273 lines
12 KiB
TeX
273 lines
12 KiB
TeX
\section{Built-in Exceptions}
|
|
\label{module-exceptions}
|
|
\stmodindex{exceptions}
|
|
|
|
Exceptions can be class objects or string objects. While
|
|
traditionally, most exceptions have been string objects, in Python
|
|
1.5, all standard exceptions have been converted to class objects,
|
|
and users are encouraged to the the same. The source code for those
|
|
exceptions is present in the standard library module
|
|
\code{exceptions}; this module never needs to be imported explicitly.
|
|
|
|
For backward compatibility, when Python is invoked with the \code{-X}
|
|
option, the standard exceptions are strings. This may be needed to
|
|
run some code that breaks because of the different semantics of class
|
|
based exceptions. The \code{-X} option will become obsolete in future
|
|
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
|
|
exception handler or to report an error condition ``just like'' the
|
|
situation in which the interpreter raises the same exception; but
|
|
beware that there is nothing to prevent user code from raising an
|
|
inappropriate error.
|
|
|
|
\setindexsubitem{(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. When used
|
|
as a sequence, this accesses the arguments given to the constructor
|
|
(handy for backward compatibility with old code).
|
|
\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}.
|
|
\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}
|
|
|
|
\setindexsubitem{(built-in exception)}
|
|
|
|
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}
|
|
% xref to attribute reference?
|
|
Raised when an attribute reference or assignment fails. (When an
|
|
object does not support attribute references or attribute assignments
|
|
at all, \code{TypeError} is raised.)
|
|
\end{excdesc}
|
|
|
|
\begin{excdesc}{EOFError}
|
|
% XXXJH xrefs here
|
|
Raised when one of the built-in functions (\code{input()} or
|
|
\code{raw_input()}) hits an end-of-file condition (\EOF{}) without
|
|
reading any data.
|
|
% XXXJH xrefs here
|
|
(N.B.: the \code{read()} and \code{readline()} methods of file
|
|
objects return an empty string when they hit \EOF{}.) No associated value.
|
|
\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}
|
|
% XXXJH xrefs here
|
|
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
|
|
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}
|
|
|
|
\begin{excdesc}{ImportError}
|
|
% XXXJH xref to import statement?
|
|
Raised when an \code{import} statement fails to find the module
|
|
definition or when a \code{from {\rm \ldots} import} fails to find a
|
|
name that is to be imported.
|
|
\end{excdesc}
|
|
|
|
\begin{excdesc}{IndexError}
|
|
% XXXJH xref to sequences
|
|
Raised when a sequence subscript is out of range. (Slice indices are
|
|
silently truncated to fall in the allowed range; if an index is not a
|
|
plain integer, \code{TypeError} is raised.)
|
|
\end{excdesc}
|
|
|
|
\begin{excdesc}{KeyError}
|
|
% XXXJH xref to mapping objects?
|
|
Raised when a mapping (dictionary) key is not found in the set of
|
|
existing keys.
|
|
\end{excdesc}
|
|
|
|
\begin{excdesc}{KeyboardInterrupt}
|
|
Raised when the user hits the interrupt key (normally
|
|
\kbd{Control-C} or \kbd{DEL}). During execution, a check for
|
|
interrupts is made regularly.
|
|
% XXXJH xrefs here
|
|
Interrupts typed when a built-in function \function{input()} or
|
|
\function{raw_input()}) is waiting for input also raise this
|
|
exception. This exception has no associated value.
|
|
\end{excdesc}
|
|
|
|
\begin{excdesc}{MemoryError}
|
|
Raised when an operation runs out of memory but the situation may
|
|
still be rescued (by deleting some objects). The associated value is
|
|
a string indicating what kind of (internal) operation ran out of memory.
|
|
Note that because of the underlying memory management architecture
|
|
(\C{}'s \code{malloc()} function), the interpreter may not always be able
|
|
to completely recover from this situation; it nevertheless raises an
|
|
exception so that a stack traceback can be printed, in case a run-away
|
|
program was the cause.
|
|
\end{excdesc}
|
|
|
|
\begin{excdesc}{NameError}
|
|
Raised when a local or global name is not found. This applies only
|
|
to unqualified names. The associated value is the name that could
|
|
not be found.
|
|
\end{excdesc}
|
|
|
|
\begin{excdesc}{OverflowError}
|
|
% XXXJH reference to long's and/or int's?
|
|
Raised when the result of an arithmetic operation is too large to be
|
|
represented. This cannot occur for long integers (which would rather
|
|
raise \code{MemoryError} than give up). Because of the lack of
|
|
standardization of floating point exception handling in \C{}, most
|
|
floating point operations also aren't checked. For plain integers,
|
|
all operations that can overflow are checked except left shift, where
|
|
typical applications prefer to drop bits than raise an exception.
|
|
\end{excdesc}
|
|
|
|
\begin{excdesc}{RuntimeError}
|
|
Raised when an error is detected that doesn't fall in any of the
|
|
other categories. The associated value is a string indicating what
|
|
precisely went wrong. (This exception is mostly a relic from a
|
|
previous version of the interpreter; it is not used very much any
|
|
more.)
|
|
\end{excdesc}
|
|
|
|
\begin{excdesc}{SyntaxError}
|
|
% XXXJH xref to these functions?
|
|
Raised when the parser encounters a syntax error. This may occur in
|
|
an \code{import} statement, in an \code{exec} statement, in a call
|
|
to the built-in function \code{eval()} or \code{input()}, or
|
|
when reading the initial script or standard input (also
|
|
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}
|
|
|
|
\begin{excdesc}{SystemError}
|
|
Raised when the interpreter finds an internal error, but the
|
|
situation does not look so serious to cause it to abandon all hope.
|
|
The associated value is a string indicating what went wrong (in
|
|
low-level terms).
|
|
|
|
You should report this to the author or maintainer of your Python
|
|
interpreter. Be sure to report the version string of the Python
|
|
interpreter (\code{sys.version}; it is also printed at the start of an
|
|
interactive Python session), the exact error message (the exception's
|
|
associated value) and if possible the source of the program that
|
|
triggered the error.
|
|
\end{excdesc}
|
|
|
|
\begin{excdesc}{SystemExit}
|
|
% XXXJH xref to module sys?
|
|
This exception is raised by the \code{sys.exit()} function. When it
|
|
is not handled, the Python interpreter exits; no stack traceback is
|
|
printed. If the associated value is a plain integer, it specifies the
|
|
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
|
|
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
|
|
clean-up handlers (\code{finally} clauses of \code{try} statements)
|
|
can be executed, and so that a debugger can execute a script without
|
|
running the risk of losing control. The \code{os._exit()} function
|
|
can be used if it is absolutely positively necessary to exit
|
|
immediately (e.g., after a \code{fork()} in the child process).
|
|
\end{excdesc}
|
|
|
|
\begin{excdesc}{TypeError}
|
|
Raised when a built-in operation or function is applied to an object
|
|
of inappropriate type. The associated value is a string giving
|
|
details about the type mismatch.
|
|
\end{excdesc}
|
|
|
|
\begin{excdesc}{ValueError}
|
|
Raised when a built-in operation or function receives an argument
|
|
that has the right type but an inappropriate value, and the
|
|
situation is not described by a more precise exception such as
|
|
\code{IndexError}.
|
|
\end{excdesc}
|
|
|
|
\begin{excdesc}{ZeroDivisionError}
|
|
Raised when the second argument of a division or modulo operation is
|
|
zero. The associated value is a string indicating the type of the
|
|
operands and the operation.
|
|
\end{excdesc}
|