Exceptions should be class objects.  
The exceptions are defined in the module exceptions.  This
module never needs to be imported explicitly: the exceptions are
provided in the built-in namespace as well as the exceptions
module.
Note:
In past versions of Python string exceptions were supported.  In
Python 1.5 and newer versions, all standard exceptions have been
converted to class objects and users are encouraged to do the same.
String exceptions will raise a 
PendingDeprecationWarning.
In future versions, support for string exceptions will be removed.
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 built-in exception classes can be sub-classed to define new
exceptions; programmers are encouraged to at least derive new
exceptions from the Exception base class.  More
information on defining exceptions is available in the
Python Tutorial under the heading
``User-defined Exceptions.''
The following exceptions are only used as base classes for other
exceptions.
- exception 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.
- exception StandardError
- 
The base class for all built-in exceptions except
StopIteration and SystemExit.
StandardError itself is derived from the root class
Exception.
- exception ArithmeticError
- 
The base class for those built-in exceptions that are raised for
various arithmetic errors: OverflowError,
ZeroDivisionError, FloatingPointError.
- exception 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.  This can be raised directly by
sys.setdefaultencoding().
- exception EnvironmentError
- 
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.
New in version 1.5.2.
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 Nonewhen this exception is
created with other than 3 arguments.  The errno and
strerror attributes are alsoNonewhen 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.
- exception AssertionError
- 
Raised when an assert statement fails.
- exception 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.)
- exception 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.)
- exception 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
  pyconfig.h file.
- exception IOError
- 
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 from EnvironmentError.  See the
  discussion above for more information on exception instance
  attributes.
 
- exception ImportError
- 
Raised when an import statement fails to find the module
  definition or when a from  ... importfails to find a
  name that is to be imported.
- exception 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.)
- exception KeyError
- 
Raised when a mapping (dictionary) key is not found in the set of
  existing keys.
- exception KeyboardInterrupt
- 
  Raised when the user hits the interrupt key (normally
  Control-C or Delete).  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.
- 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.
- exception NameError
- 
  Raised when a local or global name is not found.  This applies only
  to unqualified names.  The associated value is an error message that
  includes the name that could not be found.
- exception NotImplementedError
- 
  This exception is derived from RuntimeError.  In user
  defined base classes, abstract methods should raise this exception
  when they require derived classes to override the method.
  
New in version 1.5.2.
- exception OSError
- 
  This class is derived from EnvironmentError and is used
  primarily as the os module's os.errorexception.
  See EnvironmentError above for a description of the
  possible associated values.
  
New in version 1.5.2.
- exception 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.
- exception ReferenceError
- 
  This exception is raised when a weak reference proxy, created by the
  weakref.proxy() function, is used to access
  an attribute of the referent after it has been garbage collected.
  For more information on weak references, see the weakref
  module.
  
New in version 2.2:
Previously known as the
                weakref.ReferenceError
                exception.
- 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.)
- exception StopIteration
- 
  Raised by an iterator's next() method to signal that there
  are no further values.
  This is derived from Exception rather than
  StandardError, since this is not considered an error in
  its normal application.
  
New in version 2.2.
- exception 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).
Instances of this class have attributes filename,
  lineno, offset and text for easier access
  to the details.  str() of the exception instance returns
  only the message.
 
- exception 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 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.
 
- exception 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.
Instances have an attribute code which is set to the
  proposed exit status or error message (defaulting to None).
  Also, this exception derives directly from Exception and
  not StandardError, since it is not technically an error.
 
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 (for example, in the child process after a call to
  fork()).
 
- exception TypeError
- 
  Raised when an operation or function is applied to an object
  of inappropriate type.  The associated value is a string giving
  details about the type mismatch.
- exception UnboundLocalError
- 
  Raised when a reference is made to a local variable in a function or
  method, but no value has been bound to that variable.  This is a
  subclass of NameError.
New in version 2.0.
- exception UnicodeError
- 
  Raised when a Unicode-related encoding or decoding error occurs.  It
  is a subclass of ValueError.
New in version 2.0.
- exception UnicodeEncodeError
- 
  Raised when a Unicode-related error occurs during encoding.  It
  is a subclass of UnicodeError.
New in version 2.3.
- exception UnicodeDecodeError
- 
  Raised when a Unicode-related error occurs during decoding.  It
  is a subclass of UnicodeError.
New in version 2.3.
- exception UnicodeTranslateError
- 
  Raised when a Unicode-related error occurs during translating.  It
  is a subclass of UnicodeError.
New in version 2.3.
- exception 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.
- exception WindowsError
- 
  Raised when a Windows-specific error occurs or when the error number
  does not correspond to an errno value.  The
  errno and strerror values are created from the
  return values of the GetLastError() and
  FormatMessage() functions from the Windows Platform API.
  This is a subclass of OSError.
New in version 2.0.
- exception 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.
The following exceptions are used as warning categories; see the
warnings module for more information.
- exception Warning
- 
Base class for warning categories.
- exception UserWarning
- 
Base class for warnings generated by user code.
- exception DeprecationWarning
- 
Base class for warnings about deprecated features.
- exception PendingDeprecationWarning
- 
Base class for warnings about features which will be deprecated in the future.
- exception SyntaxWarning
- 
Base class for warnings about dubious syntax
- exception RuntimeWarning
- 
Base class for warnings about dubious runtime behavior.
- exception FutureWarning
- 
Base class for warnings about constructs that will change semantically
in the future.
The class hierarchy for built-in exceptions is:
    Exception
     +-- SystemExit
     +-- StopIteration
     +-- StandardError
     |    +-- KeyboardInterrupt
     |    +-- ImportError
     |    +-- EnvironmentError
     |    |    +-- IOError
     |    |    +-- OSError
     |    |         +-- WindowsError
     |    +-- EOFError
     |    +-- RuntimeError
     |    |    +-- NotImplementedError
     |    +-- NameError
     |    |    +-- UnboundLocalError
     |    +-- AttributeError
     |    +-- SyntaxError
     |    |    +-- IndentationError
     |    |         +-- TabError
     |    +-- TypeError
     |    +-- AssertionError
     |    +-- LookupError
     |    |    +-- IndexError
     |    |    +-- KeyError
     |    +-- ArithmeticError
     |    |    +-- OverflowError
     |    |    +-- ZeroDivisionError
     |    |    +-- FloatingPointError
     |    +-- ValueError
     |    |    +-- UnicodeError
     |    |        +-- UnicodeEncodeError
     |    |        +-- UnicodeDecodeError
     |    |        +-- UnicodeTranslateError
     |    +-- ReferenceError
     |    +-- SystemError
     |    +-- MemoryError
     +---Warning
	  +-- UserWarning
	  +-- DeprecationWarning
	  +-- PendingDeprecationWarning
	  +-- SyntaxWarning
	  +-- OverflowWarning (not generated in 2.4; won't exist in 2.5)
	  +-- RuntimeWarning
	  +-- FutureWarning
Release 2.4.3, documentation updated on 29 March 2006.
 
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