2.2 Built-in Exceptions

 

Exceptions can be class objects or string objects. While traditionally, most exceptions have been string objects, in Python 1.5, all standard exceptions have been converted to class objects, and users are encouraged to the the same. The source code for those exceptions is present in the standard library module exceptions; this module never needs to be imported explicitly.

For backward compatibility, when Python is invoked with the -X option, the standard exceptions are strings. This may be needed to run some code that breaks because of the different semantics of class based exceptions. The -X option will become obsolete in future Python versions, so the recommended solution is to fix the code.

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 derived from the root class Exception, that variable receives the exception instance, and the associated value is present as the exception instance's args attribute; this is a tuple even if the second argument to raise was not (then it is a singleton tuple).  

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. When string-based standard exceptions are used, they are tuples containing the directly derived classes.

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).

StandardError

The base class for built-in exceptions. All built-in exceptions are derived from this class, which is itself 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 thise exceptions that are raised when a key or index used on a mapping or sequence is invalid: IndexError, KeyError.

The following exceptions are the exceptions that are actually raised. They are class objects, except when the -X option is used to revert back to string-based standard exceptions.

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.) No associated value.

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''.

When class exceptions are used, and this exception is instantiated as IOError(errno, strerror), the instance has two additional attributes errno and strerror set to the error code and the error message, respectively. These attributes default to None.

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. No associated value.

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.

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.

When class exceptions are used, the instance has an attribute code which is set to the proposed exit status or error message (defaulting to None). 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.

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.

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.