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(python2.1-lib.info)Built-in Exceptions
Built-in Exceptions
===================
Standard exceptions classes.
Exceptions can be class objects or string objects. Though most
exceptions have been string objects in past versions of Python, in
Python 1.5 and newer versions, all standard exceptions have been
converted to class objects, and users are encouraged to do the same.
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.
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.
`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 all built-in exceptions except `SystemExit'.
`StandardError' itself is 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'. This can be raised directly by
`sys.setdefaultencoding()'.
`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. _Added in
Python 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.
`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'
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.
`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.
`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. _Added in
Python version 1.5.2_
`OSError'
This class is derived from `EnvironmentError' and is used
primarily as the `os' module's `os.error' exception. See
`EnvironmentError' above for a description of the possible
associated values. _Added in Python version 1.5.2_
`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.
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 (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.
`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'. _Added in Python version 2.0_
`UnicodeError'
Raised when a Unicode-related encoding or decoding error occurs.
It is a subclass of `ValueError'. _Added in Python version 2.0_
`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'.
`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'. _Added in Python
version 2.0_
`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.
`Warning'
Base class for warning categories.
`UserWarning'
Base class for warnings generated by user code.
`DeprecationWarning'
Base class for warnings about deprecated features.
`SyntaxWarning'
Base class for warnings about dubious syntax
`RuntimeWarning'
Base class for warnings about dubious runtime behavior.
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