ``file not found'' or
 151   ``disk full''.
 152
 153   This class is derived from \exception{EnvironmentError}.  See the
 154   discussion above for more information on exception instance
 155   attributes.
 156 \end{excdesc}
 157
 158 \begin{excdesc}{ImportError}
 159 % XXXJH xref to import statement?
 160   Raised when an \keyword{import} statement fails to find the module
 161   definition or when a \code{from \textrm{\ldots} import} fails to find a
 162   name that is to be imported.
 163 \end{excdesc}
 164
 165 \begin{excdesc}{IndexError}
 166 % XXXJH xref to sequences
 167   Raised when a sequence subscript is out of range.  (Slice indices are
 168   silently truncated to fall in the allowed range; if an index is not a
 169   plain integer, \exception{TypeError} is raised.)
 170 \end{excdesc}
 171
 172 \begin{excdesc}{KeyError}
 173 % XXXJH xref to mapping objects?
 174   Raised when a mapping (dictionary) key is not found in the set of
 175   existing keys.
 176 \end{excdesc}
 177
 178 \begin{excdesc}{KeyboardInterrupt}
 179   Raised when the user hits the interrupt key (normally
 180   \kbd{Control-C} or \kbd{DEL}).  During execution, a check for
 181   interrupts is made regularly.
 182 % XXXJH xrefs here
 183   Interrupts typed when a built-in function \function{input()} or
 184   \function{raw_input()}) is waiting for input also raise this
 185   exception.
 186 \end{excdesc}
 187
 188 \begin{excdesc}{MemoryError}
 189   Raised when an operation runs out of memory but the situation may
 190   still be rescued (by deleting some objects).  The associated value is
 191   a string indicating what kind of (internal) operation ran out of memory.
 192   Note that because of the underlying memory management architecture
 193   (C's \cfunction{malloc()} function), the interpreter may not
 194   always be able to completely recover from this situation; it
 195   nevertheless raises an exception so that a stack traceback can be
 196   printed, in case a run-away program was the cause.
 197 \end{excdesc}
 198
 199 \begin{excdesc}{NameError}
 200   Raised when a local or global name is not found.  This applies only
 201   to unqualified names.  The associated value is the name that could
 202   not be found.
 203 \end{excdesc}
 204
 205 \begin{excdesc}{NotImplementedError}
 206   This exception is derived from \exception{RuntimeError}.  In user
 207   defined base classes, abstract methods should raise this exception
 208   when they require derived classes to override the method.
 209   \versionadded{1.5.2}
 210 \end{excdesc}
 211
 212 \begin{excdesc}{OSError}
 213   %xref for os module
 214   This class is derived from \exception{EnvironmentError} and is used
 215   primarily as the \refmodule{os} module's \code{os.error} exception.
 216   See \exception{EnvironmentError} above for a description of the
 217   possible associated values.
 218   \versionadded{1.5.2}
 219 \end{excdesc}
 220
 221 \begin{excdesc}{OverflowError}
 222 % XXXJH reference to long's and/or int's?
 223   Raised when the result of an arithmetic operation is too large to be
 224   represented.  This cannot occur for long integers (which would rather
 225   raise \exception{MemoryError} than give up).  Because of the lack of
 226   standardization of floating point exception handling in C, most
 227   floating point operations also aren't checked.  For plain integers,
 228   all operations that can overflow are checked except left shift, where
 229   typical applications prefer to drop bits than raise an exception.
 230 \end{excdesc}
 231
 232 \begin{excdesc}{RuntimeError}
 233   Raised when an error is detected that doesn't fall in any of the
 234   other categories.  The associated value is a string indicating what
 235   precisely went wrong.  (This exception is mostly a relic from a
 236   previous version of the interpreter; it is not used very much any
 237   more.)
 238 \end{excdesc}
 239
 240 \begin{excdesc}{SyntaxError}
 241 % XXXJH xref to these functions?
 242   Raised when the parser encounters a syntax error.  This may occur in
 243   an \keyword{import} statement, in an \keyword{exec} statement, in a call
 244   to the built-in function \function{eval()} or \function{input()}, or
 245   when reading the initial script or standard input (also
 246   interactively).
 247
 248 When class exceptions are used, instances of this class have
 249 atttributes \member{filename}, \member{lineno}, \member{offset} and
 250 \member{text} for easier access to the details; for string exceptions,
 251 the associated value is usually a tuple of the form
 252 \code{(message, (filename, lineno, offset, text))}.
 253 For class exceptions, \function{str()} returns only the message.
 254 \end{excdesc}
 255
 256 \begin{excdesc}{SystemError}
 257   Raised when the interpreter finds an internal error, but the
 258   situation does not look so serious to cause it to abandon all hope.
 259   The associated value is a string indicating what went wrong (in
 260   low-level terms).
 261
 262   You should report this to the author or maintainer of your Python
 263   interpreter.  Be sure to report the version string of the Python
 264   interpreter (\code{sys.version}; it is also printed at the start of an
 265   interactive Python session), the exact error message (the exception's
 266   associated value) and if possible the source of the program that
 267   triggered the error.
 268 \end{excdesc}
 269
 270 \begin{excdesc}{SystemExit}
 271 % XXXJH xref to module sys?
 272   This exception is raised by the \function{sys.exit()} function.  When it
 273   is not handled, the Python interpreter exits; no stack traceback is
 274   printed.  If the associated value is a plain integer, it specifies the
 275   system exit status (passed to C's \cfunction{exit()} function); if it is
 276   \code{None}, the exit status is zero; if it has another type (such as
 277   a string), the object's value is printed and the exit status is one.
 278
 279   Instances have an attribute \member{code} which is set to the
 280   proposed exit status or error message (defaulting to \code{None}).
 281   Also, this exception derives directly from \exception{Exception} and
 282   not \exception{StandardError}, since it is not technically an error.
 283
 284   A call to \function{sys.exit()} is translated into an exception so that
 285   clean-up handlers (\keyword{finally} clauses of \keyword{try} statements)
 286   can be executed, and so that a debugger can execute a script without
 287   running the risk of losing control.  The \function{os._exit()} function
 288   can be used if it is absolutely positively necessary to exit
 289   immediately (e.g., after a \function{fork()} in the child process).
 290 \end{excdesc}
 291
 292 \begin{excdesc}{TypeError}
 293   Raised when a built-in operation or function is applied to an object
 294   of inappropriate type.  The associated value is a string giving
 295   details about the type mismatch.
 296 \end{excdesc}
 297
 298 \begin{excdesc}{UnboundLocalError}
 299   Raised when a reference is made to a local variable in a function or
 300   method, but no value has been bound to that variable.  This is a
 301   subclass of \exception{NameError}.
 302 \versionadded{2.0}
 303 \end{excdesc}
 304
 305 \begin{excdesc}{UnicodeError}
 306   Raised when a Unicode-related encoding or decoding error occurs.  It
 307   is a subclass of \exception{ValueError}.
 308 \versionadded{2.0}
 309 \end{excdesc}
 310
 311 \begin{excdesc}{ValueError}
 312   Raised when a built-in operation or function receives an argument
 313   that has the right type but an inappropriate value, and the
 314   situation is not described by a more precise exception such as
 315   \exception{IndexError}.
 316 \end{excdesc}
 317
 318 \begin{excdesc}{WindowsError}
 319   Raised when a Windows-specific error occurs or when the error number
 320   does not correspond to an \cdata{errno} value.  The
 321   \member{errno} and \member{strerror} values are created from the
 322   return values of the \cfunction{GetLastError()} and
 323   \cfunction{FormatMessage()} functions from the Windows Platform API.
 324   This is a subclass of \exception{OSError}.
 325 \versionadded{2.0}
 326 \end{excdesc}
 327
 328 \begin{excdesc}{ZeroDivisionError}
 329   Raised when the second argument of a division or modulo operation is
 330   zero.  The associated value is a string indicating the type of the
 331   operands and the operation.
 332 \end{excdesc}
 333
 334
 335 \setindexsubitem{(built-in warning category)}
 336
 337 The following exceptions are used as warning categories; see the
 338 \module{warnings} module for more information.
 339
 340 \begin{excdesc}{Warning}
 341 Base class for warning categories.
 342 \end{excdesc}
 343
 344 \begin{excdesc}{UserWarning}
 345 Base class for warnings generated by user code.
 346 \end{excdesc}
 347
 348 \begin{excdesc}{DeprecationWarning}
 349 Base class for warnings about deprecated features.
 350 \end{excdesc}
 351
 352 \begin{excdesc}{SyntaxWarning}
 353 Base class for warnings about dubious syntax
 354 \end{excdesc}
 355
 356 \begin{excdesc}{RuntimeWarning}
 357 Base class for warnings about dubious runtime behavior.
 358 \end{excdesc}