Exceptions can be class objects or string objects. While
traditionally, most exceptions have been string objects, in Python
1.5a4, 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
exceptions
; this module never needs to be imported explicitly.
For backward compatibility, when Python is invoked with the -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 -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 try
statement with anexcept
clause that mentions a particular class, that clause also handles
any exception classes derived from that class (but not exception
classes from which it is derived). Two exception classes
that are not related via subclassing are never equivalent, even if
they have the same name.
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 raise
statement. For string exceptions, the associated value itself will be
stored in the variable named as the second argument of the
except
clause (if any). For class exceptions derived from
the root class Exception
, that variable receives the exception
instance, and the associated value is present as the exception
instance's args
attribute; this is a tuple even if the second
argument to raise
was not (then it is a singleton tuple).
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.
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.
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).
Exception
.
OverflowError
,
ZeroDivisionError
, FloatingPointError
.
IndexError
,
KeyError
.
The following exceptions are the exceptions that are actually raised.
They are class objects, except when the -X
option is used to
revert back to string-based standard exceptions.
assert
statement fails.
TypeError
is raised.)
input()
or
raw_input()
) hits an end-of-file condition (EOF) without
reading any data.
(N.B.: the read()
and readline()
methods of file
objects return an empty string when they hit EOF.) No associated value.
-with-fpectl
option, or the WANT_SIGFPE_HANDLER
symbol is defined in the `config.h
' file.
print
statement, the
built-in 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
IOError(errno, strerror)
, the instance has two additional
attributes errno
and strerror
set to the error code and
the error message, respectively. These attributes default to
None
.
import
statement fails to find the module
definition or when a from ... import
fails to find a
name that is to be imported.
TypeError
is raised.)
input()
or
raw_input()
) is waiting for input also raise this exception. No
associated value.
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.
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.
import
statement, in an exec
statement, in a call
to the built-in function eval()
or input()
, or
when reading the initial script or standard input (also
interactively).
When class exceptions are used, instances of this class have
atttributes filename
, lineno
, offset
and
text
for easier access to the details; for string exceptions,
the associated value is usually a tuple of the form
(message, (filename, lineno, offset, text))
.
For class exceptions, str()
returns only the message.
You should report this to the author or maintainer of your Python
interpreter. Be sure to report the version string of the Python
interpreter (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.
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 exit()
function); if it is
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
which is set to the proposed exit status or error message
(defaulting to None
).
A call to sys.exit()
is translated into an exception so that
clean-up handlers (finally
clauses of try
statements)
can be executed, and so that a debugger can execute a script without
running the risk of losing control. The os._exit()
function
can be used if it is absolutely positively necessary to exit
immediately (e.g., after a fork()
in the child process).
IndexError
.