diff options
Diffstat (limited to 'Doc/c-api/exceptions.rst')
| -rw-r--r-- | Doc/c-api/exceptions.rst | 361 |
1 files changed, 207 insertions, 154 deletions
diff --git a/Doc/c-api/exceptions.rst b/Doc/c-api/exceptions.rst index c2df767..6ae723b 100644 --- a/Doc/c-api/exceptions.rst +++ b/Doc/c-api/exceptions.rst @@ -9,13 +9,19 @@ 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 :c:data:`errno` variable: +exception handling. It works somewhat like the POSIX :c:data:`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 :c:func:`PyArg_\*` functions return ``1`` for success and -``0`` for failure). +C API functions don't clear this on success, but will set it to indicate the +cause of the error on failure. Most C API 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 :c:func:`PyArg_\*` functions +return ``1`` for success and ``0`` for failure). + +Concretely, the error indicator consists of three object pointers: the +exception's type, the exception's value, and the traceback object. Any +of those pointers can be NULL if non-set (although some combinations are +forbidden, for example you can't have a non-NULL traceback if the exception +type is NULL). 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 @@ -27,12 +33,21 @@ 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 result -of ``sys.exc_info()``. API functions exist to interact with the error indicator -in various ways. There is a separate error indicator for each thread. +.. note:: + The error indicator is **not** the result of :func:`sys.exc_info()`. + The former corresponds to an exception that is not yet caught (and is + therefore still propagating), while the latter returns an exception after + it is caught (and has therefore stopped propagating). -.. XXX Order of these should be more thoughtful. - Either alphabetical or some kind of structure. + +Printing and clearing +===================== + + +.. c:function:: void PyErr_Clear() + + Clear the error indicator. If the error indicator is not set, there is no + effect. .. c:function:: void PyErr_PrintEx(int set_sys_last_vars) @@ -51,127 +66,24 @@ in various ways. There is a separate error indicator for each thread. Alias for ``PyErr_PrintEx(1)``. -.. c:function:: 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 :c:func:`PyErr_Set\*` - functions or to :c:func:`PyErr_Restore`). If not set, return *NULL*. You do not - own a reference to the return value, so you do not need to :c:func:`Py_DECREF` - it. - - .. note:: - - Do not compare the return value to a specific exception; use - :c:func:`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 be a subclass of the expected exception.) - - -.. c:function:: 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. - - -.. c:function:: 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. - - -.. c:function:: void PyErr_NormalizeException(PyObject**exc, PyObject**val, PyObject**tb) - - Under certain circumstances, the values returned by :c:func:`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. - - .. note:: - - This function *does not* implicitly set the ``__traceback__`` - attribute on the exception value. If setting the traceback - appropriately is desired, the following additional snippet is needed:: - - if (tb != NULL) { - PyException_SetTraceback(val, tb); - } - - -.. c:function:: void PyErr_Clear() - - Clear the error indicator. If the error indicator is not set, there is no - effect. - - -.. c:function:: 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. - - -.. c:function:: 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 :c:func:`PyErr_Fetch` to save the current - exception state. - - -.. c:function:: void PyErr_GetExcInfo(PyObject **ptype, PyObject **pvalue, PyObject **ptraceback) - - Retrieve the exception info, as known from ``sys.exc_info()``. This refers - to an exception that was already caught, not to an exception that was - freshly raised. Returns new references for the three objects, any of which - may be *NULL*. Does not modify the exception info state. - - .. note:: - - This function is not normally used by code that wants to handle exceptions. - Rather, it can be used when code needs to save and restore the exception - state temporarily. Use :c:func:`PyErr_SetExcInfo` to restore or clear the - exception state. - - .. versionadded:: 3.3 - +.. c:function:: void PyErr_WriteUnraisable(PyObject *obj) -.. c:function:: void PyErr_SetExcInfo(PyObject *type, PyObject *value, PyObject *traceback) + 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. - Set the exception info, as known from ``sys.exc_info()``. This refers - to an exception that was already caught, not to an exception that was - freshly raised. This function steals the references of the arguments. - To clear the exception state, pass *NULL* for all three arguments. - For general rules about the three arguments, see :c:func:`PyErr_Restore`. + 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. - .. note:: - This function is not normally used by code that wants to handle exceptions. - Rather, it can be used when code needs to save and restore the exception - state temporarily. Use :c:func:`PyErr_GetExcInfo` to read the exception - state. +Raising exceptions +================== - .. versionadded:: 3.3 +These functions help you set the current thread's error indicator. +For convenience, some of these functions will always return a +NULL pointer for use in a ``return`` statement. .. c:function:: void PyErr_SetString(PyObject *type, const char *message) @@ -197,6 +109,14 @@ in various ways. There is a separate error indicator for each thread. string. +.. c:function:: PyObject* PyErr_FormatV(PyObject *exception, const char *format, va_list vargs) + + Same as :c:func:`PyErr_Format`, but taking a :c:type:`va_list` argument rather + than a variable number of arguments. + + .. versionadded:: 3.5 + + .. c:function:: void PyErr_SetNone(PyObject *type) This is a shorthand for ``PyErr_SetObject(type, Py_None)``. @@ -346,6 +266,22 @@ in various ways. There is a separate error indicator for each thread. use. +Issuing warnings +================ + +Use these functions to issue warnings from C code. They mirror similar +functions exported by the Python :mod:`warnings` module. They normally +print 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 they will raise an exception. It is also possible that +the functions raise an exception because of a problem with the warning machinery. +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, :c:func:`Py_DECREF` owned references and return +an error value). + .. c:function:: int PyErr_WarnEx(PyObject *category, const char *message, Py_ssize_t stack_level) Issue a warning message. The *category* argument is a warning category (see @@ -355,18 +291,6 @@ in various ways. There is a separate error indicator for each thread. is the function calling :c:func:`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, :c:func:`Py_DECREF` owned references and return - an error value). - Warning categories must be subclasses of :c:data:`Warning`; the default warning category is :c:data:`RuntimeWarning`. The standard Python warning categories are available as global variables whose names are ``PyExc_`` followed by the Python @@ -410,6 +334,139 @@ in various ways. There is a separate error indicator for each thread. .. versionadded:: 3.2 +Querying the error indicator +============================ + +.. c:function:: 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 :c:func:`PyErr_Set\*` + functions or to :c:func:`PyErr_Restore`). If not set, return *NULL*. You do not + own a reference to the return value, so you do not need to :c:func:`Py_DECREF` + it. + + .. note:: + + Do not compare the return value to a specific exception; use + :c:func:`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 be a subclass of the expected exception.) + + +.. c:function:: 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. + + +.. c:function:: int PyErr_GivenExceptionMatches(PyObject *given, PyObject *exc) + + Return true if the *given* exception matches the exception type 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 exception types in the tuple (and + recursively in subtuples) are searched for a match. + + +.. c:function:: 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 catch exceptions or + by code that needs to save and restore the error indicator temporarily, e.g.:: + + { + PyObject **type, **value, **traceback; + PyErr_Fetch(&type, &value, &traceback); + + /* ... code that might produce other errors ... */ + + PyErr_Restore(type, value, traceback); + } + + +.. c:function:: 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 :c:func:`PyErr_Fetch` to save the current + error indicator. + + +.. c:function:: void PyErr_NormalizeException(PyObject**exc, PyObject**val, PyObject**tb) + + Under certain circumstances, the values returned by :c:func:`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. + + .. note:: + + This function *does not* implicitly set the ``__traceback__`` + attribute on the exception value. If setting the traceback + appropriately is desired, the following additional snippet is needed:: + + if (tb != NULL) { + PyException_SetTraceback(val, tb); + } + + +.. c:function:: void PyErr_GetExcInfo(PyObject **ptype, PyObject **pvalue, PyObject **ptraceback) + + Retrieve the exception info, as known from ``sys.exc_info()``. This refers + to an exception that was *already caught*, not to an exception that was + freshly raised. Returns new references for the three objects, any of which + may be *NULL*. Does not modify the exception info state. + + .. note:: + + This function is not normally used by code that wants to handle exceptions. + Rather, it can be used when code needs to save and restore the exception + state temporarily. Use :c:func:`PyErr_SetExcInfo` to restore or clear the + exception state. + + .. versionadded:: 3.3 + + +.. c:function:: void PyErr_SetExcInfo(PyObject *type, PyObject *value, PyObject *traceback) + + Set the exception info, as known from ``sys.exc_info()``. This refers + to an exception that was *already caught*, not to an exception that was + freshly raised. This function steals the references of the arguments. + To clear the exception state, pass *NULL* for all three arguments. + For general rules about the three arguments, see :c:func:`PyErr_Restore`. + + .. note:: + + This function is not normally used by code that wants to handle exceptions. + Rather, it can be used when code needs to save and restore the exception + state temporarily. Use :c:func:`PyErr_GetExcInfo` to read the exception + state. + + .. versionadded:: 3.3 + + +Signal Handling +=============== + + .. c:function:: int PyErr_CheckSignals() .. index:: @@ -443,13 +500,21 @@ in various ways. There is a separate error indicator for each thread. .. c:function:: 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 utility function specifies a file descriptor to which the signal number + is written as a single byte whenever a signal is received. *fd* must be + non-blocking. 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. + .. versionchanged:: 3.5 + On Windows, the function now also supports socket handles. + + +Exception Classes +================= .. c:function:: PyObject* PyErr_NewException(const char *name, PyObject *base, PyObject *dict) @@ -475,18 +540,6 @@ in various ways. There is a separate error indicator for each thread. .. versionadded:: 3.2 -.. c:function:: 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. - - Exception Objects ================= |