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 sub-classed to define new exceptions; programmers are encouraged to at least derive new exceptions from the Exception class and not 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.
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. All arguments are stored in args as a tuple.
New in version 2.5.
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.
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.
Raise 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.
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.
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.
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.
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.
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.
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.
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.
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.
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 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.
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.
Raised when a Unicode-related encoding or decoding error occurs. It is a subclass of ValueError.
New in version 2.0.
Raised when a Unicode-related error occurs during encoding. It is a subclass of UnicodeError.
New in version 2.3.
Raised when a Unicode-related error occurs during decoding. It is a subclass of UnicodeError.
New in version 2.3.
Raised when a Unicode-related error occurs during translating. It is a subclass of UnicodeError.
New in version 2.3.
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.
The following exceptions are used as warning categories; see the warnings module for more information.
Base class for warnings about probable mistakes in module imports.
New in version 2.5.
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