271 lines
12 KiB
TeX
271 lines
12 KiB
TeX
\section{Built-in Exceptions}
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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 the the same. The source code for those
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exceptions is present in the standard library module
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\code{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, the standard exceptions are strings. This may be needed to
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run some code that breaks because of the different semantics of class
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based 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 \code{try} statement with an \code{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 \code{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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\code{except} clause (if any). For class exceptions derived from
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the root class \code{Exception}, that variable receives the exception
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instance, and the associated value is present as the exception
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instance's \code{args} attribute; this is a tuple even if the second
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argument to \code{raise} was not (then it is a singleton tuple).
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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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\renewcommand{\indexsubitem}{(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 \code{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).
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\end{excdesc}
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\begin{excdesc}{StandardError}
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The base class for built-in exceptions. All built-in exceptions are
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derived from this class, which is itself derived from the root class
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\code{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: \code{OverflowError},
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\code{ZeroDivisionError}, \code{FloatingPointError}.
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\end{excdesc}
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\begin{excdesc}{LookupError}
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The base class for thise exceptions that are raised when a key or
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index used on a mapping or sequence is invalid: \code{IndexError},
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\code{KeyError}.
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\end{excdesc}
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\renewcommand{\indexsubitem}{(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 \code{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, \code{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 (\code{input()} or
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\code{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 \code{read()} and \code{readline()} methods of file
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objects return an empty string when they hit \EOF{}.) No associated value.
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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 with
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the \code{--with-fpectl} option, or the \code{WANT_SIGFPE_HANDLER}
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symbol is defined in the \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 \code{print} statement, the
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built-in \code{open()} function or a method of a file object) fails
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for an I/O-related reason, e.g., ``file not found'' or ``disk full''.
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When class exceptions are used, and this exception is instantiated as
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\code{IOError(errno, strerror)}, the instance has two additional
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attributes \code{errno} and \code{strerror} set to the error code and
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the error message, respectively. These attributes default to
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\code{None}.
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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 \code{import} statement fails to find the module
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definition or when a \code{from {\rm \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, \code{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
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\key{DEL}). During execution, a check for interrupts is made regularly.
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% XXXJH xrefs here
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Interrupts typed when a built-in function \code{input()} or
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\code{raw_input()}) is waiting for input also raise this exception. No
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associated value.
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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 \code{malloc()} function), the interpreter may not always be able
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to completely recover from this situation; it nevertheless raises an
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exception so that a stack traceback can be printed, in case a run-away
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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}{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 \code{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 \code{import} statement, in an \code{exec} statement, in a call
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to the built-in function \code{eval()} or \code{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 \code{filename}, \code{lineno}, \code{offset} and
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\code{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, \code{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 \code{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 \code{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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\code{code} which is set to the proposed exit status or error message
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(defaulting to \code{None}).
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A call to \code{sys.exit()} is translated into an exception so that
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clean-up handlers (\code{finally} clauses of \code{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 \code{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 \code{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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\code{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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