# 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 do 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, most of the standard exceptions are strings2.7. This option may be used to run 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, 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 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). The arguments are also available on the instance's `args` attribute, as a tuple.

`StandardError`
  The base class for all built-in exceptions except `SystemExit`. `StandardError` itself is 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 the exceptions that are raised when a key or index used on a mapping or sequence is invalid: `IndexError`, `KeyError`.

`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. 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.)

`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''.

  This class is derived from `EnvironmentError`. See the discussion above for more information on exception instance attributes.

`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.

`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.

`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.

`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.

`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`). 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 (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.

#### Footnotes

... strings 2.7
  For forward-compatibility the new exceptions `Exception`, `LookupError`, `ArithmeticError`, `EnvironmentError`, and `StandardError` are tuples.