323 lines
14 KiB
TeX
323 lines
14 KiB
TeX
\section{Built-in Exceptions}
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\declaremodule{standard}{exceptions}
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\modulesynopsis{Standard exceptions classes.}
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Exceptions can be class objects or string objects. While
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traditionally, most exceptions have been string objects, in Python
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1.5, all standard exceptions have been converted to class objects,
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and users are encouraged to do the same. The source code for those
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exceptions is present in the standard library module
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\module{exceptions}; this module never needs to be imported explicitly.
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For backward compatibility, when Python is invoked with the \code{-X}
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option, most of the standard exceptions are strings\footnote{For
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forward-compatibility the new exceptions \exception{Exception},
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\exception{LookupError},
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\exception{ArithmeticError}, \exception{EnvironmentError}, and
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\exception{StandardError} are tuples.}. This option may be used to
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run code that breaks because of the different semantics of class based
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exceptions. The \code{-X} option will become obsolete in future
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Python versions, so the recommended solution is to fix the code.
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Two distinct string objects with the same value are considered different
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exceptions. This is done to force programmers to use exception names
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rather than their string value when specifying exception handlers.
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The string value of all built-in exceptions is their name, but this is
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not a requirement for user-defined exceptions or exceptions defined by
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library modules.
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For class exceptions, in a \keyword{try} statement with an \keyword{except}
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clause that mentions a particular class, that clause also handles
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any exception classes derived from that class (but not exception
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classes from which \emph{it} is derived). Two exception classes
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that are not related via subclassing are never equivalent, even if
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they have the same name.
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\stindex{try}
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\stindex{except}
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The built-in exceptions listed below can be generated by the
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interpreter or built-in functions. Except where mentioned, they have
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an ``associated value'' indicating the detailed cause of the error.
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This may be a string or a tuple containing several items of
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information (e.g., an error code and a string explaining the code).
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The associated value is the second argument to the \keyword{raise}
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statement. For string exceptions, the associated value itself will be
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stored in the variable named as the second argument of the
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\keyword{except} clause (if any). For class exceptions, that variable
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receives the exception instance. If the exception class is derived
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from the standard root class \exception{Exception}, the associated
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value is present as the exception instance's \member{args} attribute,
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and possibly on other attributes as well.
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\stindex{raise}
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User code can raise built-in exceptions. This can be used to test an
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exception handler or to report an error condition ``just like'' the
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situation in which the interpreter raises the same exception; but
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beware that there is nothing to prevent user code from raising an
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inappropriate error.
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\setindexsubitem{(built-in exception base class)}
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The following exceptions are only used as base classes for other
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exceptions. When string-based standard exceptions are used, they
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are tuples containing the directly derived classes.
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\begin{excdesc}{Exception}
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The root class for exceptions. All built-in exceptions are derived
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from this class. All user-defined exceptions should also be derived
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from this class, but this is not (yet) enforced. The \function{str()}
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function, when applied to an instance of this class (or most derived
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classes) returns the string value of the argument or arguments, or an
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empty string if no arguments were given to the constructor. When used
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as a sequence, this accesses the arguments given to the constructor
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(handy for backward compatibility with old code). The arguments are
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also available on the instance's \member{args} attribute, as a tuple.
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\end{excdesc}
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\begin{excdesc}{StandardError}
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The base class for all built-in exceptions except
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\exception{SystemExit}. \exception{StandardError} itself is derived
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from the root class
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\exception{Exception}.
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\end{excdesc}
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\begin{excdesc}{ArithmeticError}
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The base class for those built-in exceptions that are raised for
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various arithmetic errors: \exception{OverflowError},
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\exception{ZeroDivisionError}, \exception{FloatingPointError}.
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\end{excdesc}
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\begin{excdesc}{LookupError}
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The base class for the exceptions that are raised when a key or
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index used on a mapping or sequence is invalid: \exception{IndexError},
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\exception{KeyError}.
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\end{excdesc}
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\begin{excdesc}{EnvironmentError}
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The base class for exceptions that
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can occur outside the Python system: \exception{IOError},
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\exception{OSError}. When exceptions of this type are created with a
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2-tuple, the first item is available on the instance's \member{errno}
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attribute (it is assumed to be an error number), and the second item
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is available on the \member{strerror} attribute (it is usually the
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associated error message). The tuple itself is also available on the
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\member{args} attribute.
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\versionadded{1.5.2}
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When an \exception{EnvironmentError} exception is instantiated with a
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3-tuple, the first two items are available as above, while the third
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item is available on the \member{filename} attribute. However, for
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backwards compatibility, the \member{args} attribute contains only a
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2-tuple of the first two constructor arguments.
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The \member{filename} attribute is \code{None} when this exception is
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created with other than 3 arguments. The \member{errno} and
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\member{strerror} attributes are also \code{None} when the instance was
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created with other than 2 or 3 arguments. In this last case,
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\member{args} contains the verbatim constructor arguments as a tuple.
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\end{excdesc}
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\setindexsubitem{(built-in exception)}
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The following exceptions are the exceptions that are actually raised.
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They are class objects, except when the \code{-X} option is used to
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revert back to string-based standard exceptions.
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\begin{excdesc}{AssertionError}
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Raised when an \keyword{assert} statement fails.
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\stindex{assert}
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\end{excdesc}
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\begin{excdesc}{AttributeError}
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% xref to attribute reference?
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Raised when an attribute reference or assignment fails. (When an
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object does not support attribute references or attribute assignments
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at all, \exception{TypeError} is raised.)
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\end{excdesc}
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\begin{excdesc}{EOFError}
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% XXXJH xrefs here
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Raised when one of the built-in functions (\function{input()} or
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\function{raw_input()}) hits an end-of-file condition (\EOF{}) without
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reading any data.
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% XXXJH xrefs here
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(N.B.: the \method{read()} and \method{readline()} methods of file
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objects return an empty string when they hit \EOF{}.)
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\end{excdesc}
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\begin{excdesc}{FloatingPointError}
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Raised when a floating point operation fails. This exception is
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always defined, but can only be raised when Python is configured
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with the \code{--with-fpectl} option, or the
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\constant{WANT_SIGFPE_HANDLER} symbol is defined in the
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\file{config.h} file.
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\end{excdesc}
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\begin{excdesc}{IOError}
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% XXXJH xrefs here
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Raised when an I/O operation (such as a \keyword{print} statement,
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the built-in \function{open()} function or a method of a file
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object) fails for an I/O-related reason, e.g., ``file not found'' or
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``disk full''.
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This class is derived from \exception{EnvironmentError}. See the
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discussion above for more information on exception instance
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attributes.
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\end{excdesc}
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\begin{excdesc}{ImportError}
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% XXXJH xref to import statement?
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Raised when an \keyword{import} statement fails to find the module
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definition or when a \code{from \textrm{\ldots} import} fails to find a
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name that is to be imported.
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\end{excdesc}
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\begin{excdesc}{IndexError}
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% XXXJH xref to sequences
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Raised when a sequence subscript is out of range. (Slice indices are
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silently truncated to fall in the allowed range; if an index is not a
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plain integer, \exception{TypeError} is raised.)
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\end{excdesc}
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\begin{excdesc}{KeyError}
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% XXXJH xref to mapping objects?
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Raised when a mapping (dictionary) key is not found in the set of
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existing keys.
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\end{excdesc}
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\begin{excdesc}{KeyboardInterrupt}
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Raised when the user hits the interrupt key (normally
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\kbd{Control-C} or \kbd{DEL}). During execution, a check for
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interrupts is made regularly.
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% XXXJH xrefs here
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Interrupts typed when a built-in function \function{input()} or
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\function{raw_input()}) is waiting for input also raise this
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exception.
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\end{excdesc}
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\begin{excdesc}{MemoryError}
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Raised when an operation runs out of memory but the situation may
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still be rescued (by deleting some objects). The associated value is
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a string indicating what kind of (internal) operation ran out of memory.
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Note that because of the underlying memory management architecture
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(\C{}'s \cfunction{malloc()} function), the interpreter may not
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always be able to completely recover from this situation; it
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nevertheless raises an exception so that a stack traceback can be
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printed, in case a run-away program was the cause.
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\end{excdesc}
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\begin{excdesc}{NameError}
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Raised when a local or global name is not found. This applies only
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to unqualified names. The associated value is the name that could
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not be found.
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\end{excdesc}
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\begin{excdesc}{NotImplementedError}
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This exception is derived from \exception{RuntimeError}. In user
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defined base classes, abstract methods should raise this exception
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when they require derived classes to override the method.
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\versionadded{1.5.2}
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\end{excdesc}
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\begin{excdesc}{OSError}
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%xref for os module
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This class is derived from \exception{EnvironmentError} and is used
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primarily as the \refmodule{os} module's \code{os.error} exception.
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See \exception{EnvironmentError} above for a description of the
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possible associated values.
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\versionadded{1.5.2}
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\end{excdesc}
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\begin{excdesc}{OverflowError}
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% XXXJH reference to long's and/or int's?
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Raised when the result of an arithmetic operation is too large to be
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represented. This cannot occur for long integers (which would rather
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raise \exception{MemoryError} than give up). Because of the lack of
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standardization of floating point exception handling in \C{}, most
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floating point operations also aren't checked. For plain integers,
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all operations that can overflow are checked except left shift, where
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typical applications prefer to drop bits than raise an exception.
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\end{excdesc}
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\begin{excdesc}{RuntimeError}
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Raised when an error is detected that doesn't fall in any of the
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other categories. The associated value is a string indicating what
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precisely went wrong. (This exception is mostly a relic from a
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previous version of the interpreter; it is not used very much any
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more.)
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\end{excdesc}
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\begin{excdesc}{SyntaxError}
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% XXXJH xref to these functions?
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Raised when the parser encounters a syntax error. This may occur in
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an \keyword{import} statement, in an \keyword{exec} statement, in a call
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to the built-in function \function{eval()} or \function{input()}, or
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when reading the initial script or standard input (also
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interactively).
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When class exceptions are used, instances of this class have
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atttributes \member{filename}, \member{lineno}, \member{offset} and
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\member{text} for easier access to the details; for string exceptions,
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the associated value is usually a tuple of the form
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\code{(message, (filename, lineno, offset, text))}.
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For class exceptions, \function{str()} returns only the message.
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\end{excdesc}
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\begin{excdesc}{SystemError}
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Raised when the interpreter finds an internal error, but the
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situation does not look so serious to cause it to abandon all hope.
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The associated value is a string indicating what went wrong (in
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low-level terms).
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You should report this to the author or maintainer of your Python
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interpreter. Be sure to report the version string of the Python
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interpreter (\code{sys.version}; it is also printed at the start of an
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interactive Python session), the exact error message (the exception's
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associated value) and if possible the source of the program that
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triggered the error.
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\end{excdesc}
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\begin{excdesc}{SystemExit}
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% XXXJH xref to module sys?
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This exception is raised by the \function{sys.exit()} function. When it
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is not handled, the Python interpreter exits; no stack traceback is
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printed. If the associated value is a plain integer, it specifies the
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system exit status (passed to \C{}'s \cfunction{exit()} function); if it is
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\code{None}, the exit status is zero; if it has another type (such as
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a string), the object's value is printed and the exit status is one.
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When class exceptions are used, the instance has an attribute
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\member{code} which is set to the proposed exit status or error message
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(defaulting to \code{None}). Also, this exception derives directly
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from \exception{Exception} and not \exception{StandardError}, since it
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is not technically an error.
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A call to \function{sys.exit()} is translated into an exception so that
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clean-up handlers (\keyword{finally} clauses of \keyword{try} statements)
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can be executed, and so that a debugger can execute a script without
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running the risk of losing control. The \function{os._exit()} function
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can be used if it is absolutely positively necessary to exit
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immediately (e.g., after a \function{fork()} in the child process).
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\end{excdesc}
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\begin{excdesc}{TypeError}
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Raised when a built-in operation or function is applied to an object
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of inappropriate type. The associated value is a string giving
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details about the type mismatch.
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\end{excdesc}
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\begin{excdesc}{ValueError}
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Raised when a built-in operation or function receives an argument
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that has the right type but an inappropriate value, and the
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situation is not described by a more precise exception such as
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\exception{IndexError}.
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\end{excdesc}
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\begin{excdesc}{ZeroDivisionError}
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Raised when the second argument of a division or modulo operation is
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zero. The associated value is a string indicating the type of the
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operands and the operation.
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\end{excdesc}
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