6. Built-in Exceptions
**********************

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.

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.
If the exception class is derived from the standard root class
"BaseException", the associated value is present as the exception
instance’s "args" attribute.

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 subclassed to define new
exceptions; programmers are encouraged to derive new exceptions from
the "Exception" class or one of its subclasses, and not from
"BaseException".  More information on defining exceptions is available
in the Python Tutorial under User-defined Exceptions.

The following exceptions are only used as base classes for other
exceptions.

exception BaseException

   The base class for all built-in exceptions.  It is not meant to be
   directly inherited by user-defined classes (for that, use
   "Exception").  If "str()" or "unicode()" is called on an instance
   of this class, the representation of the argument(s) to the
   instance are returned, or the empty string when there were no
   arguments.

   New in version 2.5.

   args

      The tuple of arguments given to the exception constructor.  Some
      built-in exceptions (like "IOError") expect a certain number of
      arguments and assign a special meaning to the elements of this
      tuple, while others are usually called only with a single string
      giving an error message.

exception Exception

   All built-in, non-system-exiting exceptions are derived from this
   class.  All user-defined exceptions should also be derived from
   this class.

   Changed in version 2.5: Changed to inherit from "BaseException".

exception StandardError

   The base class for all built-in exceptions except "StopIteration",
   "GeneratorExit", "KeyboardInterrupt" and "SystemExit".
   "StandardError" itself is derived from "Exception".

exception ArithmeticError

   The base class for those built-in exceptions that are raised for
   various arithmetic errors: "OverflowError", "ZeroDivisionError",
   "FloatingPointError".

exception BufferError

   Raised when a buffer related operation cannot be performed.

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 "codecs.lookup()".

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 "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.

exception AssertionError

   Raised when an "assert" statement fails.

exception AttributeError

   Raised when an attribute reference (see Attribute references) 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 "file.read()" and "file.readline()" methods
   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 GeneratorExit

   Raised when a *generator*’s "close()" method is called.  It
   directly inherits from "BaseException" instead of "StandardError"
   since it is technically not an error.

   New in version 2.5.

   Changed in version 2.6: Changed to inherit from "BaseException".

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.

   Changed in version 2.6: Changed "socket.error" to use this as a
   base class.

exception 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.

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. The
   exception inherits from "BaseException" so as to not be
   accidentally caught by code that catches "Exception" and thus
   prevent the interpreter from exiting.

   Changed in version 2.5: Changed to inherit from "BaseException".

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 exception is derived from "EnvironmentError".  It is raised
   when a function returns a system-related error (not for illegal
   argument types or other incidental errors).  The "errno" attribute
   is a numeric error code from "errno", and the "strerror" attribute
   is the corresponding string, as would be printed by the C function
   "perror()". See the module "errno", which contains names for the
   error codes defined by the underlying operating system.

   For exceptions that involve a file system path (such as "chdir()"
   or "unlink()"), the exception instance will contain a third
   attribute, "filename", which is the file name passed to the
   function.

   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) and for most operations
   with plain integers, which return a long integer instead.  Because
   of the lack of standardization of floating point exception handling
   in C, most floating point operations also aren’t checked.

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.

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 IndentationError

   Base class for syntax errors related to incorrect indentation.
   This is a subclass of "SyntaxError".

exception TabError

   Raised when indentation contains an inconsistent use of tabs and
   spaces. This is a subclass of "IndentationError".

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 "BaseException" 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
   "os.fork()").

   The exception inherits from "BaseException" instead of
   "StandardError" or "Exception" so that it is not accidentally
   caught by code that catches "Exception".  This allows the exception
   to properly propagate up and cause the interpreter to exit.

   Changed in version 2.5: Changed to inherit from "BaseException".

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".

   "UnicodeError" has attributes that describe the encoding or
   decoding error.  For example, "err.object[err.start:err.end]" gives
   the particular invalid input that the codec failed on.

   encoding

      The name of the encoding that raised the error.

   reason

      A string describing the specific codec error.

   object

      The object the codec was attempting to encode or decode.

   start

      The first index of invalid data in "object".

   end

      The index after the last invalid data in "object".

   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 an 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 VMSError

   Only available on VMS.  Raised when a VMS-specific error occurs.

exception WindowsError

   Raised when a Windows-specific error occurs or when the error
   number does not correspond to an "errno" value.  The "winerror" and
   "strerror" values are created from the return values of the
   "GetLastError()" and "FormatMessage()" functions from the Windows
   Platform API. The "errno" value maps the "winerror" value to
   corresponding "errno.h" values. This is a subclass of "OSError".

   New in version 2.0.

   Changed in version 2.5: Previous versions put the "GetLastError()"
   codes into "errno".

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.

exception ImportWarning

   Base class for warnings about probable mistakes in module imports.

   New in version 2.5.

exception UnicodeWarning

   Base class for warnings related to Unicode.

   New in version 2.5.

exception BytesWarning

   Base class for warnings related to bytes and bytearray.

   New in version 2.6.


6.1. Exception hierarchy
========================

The class hierarchy for built-in exceptions is:

   BaseException
    +-- SystemExit
    +-- KeyboardInterrupt
    +-- GeneratorExit
    +-- Exception
         +-- StopIteration
         +-- StandardError
         |    +-- BufferError
         |    +-- ArithmeticError
         |    |    +-- FloatingPointError
         |    |    +-- OverflowError
         |    |    +-- ZeroDivisionError
         |    +-- AssertionError
         |    +-- AttributeError
         |    +-- EnvironmentError
         |    |    +-- IOError
         |    |    +-- OSError
         |    |         +-- WindowsError (Windows)
         |    |         +-- VMSError (VMS)
         |    +-- EOFError
         |    +-- ImportError
         |    +-- LookupError
         |    |    +-- IndexError
         |    |    +-- KeyError
         |    +-- MemoryError
         |    +-- NameError
         |    |    +-- UnboundLocalError
         |    +-- ReferenceError
         |    +-- RuntimeError
         |    |    +-- NotImplementedError
         |    +-- SyntaxError
         |    |    +-- IndentationError
         |    |         +-- TabError
         |    +-- SystemError
         |    +-- TypeError
         |    +-- ValueError
         |         +-- UnicodeError
         |              +-- UnicodeDecodeError
         |              +-- UnicodeEncodeError
         |              +-- UnicodeTranslateError
         +-- Warning
              +-- DeprecationWarning
              +-- PendingDeprecationWarning
              +-- RuntimeWarning
              +-- SyntaxWarning
              +-- UserWarning
              +-- FutureWarning
   	   +-- ImportWarning
   	   +-- UnicodeWarning
   	   +-- BytesWarning
