\exception{SystemExit}\withsubitem{(built-in
exception)}{\ttindex{SystemExit}}.
-Exceptions are identified by class instances.
-Selection of a matching except clause is based on object identity.
-The \keyword{except} clause must reference the same class or a base
-class of it.
-
-When an exception is raised, an object (maybe \code{None}) is passed
-as the exception's \emph{value}; this object does not affect the
-selection of an exception handler, but is passed to the selected
-exception handler as additional information. For class exceptions,
-this object must be an instance of the exception class being raised.
+Exceptions are identified by class instances. The \keyword{except}
+clause is selected depending on the class of the instance: it must
+reference the class of the instance or a base class thereof. The
+instance can be received by the handler and can carry additional
+information about the exceptional condition.
+
+Exceptions can also be identified by strings, in which case the
+\keyword{except} clause is selected by object identity. An arbitrary
+value can be raised along with the identifying string which can be
+passed to the handler.
+
+\deprecated{2.5}{String exceptions should not be used in new code.
+They will not be supported in a future version of Python. Old code
+should be rewritten to use class exceptions instead.}
\begin{notice}[warning]
Messages to exceptions are not part of the Python API. Their contents may
except clause with an expression, that expression is evaluated, and the
clause matches the exception if the resulting object is ``compatible''
with the exception. An object is compatible with an exception if it
-is either the object that identifies the exception, or (for exceptions
-that are classes) it is a base class of the exception, or it is a
-tuple containing an item that is compatible with the exception. Note
-that the object identities must match, i.e. it must be the same
-object, not just an object with the same value.
+is the class or a base class of the exception object, a tuple
+containing an item compatible with the exception, or, in the
+(deprecated) case of string exceptions, is the raised string itself
+(note that the object identities must match, i.e. it must be the same
+string object, not just a string with the same value).
\kwindex{except}
If no except clause matches the exception, the search for an exception
on the call stack (it is treated as if the entire \keyword{try} statement
raised the exception).
-When a matching except clause is found, the exception's parameter is
-assigned to the target specified in that except clause, if present,
-and the except clause's suite is executed. All except clauses must
-have an executable block. When the end of this block
-is reached, execution continues normally after the entire try
-statement. (This means that if two nested handlers exist for the same
-exception, and the exception occurs in the try clause of the inner
-handler, the outer handler will not handle the exception.)
+When a matching except clause is found, the exception is assigned to
+the target specified in that except clause, if present, and the except
+clause's suite is executed. All except clauses must have an
+executable block. When the end of this block is reached, execution
+continues normally after the entire try statement. (This means that
+if two nested handlers exist for the same exception, and the exception
+occurs in the try clause of the inner handler, the outer handler will
+not handle the exception.)
Before an except clause's suite is executed, details about the
exception are assigned to three variables in the
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