Next: Built-in Functions Prev: Types Up: Built-in Objects Top: Top

2.2. Built-in Exceptions

Exceptions are string objects. Two distinct string objects with the same value are 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.

The following exceptions 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).

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.

AttributeError -- built-in exception: Raised when an attribute reference or assignment fails. (When an object does not support attributes references or attribute assignments at all, TypeError is raised.)

EOFError -- built-in exception: 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.

IOError -- built-in exception: 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', `disk full'.

ImportError -- built-in exception: 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 -- built-in exception: 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 -- built-in exception: Raised when a mapping (dictionary) key is not found in the set of existing keys.

KeyboardInterrupt -- built-in exception: 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 -- built-in exception: 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 -- built-in exception: 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 -- built-in exception: 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 -- built-in exception: 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 a relic from a previous version of the interpreter; it is not used any more except by some extension modules that haven't been converted to define their own exceptions yet.)

SyntaxError -- built-in exception: 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).

SystemError -- built-in exception: 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 -- built-in exception: 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.
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 posix._exit() function can be used if it is absolutely positively necessary to exit immediately (e.g., after a fork() in the child process).

TypeError -- built-in exception: 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 -- built-in exception: 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 -- built-in exception: 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.