2.3 Built-in Exceptions

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 as well as the exceptions module.

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 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 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 ... 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.

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 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.error exception. 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 atttributes 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 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.

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 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 SyntaxWarning
Base class for warnings about dubious syntax

exception RuntimeWarning
Base class for warnings about dubious runtime behavior.

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