Documented new exceptions and exception classes.

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Guido van Rossum 1997-10-05 18:51:26 +00:00
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@ -1,25 +1,99 @@
\section{Built-in Exceptions} \section{Built-in Exceptions}
Exceptions are string objects. Two distinct string objects with the Exceptions can be class objects or string objects. While
same value are different exceptions. This is done to force programmers traditionally, most exceptions have been string objects, in Python
to use exception names rather than their string value when specifying 1.5a4, all standard exceptions have been converted to class objects,
exception handlers. The string value of all built-in exceptions is and users are encouraged to the the same. The source code for those
their name, but this is not a requirement for user-defined exceptions exceptions is present in the standard library module
or exceptions defined by library modules. \code{exceptions}; this module never needs to be imported explicitly.
The following exceptions can be generated by the interpreter or For backward compatibility, when Python is invoked with the \code{-X}
built-in functions. Except where mentioned, they have an `associated option, the standard exceptions are strings. This may be needed to
value' indicating the detailed cause of the error. This may be a run some code that breaks because of the different semantics of class
string or a tuple containing several items of information (e.g., an based exceptions. The \code{-X} option will become obsolete in future
error code and a string explaining the code). Python versions, so the recommended solution is to fix the code.
Two distinct string objects with the same value are considered different
exceptions. This is done to force programmers to use exception names
rather than their string value when specifying exception handlers.
The string value of all built-in exceptions is their name, but this is
not a requirement for user-defined exceptions or exceptions defined by
library modules.
For class exceptions, in a \code{try} statement with an\code{except}
clause that mentions a particular class, that clause also handles
any exception classes derived from that class (but not exception
classes from which \emph{it} is derived). Two exception classes
that are not related via subclassing are never equivalent, even if
they have the same name.
\stindex{try}
\stindex{except}
The built-in exceptions listed below can be generated by the
interpreter or built-in functions. Except where mentioned, they have
an ``associated value'' indicating the detailed cause of the error.
This may be a string or a tuple containing several items of
information (e.g., an error code and a string explaining the code).
The associated value is the second argument to the \code{raise}
statement. For string exceptions, the associated value itself will be
stored in the variable named as the second argument of the
\code{except} clause (if any). For class exceptions derived from
the root class \code{Exception}, that variable receives the exception
instance, and the associated value is present as the exception
instance's \code{args} attribute; this is a tuple even if the second
argument to \code{raise} was not (then it is a singleton tuple).
\stindex{raise}
User code can raise built-in exceptions. This can be used to test an User code can raise built-in exceptions. This can be used to test an
exception handler or to report an error condition `just like' the exception handler or to report an error condition ``just like'' the
situation in which the interpreter raises the same exception; but situation in which the interpreter raises the same exception; but
beware that there is nothing to prevent user code from raising an beware that there is nothing to prevent user code from raising an
inappropriate error. inappropriate error.
\renewcommand{\indexsubitem}{(built-in exception)} \renewcommand{\indexsubitem}{(built-in exception base class)}
The following exceptions are only used as base classes for other
exceptions. When string-based standard exceptions are used, they
are tuples containing the directly derived classes.
\begin{excdesc}{Exception}
The root class for exceptions. All built-in exceptions are derived
from this class. All user-defined exceptions should also be derived
from this class, but this is not (yet) enforced. The \code{str()}
function, when applied to an instance of this class (or most derived
classes) returns the string value of the argument or arguments, or an
empty string if no arguments were given to the constructor.
\end{excdesc}
\begin{excdesc}{StandardError}
The base class for built-in exceptions. All built-in exceptions are
derived from this class, which is itself derived from the root class
\code{Exception}. For backward compatibility, when used as a
sequence, this accesses the arguments given to the constructor.
\end{excdesc}
\begin{excdesc}{ArithmeticError}
The base class for those built-in exceptions that are raised for
various arithmetic errors: \code{OverflowError},
\code{ZeroDivisionError}, \code{FloatingPointError}.
\end{excdesc}
\begin{excdesc}{LookupError}
The base class for thise exceptions that are raised when a key or
index used on a mapping or sequence is invalid: \code{IndexError},
\code{KeyError}.
\end{excdesc}
\renewcommand{\indexsubitem}{(built-in exception base class)}
The following exceptions are the exceptions that are actually raised.
They are class objects, except when the \code{-X} option is used to
revert back to string-based standard exceptions.
\begin{excdesc}{AssertionError}
Raised when an \code{assert} statement fails.
\stindex{assert}
\end{excdesc}
\begin{excdesc}{AttributeError} \begin{excdesc}{AttributeError}
% xref to attribute reference? % xref to attribute reference?
@ -38,11 +112,24 @@ inappropriate error.
objects return an empty string when they hit \EOF{}.) No associated value. objects return an empty string when they hit \EOF{}.) No associated value.
\end{excdesc} \end{excdesc}
\begin{excdesc}{FloatingPointError}
Raised when a floating point operation fails. This exception is
always defined, but can only be raised when Python is configured with
the \code{--with-fpectl} option, or the \code{WANT_SIGFPE_HANDLER}
symbol is defined in the \file{config.h} file.
\end{excdesc}
\begin{excdesc}{IOError} \begin{excdesc}{IOError}
% XXXJH xrefs here % XXXJH xrefs here
Raised when an I/O operation (such as a \code{print} statement, the Raised when an I/O operation (such as a \code{print} statement, the
built-in \code{open()} function or a method of a file object) fails built-in \code{open()} function or a method of a file object) fails
for an I/O-related reason, e.g., `file not found', `disk full'. for an I/O-related reason, e.g., ``file not found'' or ``disk full''.
When class exceptions are used, and this exception is instantiated as
\code{IOError(errno, strerror)}, the instance has two additional
attributes \code{errno} and \code{strerror} set to the error code and
the error message, respectively. These attributes default to
\code{None}.
\end{excdesc} \end{excdesc}
\begin{excdesc}{ImportError} \begin{excdesc}{ImportError}
@ -106,10 +193,9 @@ inappropriate error.
\begin{excdesc}{RuntimeError} \begin{excdesc}{RuntimeError}
Raised when an error is detected that doesn't fall in any of the Raised when an error is detected that doesn't fall in any of the
other categories. The associated value is a string indicating what other categories. The associated value is a string indicating what
precisely went wrong. (This exception is a relic from a previous precisely went wrong. (This exception is mostly a relic from a
version of the interpreter; it is not used any more except by some previous version of the interpreter; it is not used very much any
extension modules that haven't been converted to define their own more.)
exceptions yet.)
\end{excdesc} \end{excdesc}
\begin{excdesc}{SyntaxError} \begin{excdesc}{SyntaxError}
@ -119,6 +205,13 @@ inappropriate error.
to the built-in function \code{eval()} or \code{input()}, or to the built-in function \code{eval()} or \code{input()}, or
when reading the initial script or standard input (also when reading the initial script or standard input (also
interactively). interactively).
When class exceptions are used, instances of this class have
atttributes \code{filename}, \code{lineno}, \code{offset} and
\code{text} for easier access to the details; for string exceptions,
the associated value is usually a tuple of the form
\code{(message, (filename, lineno, offset, text))}.
For class exceptions, \code{str()} returns only the message.
\end{excdesc} \end{excdesc}
\begin{excdesc}{SystemError} \begin{excdesc}{SystemError}
@ -143,11 +236,15 @@ inappropriate error.
system exit status (passed to \C{}'s \code{exit()} function); if it is system exit status (passed to \C{}'s \code{exit()} function); if it is
\code{None}, the exit status is zero; if it has another type (such as \code{None}, the exit status is zero; if it has another type (such as
a string), the object's value is printed and the exit status is one. a string), the object's value is printed and the exit status is one.
When class exceptions are used, the instance has an attribute
\code{code} which is set to the proposed exit status or error message
(defaulting to \code{None}).
A call to \code{sys.exit} is translated into an exception so that A call to \code{sys.exit} is translated into an exception so that
clean-up handlers (\code{finally} clauses of \code{try} statements) clean-up handlers (\code{finally} clauses of \code{try} statements)
can be executed, and so that a debugger can execute a script without can be executed, and so that a debugger can execute a script without
running the risk of losing control. The \code{posix._exit()} function running the risk of losing control. The \code{os._exit()} function
can be used if it is absolutely positively necessary to exit can be used if it is absolutely positively necessary to exit
immediately (e.g., after a \code{fork()} in the child process). immediately (e.g., after a \code{fork()} in the child process).
\end{excdesc} \end{excdesc}

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@ -1,25 +1,99 @@
\section{Built-in Exceptions} \section{Built-in Exceptions}
Exceptions are string objects. Two distinct string objects with the Exceptions can be class objects or string objects. While
same value are different exceptions. This is done to force programmers traditionally, most exceptions have been string objects, in Python
to use exception names rather than their string value when specifying 1.5a4, all standard exceptions have been converted to class objects,
exception handlers. The string value of all built-in exceptions is and users are encouraged to the the same. The source code for those
their name, but this is not a requirement for user-defined exceptions exceptions is present in the standard library module
or exceptions defined by library modules. \code{exceptions}; this module never needs to be imported explicitly.
The following exceptions can be generated by the interpreter or For backward compatibility, when Python is invoked with the \code{-X}
built-in functions. Except where mentioned, they have an `associated option, the standard exceptions are strings. This may be needed to
value' indicating the detailed cause of the error. This may be a run some code that breaks because of the different semantics of class
string or a tuple containing several items of information (e.g., an based exceptions. The \code{-X} option will become obsolete in future
error code and a string explaining the code). Python versions, so the recommended solution is to fix the code.
Two distinct string objects with the same value are considered different
exceptions. This is done to force programmers to use exception names
rather than their string value when specifying exception handlers.
The string value of all built-in exceptions is their name, but this is
not a requirement for user-defined exceptions or exceptions defined by
library modules.
For class exceptions, in a \code{try} statement with an\code{except}
clause that mentions a particular class, that clause also handles
any exception classes derived from that class (but not exception
classes from which \emph{it} is derived). Two exception classes
that are not related via subclassing are never equivalent, even if
they have the same name.
\stindex{try}
\stindex{except}
The built-in exceptions listed below can be generated by the
interpreter or built-in functions. Except where mentioned, they have
an ``associated value'' indicating the detailed cause of the error.
This may be a string or a tuple containing several items of
information (e.g., an error code and a string explaining the code).
The associated value is the second argument to the \code{raise}
statement. For string exceptions, the associated value itself will be
stored in the variable named as the second argument of the
\code{except} clause (if any). For class exceptions derived from
the root class \code{Exception}, that variable receives the exception
instance, and the associated value is present as the exception
instance's \code{args} attribute; this is a tuple even if the second
argument to \code{raise} was not (then it is a singleton tuple).
\stindex{raise}
User code can raise built-in exceptions. This can be used to test an User code can raise built-in exceptions. This can be used to test an
exception handler or to report an error condition `just like' the exception handler or to report an error condition ``just like'' the
situation in which the interpreter raises the same exception; but situation in which the interpreter raises the same exception; but
beware that there is nothing to prevent user code from raising an beware that there is nothing to prevent user code from raising an
inappropriate error. inappropriate error.
\renewcommand{\indexsubitem}{(built-in exception)} \renewcommand{\indexsubitem}{(built-in exception base class)}
The following exceptions are only used as base classes for other
exceptions. When string-based standard exceptions are used, they
are tuples containing the directly derived classes.
\begin{excdesc}{Exception}
The root class for exceptions. All built-in exceptions are derived
from this class. All user-defined exceptions should also be derived
from this class, but this is not (yet) enforced. The \code{str()}
function, when applied to an instance of this class (or most derived
classes) returns the string value of the argument or arguments, or an
empty string if no arguments were given to the constructor.
\end{excdesc}
\begin{excdesc}{StandardError}
The base class for built-in exceptions. All built-in exceptions are
derived from this class, which is itself derived from the root class
\code{Exception}. For backward compatibility, when used as a
sequence, this accesses the arguments given to the constructor.
\end{excdesc}
\begin{excdesc}{ArithmeticError}
The base class for those built-in exceptions that are raised for
various arithmetic errors: \code{OverflowError},
\code{ZeroDivisionError}, \code{FloatingPointError}.
\end{excdesc}
\begin{excdesc}{LookupError}
The base class for thise exceptions that are raised when a key or
index used on a mapping or sequence is invalid: \code{IndexError},
\code{KeyError}.
\end{excdesc}
\renewcommand{\indexsubitem}{(built-in exception base class)}
The following exceptions are the exceptions that are actually raised.
They are class objects, except when the \code{-X} option is used to
revert back to string-based standard exceptions.
\begin{excdesc}{AssertionError}
Raised when an \code{assert} statement fails.
\stindex{assert}
\end{excdesc}
\begin{excdesc}{AttributeError} \begin{excdesc}{AttributeError}
% xref to attribute reference? % xref to attribute reference?
@ -38,11 +112,24 @@ inappropriate error.
objects return an empty string when they hit \EOF{}.) No associated value. objects return an empty string when they hit \EOF{}.) No associated value.
\end{excdesc} \end{excdesc}
\begin{excdesc}{FloatingPointError}
Raised when a floating point operation fails. This exception is
always defined, but can only be raised when Python is configured with
the \code{--with-fpectl} option, or the \code{WANT_SIGFPE_HANDLER}
symbol is defined in the \file{config.h} file.
\end{excdesc}
\begin{excdesc}{IOError} \begin{excdesc}{IOError}
% XXXJH xrefs here % XXXJH xrefs here
Raised when an I/O operation (such as a \code{print} statement, the Raised when an I/O operation (such as a \code{print} statement, the
built-in \code{open()} function or a method of a file object) fails built-in \code{open()} function or a method of a file object) fails
for an I/O-related reason, e.g., `file not found', `disk full'. for an I/O-related reason, e.g., ``file not found'' or ``disk full''.
When class exceptions are used, and this exception is instantiated as
\code{IOError(errno, strerror)}, the instance has two additional
attributes \code{errno} and \code{strerror} set to the error code and
the error message, respectively. These attributes default to
\code{None}.
\end{excdesc} \end{excdesc}
\begin{excdesc}{ImportError} \begin{excdesc}{ImportError}
@ -106,10 +193,9 @@ inappropriate error.
\begin{excdesc}{RuntimeError} \begin{excdesc}{RuntimeError}
Raised when an error is detected that doesn't fall in any of the Raised when an error is detected that doesn't fall in any of the
other categories. The associated value is a string indicating what other categories. The associated value is a string indicating what
precisely went wrong. (This exception is a relic from a previous precisely went wrong. (This exception is mostly a relic from a
version of the interpreter; it is not used any more except by some previous version of the interpreter; it is not used very much any
extension modules that haven't been converted to define their own more.)
exceptions yet.)
\end{excdesc} \end{excdesc}
\begin{excdesc}{SyntaxError} \begin{excdesc}{SyntaxError}
@ -119,6 +205,13 @@ inappropriate error.
to the built-in function \code{eval()} or \code{input()}, or to the built-in function \code{eval()} or \code{input()}, or
when reading the initial script or standard input (also when reading the initial script or standard input (also
interactively). interactively).
When class exceptions are used, instances of this class have
atttributes \code{filename}, \code{lineno}, \code{offset} and
\code{text} for easier access to the details; for string exceptions,
the associated value is usually a tuple of the form
\code{(message, (filename, lineno, offset, text))}.
For class exceptions, \code{str()} returns only the message.
\end{excdesc} \end{excdesc}
\begin{excdesc}{SystemError} \begin{excdesc}{SystemError}
@ -143,11 +236,15 @@ inappropriate error.
system exit status (passed to \C{}'s \code{exit()} function); if it is system exit status (passed to \C{}'s \code{exit()} function); if it is
\code{None}, the exit status is zero; if it has another type (such as \code{None}, the exit status is zero; if it has another type (such as
a string), the object's value is printed and the exit status is one. a string), the object's value is printed and the exit status is one.
When class exceptions are used, the instance has an attribute
\code{code} which is set to the proposed exit status or error message
(defaulting to \code{None}).
A call to \code{sys.exit} is translated into an exception so that A call to \code{sys.exit} is translated into an exception so that
clean-up handlers (\code{finally} clauses of \code{try} statements) clean-up handlers (\code{finally} clauses of \code{try} statements)
can be executed, and so that a debugger can execute a script without can be executed, and so that a debugger can execute a script without
running the risk of losing control. The \code{posix._exit()} function running the risk of losing control. The \code{os._exit()} function
can be used if it is absolutely positively necessary to exit can be used if it is absolutely positively necessary to exit
immediately (e.g., after a \code{fork()} in the child process). immediately (e.g., after a \code{fork()} in the child process).
\end{excdesc} \end{excdesc}