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 do 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, most of the standard exceptions are strings2.7. This option may be used to
run 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 an except
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, that variable
receives the exception instance. If the exception class is derived
from the standard root class Exception, the associated
value is present as the exception instance's args attribute,
and possibly on other attributes as well.
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.
- 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 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). The arguments are
also available on the instance's args attribute, as a tuple.
- 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
Exception.
- ArithmeticError
-
The base class for those built-in exceptions that are raised for
various arithmetic errors: OverflowError,
ZeroDivisionError, FloatingPointError.
- LookupError
-
The base class for the exceptions that are raised when a key or
index used on a mapping or sequence is invalid: IndexError,
KeyError.
- EnvironmentError
-
New in version 1.5.2.
The base class for exceptions that
can occur outside the Python system: IOError,
OSError. When exceptions of this type are created with a
2-tuple, the first item is available on the instance's errno
attribute (it is assumed to be an error number), and the second item
is available on the strerror attribute (it is usually the
associated error message). The tuple itself is also available on the
args attribute.
When an EnvironmentError exception is instantiated with a
3-tuple, the first two items are available as above, while the third
item is available on the filename attribute. However, for
backwards compatibility, the args attribute contains only a
2-tuple of the first two constructor arguments.
The filename attribute is None when this exception is
created with other than 3 arguments. The errno and
strerror attributes are also None when the instance was
created with other than 2 or 3 arguments. In this last case,
args contains the verbatim constructor arguments as a tuple.
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.
- AssertionError
-
Raised when an assert statement fails.
- AttributeError
-
Raised when an attribute reference or assignment fails. (When an
object does not support attribute references or attribute assignments
at all, TypeError is raised.)
- EOFError
-
Raised when one of the built-in functions (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.)
- FloatingPointError
-
Raised when a floating point operation fails. This exception is
always defined, but can only be raised when Python is configured
with the -with-fpectl option, or the
WANT_SIGFPE_HANDLER symbol is defined in the
"config.h" file.
- IOError
-
Changed in version 1.5.2.
Raised when an I/O operation (such as a 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''.
This class is derived EnvironmentError. See the
discussion above for more information on exception instance
attributes.
- ImportError
-
Raised when an import statement fails to find the module
definition or when a from
... import fails to find a
name that is to be imported.
- IndexError
-
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, TypeError is raised.)
- KeyError
-
Raised when a mapping (dictionary) key is not found in the set of
existing keys.
- KeyboardInterrupt
-
Raised when the user hits the interrupt key (normally
Control-C or DEL). During execution, a check for
interrupts is made regularly.
Interrupts typed when a built-in function input() or
raw_input()) is waiting for input also raise this
exception.
- 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 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.
- 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.
- OSError
-
New in version 1.5.2.
This class is derived from EnvironmentError and is used
primarily as the by the os module's os.error
exception. See EnvironmentError above for a description
of the possible associated values.
- OverflowError
-
Raised when the result of an arithmetic operation is too large to be
represented. This cannot occur for long integers (which would rather
raise 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.
- 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.)
- SyntaxError
-
Raised when the parser encounters a syntax error. This may occur in
an 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.
- 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 (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.
- SystemExit
-
This exception is raised by the 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).
- 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.
- 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
IndexError.
- 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.
Footnotes
- ... strings2.7
- For
forward-compatibility the new exceptions LookupError,
ArithmeticError, EnvironmentError, and
StandardError are tuples.
