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 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 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.
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