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Exception Handling

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_*() 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.

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_PrintEx(int set_sys_last_vars)

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!)

If set_sys_last_vars is nonzero, the variables sys.last_type, sys.last_value and sys.last_traceback will be set to the type, value and traceback of the printed exception, respectively.

void PyErr_Print()

Alias for PyErr_PrintEx(1).

PyObject* PyErr_Occurred()
Return value: Borrowed reference.

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.

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.

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.

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.)

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. 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.

PyObject* PyErr_Format(PyObject *exception, const char *format, ...)
Return value: Always NULL.

This function sets the error indicator and returns NULL. exception should be a Python exception class. The format and subsequent parameters help format the error message; they have the same meaning and values as in PyString_FromFormat().

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()
Return value: Always NULL.

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)
Return value: Always NULL.

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(type); when the system call returns an error.

PyObject* PyErr_SetFromErrnoWithFilenameObject(PyObject *type, PyObject *filenameObject)

Similar to PyErr_SetFromErrno(), with the additional behavior that if filenameObject 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.

PyObject* PyErr_SetFromErrnoWithFilename(PyObject *type, const char *filename)
Return value: Always NULL.

Similar to PyErr_SetFromErrnoWithFilenameObject(), but the filename is given as a C string.

PyObject* PyErr_SetFromWindowsErr(int ierr)
Return value: Always NULL.

This is a convenience function to raise WindowsError. If called with ierr of 0, the error code returned by a call to GetLastError() is used instead. It calls the Win32 function FormatMessage() to retrieve the Windows description of error code given by ierr or 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 FormatMessage()), and then calls PyErr_SetObject(PyExc_WindowsError, object). This function always returns NULL. Availability: Windows.

PyObject* PyErr_SetExcFromWindowsErr(PyObject *type, int ierr)
Return value: Always NULL.

Similar to PyErr_SetFromWindowsErr(), with an additional parameter specifying the exception type to be raised. Availability: Windows.

New in version 2.3.

PyObject* PyErr_SetFromWindowsErrWithFilenameObject(int ierr, PyObject *filenameObject)

Similar to PyErr_SetFromWindowsErr(), with the additional behavior that if filenameObject is not NULL, it is passed to the constructor of WindowsError as a third parameter. Availability: Windows.

PyObject* PyErr_SetFromWindowsErrWithFilename(int ierr, const char *filename)
Return value: Always NULL.

Similar to PyErr_SetFromWindowsErrWithFilenameObject(), but the filename is given as a C string. Availability: Windows.

PyObject* PyErr_SetExcFromWindowsErrWithFilenameObject(PyObject *type, int ierr, PyObject *filename)

Similar to PyErr_SetFromWindowsErrWithFilenameObject(), with an additional parameter specifying the exception type to be raised. Availability: Windows.

New in version 2.3.

PyObject* PyErr_SetExcFromWindowsErrWithFilename(PyObject *type, int ierr, const char *filename)
Return value: Always NULL.

Similar to PyErr_SetFromWindowsErrWithFilename(), with an additional parameter specifying the exception type to be raised. Availability: Windows.

New in version 2.3.

void PyErr_BadInternalCall()

This is a shorthand for PyErr_SetString(PyExc_SystemError, 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_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 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 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, Py_DECREF() owned references and return an error value).

Warning categories must be subclasses of PyExc_Warning; PyExc_Warning is a subclass of PyExc_Exception; the default warning category is PyExc_RuntimeWarning. The standard Python warning categories are available as global variables whose names are enumerated at Standard Warning Categories.

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_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 PyErr_Warn(), equivalent to calling PyErr_WarnEx() with a stacklevel of 1.

Deprecated; use PyErr_WarnEx() instead.

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 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_WarnPy3k(char *message, int stacklevel)

Issue a DeprecationWarning with the given message and stacklevel if the Py_Py3kWarningFlag flag is enabled.

New in version 2.6.

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 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.

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 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.

New in version 2.6.

PyObject* PyErr_NewException(char *name, PyObject *base, PyObject *dict)
Return value: New reference.

This utility function creates and returns a new exception class. The name argument must be the name of the new exception, a C string of the form module.classname. The base and dict arguments are normally NULL. This creates a class object derived from 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 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.

PyObject* PyErr_NewExceptionWithDoc(char *name, char *doc, PyObject *base, PyObject *dict)
Return value: New reference.

Same as PyErr_NewException(), except that the new exception class can easily be given a docstring: If doc is non-NULL, it will be used as the docstring for the exception class.

New in version 2.7.

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 the context in which the unraisable exception occurred. If possible, the repr of obj will be printed in the warning message.

Unicode Exception Objects

The following functions are used to create and modify Unicode exceptions from C.

PyObject* PyUnicodeDecodeError_Create(const char *encoding, const char *object, Py_ssize_t length, Py_ssize_t start, Py_ssize_t end, const char *reason)

Create a UnicodeDecodeError object with the attributes encoding, object, length, start, end and reason.

PyObject* PyUnicodeEncodeError_Create(const char *encoding, const Py_UNICODE *object, Py_ssize_t length, Py_ssize_t start, Py_ssize_t end, const char *reason)

Create a UnicodeEncodeError object with the attributes encoding, object, length, start, end and reason.

PyObject* PyUnicodeTranslateError_Create(const Py_UNICODE *object, Py_ssize_t length, Py_ssize_t start, Py_ssize_t end, const char *reason)

Create a UnicodeTranslateError object with the attributes object, length, start, end and reason.

PyObject* PyUnicodeDecodeError_GetEncoding(PyObject *exc)
PyObject* PyUnicodeEncodeError_GetEncoding(PyObject *exc)

Return the encoding attribute of the given exception object.

PyObject* PyUnicodeDecodeError_GetObject(PyObject *exc)
PyObject* PyUnicodeEncodeError_GetObject(PyObject *exc)
PyObject* PyUnicodeTranslateError_GetObject(PyObject *exc)

Return the object attribute of the given exception object.

int PyUnicodeDecodeError_GetStart(PyObject *exc, Py_ssize_t *start)
int PyUnicodeEncodeError_GetStart(PyObject *exc, Py_ssize_t *start)
int PyUnicodeTranslateError_GetStart(PyObject *exc, Py_ssize_t *start)

Get the start attribute of the given exception object and place it into *start. start must not be NULL. Return 0 on success, -1 on failure.

int PyUnicodeDecodeError_SetStart(PyObject *exc, Py_ssize_t start)
int PyUnicodeEncodeError_SetStart(PyObject *exc, Py_ssize_t start)
int PyUnicodeTranslateError_SetStart(PyObject *exc, Py_ssize_t start)

Set the start attribute of the given exception object to start. Return 0 on success, -1 on failure.

int PyUnicodeDecodeError_GetEnd(PyObject *exc, Py_ssize_t *end)
int PyUnicodeEncodeError_GetEnd(PyObject *exc, Py_ssize_t *end)
int PyUnicodeTranslateError_GetEnd(PyObject *exc, Py_ssize_t *end)

Get the end attribute of the given exception object and place it into *end. end must not be NULL. Return 0 on success, -1 on failure.

int PyUnicodeDecodeError_SetEnd(PyObject *exc, Py_ssize_t end)
int PyUnicodeEncodeError_SetEnd(PyObject *exc, Py_ssize_t end)
int PyUnicodeTranslateError_SetEnd(PyObject *exc, Py_ssize_t end)

Set the end attribute of the given exception object to end. Return 0 on success, -1 on failure.

PyObject* PyUnicodeDecodeError_GetReason(PyObject *exc)
PyObject* PyUnicodeEncodeError_GetReason(PyObject *exc)
PyObject* PyUnicodeTranslateError_GetReason(PyObject *exc)

Return the reason attribute of the given exception object.

int PyUnicodeDecodeError_SetReason(PyObject *exc, const char *reason)
int PyUnicodeEncodeError_SetReason(PyObject *exc, const char *reason)
int PyUnicodeTranslateError_SetReason(PyObject *exc, const char *reason)

Set the reason attribute of the given exception object to reason. Return 0 on success, -1 on failure.

Recursion Control

These two functions provide a way to perform safe recursive calls at the C level, both in the core and in extension modules. They are needed if the recursive code does not necessarily invoke Python code (which tracks its recursion depth automatically).

int Py_EnterRecursiveCall(const char *where)

Marks a point where a recursive C-level call is about to be performed.

If USE_STACKCHECK is defined, this function checks if the OS stack overflowed using PyOS_CheckStack(). In this is the case, it sets a MemoryError and returns a nonzero value.

The function then checks if the recursion limit is reached. If this is the case, a RuntimeError is set and a nonzero value is returned. Otherwise, zero is returned.

where should be a string such as " in instance check" to be concatenated to the RuntimeError message caused by the recursion depth limit.

void Py_LeaveRecursiveCall()

Ends a Py_EnterRecursiveCall(). Must be called once for each successful invocation of Py_EnterRecursiveCall().

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 PyObject*; they are all class objects. For completeness, here are all the variables:

C NamePython NameNotes
PyExc_BaseExceptionBaseException(1), (4)
PyExc_ExceptionException(1)
PyExc_StandardErrorStandardError(1)
PyExc_ArithmeticErrorArithmeticError(1)
PyExc_AssertionErrorAssertionError
PyExc_AttributeErrorAttributeError
PyExc_BufferErrorBufferError
PyExc_EnvironmentErrorEnvironmentError(1)
PyExc_EOFErrorEOFError
PyExc_FloatingPointErrorFloatingPointError
PyExc_GeneratorExitGeneratorExit
PyExc_ImportErrorImportError
PyExc_IndentationErrorIndentationError
PyExc_IndexErrorIndexError
PyExc_IOErrorIOError
PyExc_KeyErrorKeyError
PyExc_KeyboardInterruptKeyboardInterrupt
PyExc_LookupErrorLookupError(1)
PyExc_MemoryErrorMemoryError
PyExc_NameErrorNameError
PyExc_NotImplementedErrorNotImplementedError
PyExc_OSErrorOSError
PyExc_OverflowErrorOverflowError
PyExc_ReferenceErrorReferenceError(2)
PyExc_RuntimeErrorRuntimeError
PyExc_StopIterationStopIteration
PyExc_SyntaxErrorSyntaxError
PyExc_SystemErrorSystemError
PyExc_SystemExitSystemExit
PyExc_TabErrorTabError
PyExc_TypeErrorTypeError
PyExc_UnboundLocalErrorUnboundLocalError
PyExc_UnicodeDecodeErrorUnicodeDecodeError
PyExc_UnicodeEncodeErrorUnicodeEncodeError
PyExc_UnicodeErrorUnicodeError
PyExc_UnicodeTranslateErrorUnicodeTranslateError
PyExc_VMSErrorVMSError(5)
PyExc_ValueErrorValueError
PyExc_WindowsErrorWindowsError(3)
PyExc_ZeroDivisionErrorZeroDivisionError

Notes:

  1. This is a base class for other standard exceptions.
  2. This is the same as weakref.ReferenceError.
  3. Only defined on Windows; protect code that uses this by testing that the preprocessor macro MS_WINDOWS is defined.

  4. New in version 2.5.

  5. Only defined on VMS; protect code that uses this by testing that the preprocessor macro __VMS is defined.
Standard Warning Categories

All 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. For completeness, here are all the variables:

C NamePython NameNotes
PyExc_WarningWarning(1)
PyExc_BytesWarningBytesWarning
PyExc_DeprecationWarningDeprecationWarning
PyExc_FutureWarningFutureWarning
PyExc_ImportWarningImportWarning
PyExc_PendingDeprecationWarningPendingDeprecationWarning
PyExc_RuntimeWarningRuntimeWarning
PyExc_SyntaxWarningSyntaxWarning
PyExc_UnicodeWarningUnicodeWarning
PyExc_UserWarningUserWarning

Notes:

  1. This is a base class for other standard warning categories.
String Exceptions

Changed in version 2.6: All exceptions to be raised or caught must be derived from BaseException. Trying to raise a string exception now raises TypeError.