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(python2.1-api.info)Exception Handling


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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 `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
`PyArg_Parse*()' 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.

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.

`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!)

`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
     `PyErr_Set*()' functions or to `PyErr_Restore()').  If not set,
     return `NULL'.  You do not own a reference to the return value, so
     you do not need to `Py_DECREF()' it.  *Note:*  Do not compare the
     return value to a specific exception; use
     `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.)

`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.

`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.

`void PyErr_NormalizeException(PyObject**exc, PyObject**val, PyObject**tb)'
     Under certain circumstances, the values returned by
     `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.

`void PyErr_Clear()'
     Clear the error indicator.  If the error indicator is not set,
     there is no effect.

`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.

`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
     string or class; if it is a class, the value should be an instance
     of that 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.

`void PyErr_SetString(PyObject *type, 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. `PyExc_RuntimeError'.  You need not
     increment its reference count.  The second argument is an error
     message; it is converted to a string object.

`void PyErr_SetObject(PyObject *type, PyObject *value)'
     This function is similar to `PyErr_SetString()' but lets you
     specify an arbitrary Python object for the "value" of the
     exception.  You need not increment its reference count.

`PyObject* PyErr_Format(PyObject *exception, const char *format, ...)'
     This function sets the error indicator.  EXCEPTION should be a
     Python exception (string or class, not an instance).  FORMAT
     should be a string, containing format codes, similar to `printf'.
     The `width.precision' before a format code is parsed, but the
     width part is ignored.

     Character                          Meaning
     ------                             -----
     c                                  Character, as an `int' parameter
     d                                  Number in decimal, as an `int'
                                        parameter
     x                                  Number in hexadecimal, as an
                                        `int' parameter
     x                                  A string, as a `char *' parameter

     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.

     A new reference is returned, which is owned by the caller.

`void PyErr_SetNone(PyObject *type)'
     This is a shorthand for `PyErr_SetObject(TYPE, Py_None)'.

`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.

`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.

`PyObject* PyErr_SetFromErrno(PyObject *type)'
     This is a convenience function to raise an exception when a C
     library function has returned an error and set the C variable
     `errno'.  It constructs a tuple object whose first item is the
     integer `errno' value and whose second item is the corresponding
     error message (gotten from `strerror()' ), and then calls
     `PyErr_SetObject(TYPE, OBJECT)'.  On UNIX, when the `errno' value
     is `EINTR', indicating an interrupted system call, this calls
     `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();' when  the system call returns an error.

`PyObject* PyErr_SetFromErrnoWithFilename(PyObject *type, char *filename)'
     Similar to `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
     `IOError' and `OSError', this is used to define the `filename'
     attribute of the exception instance.

`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.

`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.

     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 `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 (e.g.
     `Py_DECREF()' owned references and return an error value).

     Warning categories must be subclasses of `Warning'; the default
     warning category is `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 `PyObject*'; they are all class objects.  Their names are
     `PyExc_Warning', `PyExc_UserWarning', `PyExc_DeprecationWarning',
     `PyExc_SyntaxWarning', and `PyExc_RuntimeWarning'.
     `PyExc_Warning' is a subclass of `PyExc_Exception'; the other
     warning categories are subclasses of `PyExc_Warning'.

     For information about warning control, see the documentation for
     the `warnings' module and the `-W' option in the command line
     documentation.  There is no C API for warning control.

`int PyErr_WarnExplicit(PyObject *category, char *message, char *filename, int lineno, char *module, PyObject *registry)'
     Issue a warning message with explicit control over all warning
     attributes.  This is a straightforward wrapper around the Python
     function `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.

`int PyErr_CheckSignals()'
     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 `signal'  module is
     supported, this can invoke a signal handler written in Python.  In
     all cases, the default effect for `SIGINT'  is to raise the
     `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.

`void PyErr_SetInterrupt()'
     This function is obsolete.  It simulates the effect of a `SIGINT'
     signal arriving -- the next time `PyErr_CheckSignals()' is called,
     `KeyboardInterrupt' will be raised.  It may be called without
     holding the interpreter lock.

`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 the
     root for all exceptions, the built-in name `Exception' (accessible
     in C as `PyExc_Exception').  The `__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 an alternate
     base class.  The DICT argument can be used to specify a dictionary
     of class variables and methods.

`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 `__del__' method.

     The function is called with a single argument OBJ that identifies
     where the context in which the unraisable exception occurred.  The
     repr of OBJ will be printed in the warning message.

Standard Exceptions
Deprecation of String Exceptions

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