--- /dev/null Thu Jan 01 00:00:00 1970 +0000
+++ b/symbian-qemu-0.9.1-12/python-2.6.1/Doc/c-api/exceptions.rst Fri Jul 31 15:01:17 2009 +0100
@@ -0,0 +1,552 @@
+.. highlightlang:: c
+
+
+.. _exceptionhandling:
+
+******************
+Exception Handling
+******************
+
+The functions described in this chapter will let you handle and raise Python
+exceptions. It is important to understand some of the basics of Python
+exception handling. It works somewhat like the Unix :cdata:`errno` variable:
+there is a global indicator (per thread) of the last error that occurred. Most
+functions don't clear this on success, but will set it to indicate the cause of
+the error on failure. Most functions also return an error indicator, usually
+*NULL* if they are supposed to return a pointer, or ``-1`` if they return an
+integer (exception: the :cfunc:`PyArg_\*` functions return ``1`` for success and
+``0`` for failure).
+
+When a function must fail because some function it called failed, it generally
+doesn't set the error indicator; the function it called already set it. It is
+responsible for either handling the error and clearing the exception or
+returning after cleaning up any resources it holds (such as object references or
+memory allocations); it should *not* continue normally if it is not prepared to
+handle the error. If returning due to an error, it is important to indicate to
+the caller that an error has been set. If the error is not handled or carefully
+propagated, additional calls into the Python/C API may not behave as intended
+and may fail in mysterious ways.
+
+.. index::
+ single: exc_type (in module sys)
+ single: exc_value (in module sys)
+ single: exc_traceback (in module sys)
+
+The error indicator consists of three Python objects corresponding to the
+Python variables ``sys.exc_type``, ``sys.exc_value`` and ``sys.exc_traceback``.
+API functions exist to interact with the error indicator in various ways. There
+is a separate error indicator for each thread.
+
+.. XXX Order of these should be more thoughtful.
+ Either alphabetical or some kind of structure.
+
+
+.. cfunction:: void PyErr_Print()
+
+ Print a standard traceback to ``sys.stderr`` and clear the error indicator.
+ Call this function only when the error indicator is set. (Otherwise it will
+ cause a fatal error!)
+
+
+.. cfunction:: PyObject* PyErr_Occurred()
+
+ Test whether the error indicator is set. If set, return the exception *type*
+ (the first argument to the last call to one of the :cfunc:`PyErr_Set\*`
+ functions or to :cfunc:`PyErr_Restore`). If not set, return *NULL*. You do not
+ own a reference to the return value, so you do not need to :cfunc:`Py_DECREF`
+ it.
+
+ .. note::
+
+ Do not compare the return value to a specific exception; use
+ :cfunc:`PyErr_ExceptionMatches` instead, shown below. (The comparison could
+ easily fail since the exception may be an instance instead of a class, in the
+ case of a class exception, or it may the a subclass of the expected exception.)
+
+
+.. cfunction:: int PyErr_ExceptionMatches(PyObject *exc)
+
+ Equivalent to ``PyErr_GivenExceptionMatches(PyErr_Occurred(), exc)``. This
+ should only be called when an exception is actually set; a memory access
+ violation will occur if no exception has been raised.
+
+
+.. cfunction:: int PyErr_GivenExceptionMatches(PyObject *given, PyObject *exc)
+
+ Return true if the *given* exception matches the exception in *exc*. If *exc*
+ is a class object, this also returns true when *given* is an instance of a
+ subclass. If *exc* is a tuple, all exceptions in the tuple (and recursively in
+ subtuples) are searched for a match. If *given* is *NULL*, a memory access
+ violation will occur.
+
+
+.. cfunction:: void PyErr_NormalizeException(PyObject**exc, PyObject**val, PyObject**tb)
+
+ Under certain circumstances, the values returned by :cfunc:`PyErr_Fetch` below
+ can be "unnormalized", meaning that ``*exc`` is a class object but ``*val`` is
+ not an instance of the same class. This function can be used to instantiate
+ the class in that case. If the values are already normalized, nothing happens.
+ The delayed normalization is implemented to improve performance.
+
+
+.. cfunction:: void PyErr_Clear()
+
+ Clear the error indicator. If the error indicator is not set, there is no
+ effect.
+
+
+.. cfunction:: void PyErr_Fetch(PyObject **ptype, PyObject **pvalue, PyObject **ptraceback)
+
+ Retrieve the error indicator into three variables whose addresses are passed.
+ If the error indicator is not set, set all three variables to *NULL*. If it is
+ set, it will be cleared and you own a reference to each object retrieved. The
+ value and traceback object may be *NULL* even when the type object is not.
+
+ .. note::
+
+ This function is normally only used by code that needs to handle exceptions or
+ by code that needs to save and restore the error indicator temporarily.
+
+
+.. cfunction:: void PyErr_Restore(PyObject *type, PyObject *value, PyObject *traceback)
+
+ Set the error indicator from the three objects. If the error indicator is
+ already set, it is cleared first. If the objects are *NULL*, the error
+ indicator is cleared. Do not pass a *NULL* type and non-*NULL* value or
+ traceback. The exception type should be a class. Do not pass an invalid
+ exception type or value. (Violating these rules will cause subtle problems
+ later.) This call takes away a reference to each object: you must own a
+ reference to each object before the call and after the call you no longer own
+ these references. (If you don't understand this, don't use this function. I
+ warned you.)
+
+ .. note::
+
+ This function is normally only used by code that needs to save and restore the
+ error indicator temporarily; use :cfunc:`PyErr_Fetch` to save the current
+ exception state.
+
+
+.. cfunction:: void PyErr_SetString(PyObject *type, const char *message)
+
+ This is the most common way to set the error indicator. The first argument
+ specifies the exception type; it is normally one of the standard exceptions,
+ e.g. :cdata:`PyExc_RuntimeError`. You need not increment its reference count.
+ The second argument is an error message; it is converted to a string object.
+
+
+.. cfunction:: void PyErr_SetObject(PyObject *type, PyObject *value)
+
+ This function is similar to :cfunc:`PyErr_SetString` but lets you specify an
+ arbitrary Python object for the "value" of the exception.
+
+
+.. cfunction:: PyObject* PyErr_Format(PyObject *exception, const char *format, ...)
+
+ This function sets the error indicator and returns *NULL*. *exception* should be
+ a Python exception (class, not an instance). *format* should be a string,
+ containing format codes, similar to :cfunc:`printf`. The ``width.precision``
+ before a format code is parsed, but the width part is ignored.
+
+ .. % This should be exactly the same as the table in PyString_FromFormat.
+ .. % One should just refer to the other.
+ .. % The descriptions for %zd and %zu are wrong, but the truth is complicated
+ .. % because not all compilers support the %z width modifier -- we fake it
+ .. % when necessary via interpolating PY_FORMAT_SIZE_T.
+ .. % %u, %lu, %zu should have "new in Python 2.5" blurbs.
+
+ +-------------------+---------------+--------------------------------+
+ | Format Characters | Type | Comment |
+ +===================+===============+================================+
+ | :attr:`%%` | *n/a* | The literal % character. |
+ +-------------------+---------------+--------------------------------+
+ | :attr:`%c` | int | A single character, |
+ | | | represented as an C int. |
+ +-------------------+---------------+--------------------------------+
+ | :attr:`%d` | int | Exactly equivalent to |
+ | | | ``printf("%d")``. |
+ +-------------------+---------------+--------------------------------+
+ | :attr:`%u` | unsigned int | Exactly equivalent to |
+ | | | ``printf("%u")``. |
+ +-------------------+---------------+--------------------------------+
+ | :attr:`%ld` | long | Exactly equivalent to |
+ | | | ``printf("%ld")``. |
+ +-------------------+---------------+--------------------------------+
+ | :attr:`%lu` | unsigned long | Exactly equivalent to |
+ | | | ``printf("%lu")``. |
+ +-------------------+---------------+--------------------------------+
+ | :attr:`%zd` | Py_ssize_t | Exactly equivalent to |
+ | | | ``printf("%zd")``. |
+ +-------------------+---------------+--------------------------------+
+ | :attr:`%zu` | size_t | Exactly equivalent to |
+ | | | ``printf("%zu")``. |
+ +-------------------+---------------+--------------------------------+
+ | :attr:`%i` | int | Exactly equivalent to |
+ | | | ``printf("%i")``. |
+ +-------------------+---------------+--------------------------------+
+ | :attr:`%x` | int | Exactly equivalent to |
+ | | | ``printf("%x")``. |
+ +-------------------+---------------+--------------------------------+
+ | :attr:`%s` | char\* | A null-terminated C character |
+ | | | array. |
+ +-------------------+---------------+--------------------------------+
+ | :attr:`%p` | void\* | The hex representation of a C |
+ | | | pointer. Mostly equivalent to |
+ | | | ``printf("%p")`` except that |
+ | | | it is guaranteed to start with |
+ | | | the literal ``0x`` regardless |
+ | | | of what the platform's |
+ | | | ``printf`` yields. |
+ +-------------------+---------------+--------------------------------+
+
+ An unrecognized format character causes all the rest of the format string to be
+ copied as-is to the result string, and any extra arguments discarded.
+
+
+.. cfunction:: void PyErr_SetNone(PyObject *type)
+
+ This is a shorthand for ``PyErr_SetObject(type, Py_None)``.
+
+
+.. cfunction:: int PyErr_BadArgument()
+
+ This is a shorthand for ``PyErr_SetString(PyExc_TypeError, message)``, where
+ *message* indicates that a built-in operation was invoked with an illegal
+ argument. It is mostly for internal use.
+
+
+.. cfunction:: PyObject* PyErr_NoMemory()
+
+ This is a shorthand for ``PyErr_SetNone(PyExc_MemoryError)``; it returns *NULL*
+ so an object allocation function can write ``return PyErr_NoMemory();`` when it
+ runs out of memory.
+
+
+.. cfunction:: PyObject* PyErr_SetFromErrno(PyObject *type)
+
+ .. index:: single: strerror()
+
+ This is a convenience function to raise an exception when a C library function
+ has returned an error and set the C variable :cdata:`errno`. It constructs a
+ tuple object whose first item is the integer :cdata:`errno` value and whose
+ second item is the corresponding error message (gotten from :cfunc:`strerror`),
+ and then calls ``PyErr_SetObject(type, object)``. On Unix, when the
+ :cdata:`errno` value is :const:`EINTR`, indicating an interrupted system call,
+ this calls :cfunc:`PyErr_CheckSignals`, and if that set the error indicator,
+ leaves it set to that. The function always returns *NULL*, so a wrapper
+ function around a system call can write ``return PyErr_SetFromErrno(type);``
+ when the system call returns an error.
+
+
+.. cfunction:: PyObject* PyErr_SetFromErrnoWithFilename(PyObject *type, const char *filename)
+
+ Similar to :cfunc:`PyErr_SetFromErrno`, with the additional behavior that if
+ *filename* is not *NULL*, it is passed to the constructor of *type* as a third
+ parameter. In the case of exceptions such as :exc:`IOError` and :exc:`OSError`,
+ this is used to define the :attr:`filename` attribute of the exception instance.
+
+
+.. cfunction:: PyObject* PyErr_SetFromWindowsErr(int ierr)
+
+ This is a convenience function to raise :exc:`WindowsError`. If called with
+ *ierr* of :cdata:`0`, the error code returned by a call to :cfunc:`GetLastError`
+ is used instead. It calls the Win32 function :cfunc:`FormatMessage` to retrieve
+ the Windows description of error code given by *ierr* or :cfunc:`GetLastError`,
+ then it constructs a tuple object whose first item is the *ierr* value and whose
+ second item is the corresponding error message (gotten from
+ :cfunc:`FormatMessage`), and then calls ``PyErr_SetObject(PyExc_WindowsError,
+ object)``. This function always returns *NULL*. Availability: Windows.
+
+
+.. cfunction:: PyObject* PyErr_SetExcFromWindowsErr(PyObject *type, int ierr)
+
+ Similar to :cfunc:`PyErr_SetFromWindowsErr`, with an additional parameter
+ specifying the exception type to be raised. Availability: Windows.
+
+ .. versionadded:: 2.3
+
+
+.. cfunction:: PyObject* PyErr_SetFromWindowsErrWithFilename(int ierr, const char *filename)
+
+ Similar to :cfunc:`PyErr_SetFromWindowsErr`, with the additional behavior that
+ if *filename* is not *NULL*, it is passed to the constructor of
+ :exc:`WindowsError` as a third parameter. Availability: Windows.
+
+
+.. cfunction:: PyObject* PyErr_SetExcFromWindowsErrWithFilename(PyObject *type, int ierr, char *filename)
+
+ Similar to :cfunc:`PyErr_SetFromWindowsErrWithFilename`, with an additional
+ parameter specifying the exception type to be raised. Availability: Windows.
+
+ .. versionadded:: 2.3
+
+
+.. cfunction:: void PyErr_BadInternalCall()
+
+ This is a shorthand for ``PyErr_SetString(PyExc_TypeError, message)``, where
+ *message* indicates that an internal operation (e.g. a Python/C API function)
+ was invoked with an illegal argument. It is mostly for internal use.
+
+
+.. cfunction:: int PyErr_WarnEx(PyObject *category, char *message, int stacklevel)
+
+ Issue a warning message. The *category* argument is a warning category (see
+ below) or *NULL*; the *message* argument is a message string. *stacklevel* is a
+ positive number giving a number of stack frames; the warning will be issued from
+ the currently executing line of code in that stack frame. A *stacklevel* of 1
+ is the function calling :cfunc:`PyErr_WarnEx`, 2 is the function above that,
+ and so forth.
+
+ This function normally prints a warning message to *sys.stderr*; however, it is
+ also possible that the user has specified that warnings are to be turned into
+ errors, and in that case this will raise an exception. It is also possible that
+ the function raises an exception because of a problem with the warning machinery
+ (the implementation imports the :mod:`warnings` module to do the heavy lifting).
+ The return value is ``0`` if no exception is raised, or ``-1`` if an exception
+ is raised. (It is not possible to determine whether a warning message is
+ actually printed, nor what the reason is for the exception; this is
+ intentional.) If an exception is raised, the caller should do its normal
+ exception handling (for example, :cfunc:`Py_DECREF` owned references and return
+ an error value).
+
+ Warning categories must be subclasses of :cdata:`Warning`; the default warning
+ category is :cdata:`RuntimeWarning`. The standard Python warning categories are
+ available as global variables whose names are ``PyExc_`` followed by the Python
+ exception name. These have the type :ctype:`PyObject\*`; they are all class
+ objects. Their names are :cdata:`PyExc_Warning`, :cdata:`PyExc_UserWarning`,
+ :cdata:`PyExc_UnicodeWarning`, :cdata:`PyExc_DeprecationWarning`,
+ :cdata:`PyExc_SyntaxWarning`, :cdata:`PyExc_RuntimeWarning`, and
+ :cdata:`PyExc_FutureWarning`. :cdata:`PyExc_Warning` is a subclass of
+ :cdata:`PyExc_Exception`; the other warning categories are subclasses of
+ :cdata:`PyExc_Warning`.
+
+ For information about warning control, see the documentation for the
+ :mod:`warnings` module and the :option:`-W` option in the command line
+ documentation. There is no C API for warning control.
+
+
+.. cfunction:: int PyErr_Warn(PyObject *category, char *message)
+
+ Issue a warning message. The *category* argument is a warning category (see
+ below) or *NULL*; the *message* argument is a message string. The warning will
+ appear to be issued from the function calling :cfunc:`PyErr_Warn`, equivalent to
+ calling :cfunc:`PyErr_WarnEx` with a *stacklevel* of 1.
+
+ Deprecated; use :cfunc:`PyErr_WarnEx` instead.
+
+
+.. cfunction:: int PyErr_WarnExplicit(PyObject *category, const char *message, const char *filename, int lineno, const char *module, PyObject *registry)
+
+ Issue a warning message with explicit control over all warning attributes. This
+ is a straightforward wrapper around the Python function
+ :func:`warnings.warn_explicit`, see there for more information. The *module*
+ and *registry* arguments may be set to *NULL* to get the default effect
+ described there.
+
+
+.. cfunction:: int PyErr_WarnPy3k(char *message, int stacklevel)
+
+ Issue a :exc:`DeprecationWarning` with the given *message* and *stacklevel*
+ if the :cdata:`Py_Py3kWarningFlag` flag is enabled.
+
+ .. versionadded:: 2.6
+
+
+.. cfunction:: int PyErr_CheckSignals()
+
+ .. index::
+ module: signal
+ single: SIGINT
+ single: KeyboardInterrupt (built-in exception)
+
+ This function interacts with Python's signal handling. It checks whether a
+ signal has been sent to the processes and if so, invokes the corresponding
+ signal handler. If the :mod:`signal` module is supported, this can invoke a
+ signal handler written in Python. In all cases, the default effect for
+ :const:`SIGINT` is to raise the :exc:`KeyboardInterrupt` exception. If an
+ exception is raised the error indicator is set and the function returns ``-1``;
+ otherwise the function returns ``0``. The error indicator may or may not be
+ cleared if it was previously set.
+
+
+.. cfunction:: void PyErr_SetInterrupt()
+
+ .. index::
+ single: SIGINT
+ single: KeyboardInterrupt (built-in exception)
+
+ This function simulates the effect of a :const:`SIGINT` signal arriving --- the
+ next time :cfunc:`PyErr_CheckSignals` is called, :exc:`KeyboardInterrupt` will
+ be raised. It may be called without holding the interpreter lock.
+
+ .. % XXX This was described as obsolete, but is used in
+ .. % thread.interrupt_main() (used from IDLE), so it's still needed.
+
+
+.. cfunction:: int PySignal_SetWakeupFd(int fd)
+
+ This utility function specifies a file descriptor to which a ``'\0'`` byte will
+ be written whenever a signal is received. It returns the previous such file
+ descriptor. The value ``-1`` disables the feature; this is the initial state.
+ This is equivalent to :func:`signal.set_wakeup_fd` in Python, but without any
+ error checking. *fd* should be a valid file descriptor. The function should
+ only be called from the main thread.
+
+
+.. cfunction:: PyObject* PyErr_NewException(char *name, PyObject *base, PyObject *dict)
+
+ This utility function creates and returns a new exception object. The *name*
+ argument must be the name of the new exception, a C string of the form
+ ``module.class``. The *base* and *dict* arguments are normally *NULL*. This
+ creates a class object derived from :exc:`Exception` (accessible in C as
+ :cdata:`PyExc_Exception`).
+
+ The :attr:`__module__` attribute of the new class is set to the first part (up
+ to the last dot) of the *name* argument, and the class name is set to the last
+ part (after the last dot). The *base* argument can be used to specify alternate
+ base classes; it can either be only one class or a tuple of classes. The *dict*
+ argument can be used to specify a dictionary of class variables and methods.
+
+
+.. cfunction:: void PyErr_WriteUnraisable(PyObject *obj)
+
+ This utility function prints a warning message to ``sys.stderr`` when an
+ exception has been set but it is impossible for the interpreter to actually
+ raise the exception. It is used, for example, when an exception occurs in an
+ :meth:`__del__` method.
+
+ The function is called with a single argument *obj* that identifies the context
+ in which the unraisable exception occurred. The repr of *obj* will be printed in
+ the warning message.
+
+
+.. _standardexceptions:
+
+Standard Exceptions
+===================
+
+All standard Python exceptions are available as global variables whose names are
+``PyExc_`` followed by the Python exception name. These have the type
+:ctype:`PyObject\*`; they are all class objects. For completeness, here are all
+the variables:
+
++------------------------------------+----------------------------+----------+
+| C Name | Python Name | Notes |
++====================================+============================+==========+
+| :cdata:`PyExc_BaseException` | :exc:`BaseException` | (1), (4) |
++------------------------------------+----------------------------+----------+
+| :cdata:`PyExc_Exception` | :exc:`Exception` | \(1) |
++------------------------------------+----------------------------+----------+
+| :cdata:`PyExc_StandardError` | :exc:`StandardError` | \(1) |
++------------------------------------+----------------------------+----------+
+| :cdata:`PyExc_ArithmeticError` | :exc:`ArithmeticError` | \(1) |
++------------------------------------+----------------------------+----------+
+| :cdata:`PyExc_LookupError` | :exc:`LookupError` | \(1) |
++------------------------------------+----------------------------+----------+
+| :cdata:`PyExc_AssertionError` | :exc:`AssertionError` | |
++------------------------------------+----------------------------+----------+
+| :cdata:`PyExc_AttributeError` | :exc:`AttributeError` | |
++------------------------------------+----------------------------+----------+
+| :cdata:`PyExc_EOFError` | :exc:`EOFError` | |
++------------------------------------+----------------------------+----------+
+| :cdata:`PyExc_EnvironmentError` | :exc:`EnvironmentError` | \(1) |
++------------------------------------+----------------------------+----------+
+| :cdata:`PyExc_FloatingPointError` | :exc:`FloatingPointError` | |
++------------------------------------+----------------------------+----------+
+| :cdata:`PyExc_IOError` | :exc:`IOError` | |
++------------------------------------+----------------------------+----------+
+| :cdata:`PyExc_ImportError` | :exc:`ImportError` | |
++------------------------------------+----------------------------+----------+
+| :cdata:`PyExc_IndexError` | :exc:`IndexError` | |
++------------------------------------+----------------------------+----------+
+| :cdata:`PyExc_KeyError` | :exc:`KeyError` | |
++------------------------------------+----------------------------+----------+
+| :cdata:`PyExc_KeyboardInterrupt` | :exc:`KeyboardInterrupt` | |
++------------------------------------+----------------------------+----------+
+| :cdata:`PyExc_MemoryError` | :exc:`MemoryError` | |
++------------------------------------+----------------------------+----------+
+| :cdata:`PyExc_NameError` | :exc:`NameError` | |
++------------------------------------+----------------------------+----------+
+| :cdata:`PyExc_NotImplementedError` | :exc:`NotImplementedError` | |
++------------------------------------+----------------------------+----------+
+| :cdata:`PyExc_OSError` | :exc:`OSError` | |
++------------------------------------+----------------------------+----------+
+| :cdata:`PyExc_OverflowError` | :exc:`OverflowError` | |
++------------------------------------+----------------------------+----------+
+| :cdata:`PyExc_ReferenceError` | :exc:`ReferenceError` | \(2) |
++------------------------------------+----------------------------+----------+
+| :cdata:`PyExc_RuntimeError` | :exc:`RuntimeError` | |
++------------------------------------+----------------------------+----------+
+| :cdata:`PyExc_SyntaxError` | :exc:`SyntaxError` | |
++------------------------------------+----------------------------+----------+
+| :cdata:`PyExc_SystemError` | :exc:`SystemError` | |
++------------------------------------+----------------------------+----------+
+| :cdata:`PyExc_SystemExit` | :exc:`SystemExit` | |
++------------------------------------+----------------------------+----------+
+| :cdata:`PyExc_TypeError` | :exc:`TypeError` | |
++------------------------------------+----------------------------+----------+
+| :cdata:`PyExc_ValueError` | :exc:`ValueError` | |
++------------------------------------+----------------------------+----------+
+| :cdata:`PyExc_WindowsError` | :exc:`WindowsError` | \(3) |
++------------------------------------+----------------------------+----------+
+| :cdata:`PyExc_ZeroDivisionError` | :exc:`ZeroDivisionError` | |
++------------------------------------+----------------------------+----------+
+
+.. index::
+ single: PyExc_BaseException
+ single: PyExc_Exception
+ single: PyExc_StandardError
+ single: PyExc_ArithmeticError
+ single: PyExc_LookupError
+ single: PyExc_AssertionError
+ single: PyExc_AttributeError
+ single: PyExc_EOFError
+ single: PyExc_EnvironmentError
+ single: PyExc_FloatingPointError
+ single: PyExc_IOError
+ single: PyExc_ImportError
+ single: PyExc_IndexError
+ single: PyExc_KeyError
+ single: PyExc_KeyboardInterrupt
+ single: PyExc_MemoryError
+ single: PyExc_NameError
+ single: PyExc_NotImplementedError
+ single: PyExc_OSError
+ single: PyExc_OverflowError
+ single: PyExc_ReferenceError
+ single: PyExc_RuntimeError
+ single: PyExc_SyntaxError
+ single: PyExc_SystemError
+ single: PyExc_SystemExit
+ single: PyExc_TypeError
+ single: PyExc_ValueError
+ single: PyExc_WindowsError
+ single: PyExc_ZeroDivisionError
+
+Notes:
+
+(1)
+ This is a base class for other standard exceptions.
+
+(2)
+ This is the same as :exc:`weakref.ReferenceError`.
+
+(3)
+ Only defined on Windows; protect code that uses this by testing that the
+ preprocessor macro ``MS_WINDOWS`` is defined.
+
+(4)
+ .. versionadded:: 2.5
+
+
+Deprecation of String Exceptions
+================================
+
+.. index:: single: BaseException (built-in exception)
+
+All exceptions built into Python or provided in the standard library are derived
+from :exc:`BaseException`.
+
+String exceptions are still supported in the interpreter to allow existing code
+to run unmodified, but this will also change in a future release.
+