xref: /third_party/python/Doc/c-api/exceptions.rst (revision 7db96d56)

.. highlight:: 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 POSIX :c:data:`errno` variable:
there is a global indicator (per thread) of the last error that occurred.  Most
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 ``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
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.

.. 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).


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)

   Print a standard traceback to ``sys.stderr`` and clear the error indicator.
   **Unless** the error is a ``SystemExit``, in that case no traceback is
   printed and the Python process will exit with the error code specified by
   the ``SystemExit`` instance.

   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 :data:`sys.last_type`,
   :data:`sys.last_value` and :data:`sys.last_traceback` will be set to the
   type, value and traceback of the printed exception, respectively.


.. c:function:: void PyErr_Print()

   Alias for ``PyErr_PrintEx(1)``.


.. c:function:: void PyErr_WriteUnraisable(PyObject *obj)

   Call :func:`sys.unraisablehook` using the current exception and *obj*
   argument.

   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. If possible,
   the repr of *obj* will be printed in the warning message.

   An exception must be set when calling this function.


Raising exceptions
==================

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)

   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. :c:data:`PyExc_RuntimeError`.  You need not increment its reference count.
   The second argument is an error message; it is decoded from ``'utf-8'``.


.. c:function:: void PyErr_SetObject(PyObject *type, PyObject *value)

   This function is similar to :c:func:`PyErr_SetString` but lets you specify an
   arbitrary Python object for the "value" of the exception.


.. c:function:: PyObject* PyErr_Format(PyObject *exception, const char *format, ...)

   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 :c:func:`PyUnicode_FromFormat`. *format* is an ASCII-encoded
   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)``.


.. c:function:: 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.


.. c:function:: 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.


.. c:function:: 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 :c:data:`errno`.  It constructs a
   tuple object whose first item is the integer :c:data:`errno` value and whose
   second item is the corresponding error message (gotten from :c:func:`strerror`),
   and then calls ``PyErr_SetObject(type, object)``.  On Unix, when the
   :c:data:`errno` value is :const:`EINTR`, indicating an interrupted system call,
   this calls :c:func:`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.


.. c:function:: PyObject* PyErr_SetFromErrnoWithFilenameObject(PyObject *type, PyObject *filenameObject)

   Similar to :c:func:`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 :exc:`OSError` exception,
   this is used to define the :attr:`filename` attribute of the
   exception instance.


.. c:function:: PyObject* PyErr_SetFromErrnoWithFilenameObjects(PyObject *type, PyObject *filenameObject, PyObject *filenameObject2)

   Similar to :c:func:`PyErr_SetFromErrnoWithFilenameObject`, but takes a second
   filename object, for raising errors when a function that takes two filenames
   fails.

   .. versionadded:: 3.4


.. c:function:: PyObject* PyErr_SetFromErrnoWithFilename(PyObject *type, const char *filename)

   Similar to :c:func:`PyErr_SetFromErrnoWithFilenameObject`, but the filename
   is given as a C string.  *filename* is decoded from the :term:`filesystem
   encoding and error handler`.


.. c:function:: PyObject* PyErr_SetFromWindowsErr(int ierr)

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

   .. availability:: Windows.


.. c:function:: PyObject* PyErr_SetExcFromWindowsErr(PyObject *type, int ierr)

   Similar to :c:func:`PyErr_SetFromWindowsErr`, with an additional parameter
   specifying the exception type to be raised.

   .. availability:: Windows.


.. c:function:: PyObject* PyErr_SetFromWindowsErrWithFilename(int ierr, const char *filename)

   Similar to :c:func:`PyErr_SetFromWindowsErrWithFilenameObject`, but the
   filename is given as a C string.  *filename* is decoded from the filesystem
   encoding (:func:`os.fsdecode`).

   .. availability:: Windows.


.. c:function:: PyObject* PyErr_SetExcFromWindowsErrWithFilenameObject(PyObject *type, int ierr, PyObject *filename)

   Similar to :c:func:`PyErr_SetFromWindowsErrWithFilenameObject`, with an
   additional parameter specifying the exception type to be raised.

   .. availability:: Windows.


.. c:function:: PyObject* PyErr_SetExcFromWindowsErrWithFilenameObjects(PyObject *type, int ierr, PyObject *filename, PyObject *filename2)

   Similar to :c:func:`PyErr_SetExcFromWindowsErrWithFilenameObject`,
   but accepts a second filename object.

   .. availability:: Windows.

   .. versionadded:: 3.4


.. c:function:: PyObject* PyErr_SetExcFromWindowsErrWithFilename(PyObject *type, int ierr, const char *filename)

   Similar to :c:func:`PyErr_SetFromWindowsErrWithFilename`, with an additional
   parameter specifying the exception type to be raised.

   .. availability:: Windows.


.. c:function:: PyObject* PyErr_SetImportError(PyObject *msg, PyObject *name, PyObject *path)

   This is a convenience function to raise :exc:`ImportError`. *msg* will be
   set as the exception's message string. *name* and *path*, both of which can
   be ``NULL``, will be set as the :exc:`ImportError`'s respective ``name``
   and ``path`` attributes.

   .. versionadded:: 3.3


.. c:function:: PyObject* PyErr_SetImportErrorSubclass(PyObject *exception, PyObject *msg, PyObject *name, PyObject *path)

   Much like :c:func:`PyErr_SetImportError` but this function allows for
   specifying a subclass of :exc:`ImportError` to raise.

   .. versionadded:: 3.6


.. c:function:: void PyErr_SyntaxLocationObject(PyObject *filename, int lineno, int col_offset)

   Set file, line, and offset information for the current exception.  If the
   current exception is not a :exc:`SyntaxError`, then it sets additional
   attributes, which make the exception printing subsystem think the exception
   is a :exc:`SyntaxError`.

   .. versionadded:: 3.4


.. c:function:: void PyErr_SyntaxLocationEx(const char *filename, int lineno, int col_offset)

   Like :c:func:`PyErr_SyntaxLocationObject`, but *filename* is a byte string
   decoded from the :term:`filesystem encoding and error handler`.

   .. versionadded:: 3.2


.. c:function:: void PyErr_SyntaxLocation(const char *filename, int lineno)

   Like :c:func:`PyErr_SyntaxLocationEx`, but the *col_offset* parameter is
   omitted.


.. c:function:: 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.


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
   below) or ``NULL``; the *message* argument is a UTF-8 encoded string.  *stack_level* 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 *stack_level* of 1
   is the function calling :c:func:`PyErr_WarnEx`, 2 is  the function above that,
   and so forth.

   Warning categories must be subclasses of :c:data:`PyExc_Warning`;
   :c:data:`PyExc_Warning` is a subclass of :c:data:`PyExc_Exception`;
   the default warning category is :c:data:`PyExc_RuntimeWarning`. The standard
   Python warning categories are available as global variables whose names are
   enumerated at :ref:`standardwarningcategories`.

   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.


.. c:function:: int PyErr_WarnExplicitObject(PyObject *category, PyObject *message, PyObject *filename, int lineno, PyObject *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.

   .. versionadded:: 3.4


.. c:function:: int PyErr_WarnExplicit(PyObject *category, const char *message, const char *filename, int lineno, const char *module, PyObject *registry)

   Similar to :c:func:`PyErr_WarnExplicitObject` except that *message* and
   *module* are UTF-8 encoded strings, and *filename* is decoded from the
   :term:`filesystem encoding and error handler`.


.. c:function:: int PyErr_WarnFormat(PyObject *category, Py_ssize_t stack_level, const char *format, ...)

   Function similar to :c:func:`PyErr_WarnEx`, but use
   :c:func:`PyUnicode_FromFormat` to format the warning message.  *format* is
   an ASCII-encoded string.

   .. versionadded:: 3.2


.. c:function:: int PyErr_ResourceWarning(PyObject *source, Py_ssize_t stack_level, const char *format, ...)

   Function similar to :c:func:`PyErr_WarnFormat`, but *category* is
   :exc:`ResourceWarning` and it passes *source* to :func:`warnings.WarningMessage`.

   .. versionadded:: 3.6


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 ``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.

   The caller must hold the GIL.

   .. 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:: PyObject* PyErr_GetHandledException(void)

   Retrieve the active exception instance, as would be returned by :func:`sys.exception`.
   This refers to an exception that was *already caught*, not to an exception that was
   freshly raised. Returns a new reference to the exception or ``NULL``.
   Does not modify the interpreter's exception 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_SetHandledException` to restore or
      clear the exception state.

   .. versionadded:: 3.11

.. c:function:: void PyErr_SetHandledException(PyObject *exc)

   Set the active exception, as known from ``sys.exception()``.  This refers
   to an exception that was *already caught*, not to an exception that was
   freshly raised.
   To clear the exception state, pass ``NULL``.

   .. 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_GetHandledException` to get the exception
      state.

   .. versionadded:: 3.11

.. c:function:: void PyErr_GetExcInfo(PyObject **ptype, PyObject **pvalue, PyObject **ptraceback)

   Retrieve the old-style representation of the exception info, as known from
   :func:`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.  This function is kept for backwards compatibility. Prefer using
   :c:func:`PyErr_GetHandledException`.

   .. 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.
   This function is kept for backwards compatibility. Prefer using
   :c:func:`PyErr_SetHandledException`.

   .. 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

   .. versionchanged:: 3.11
      The ``type`` and ``traceback`` arguments are no longer used and
      can be NULL. The interpreter now derives them from the exception
      instance (the ``value`` argument). The function still steals
      references of all three arguments.


Signal Handling
===============


.. c:function:: int PyErr_CheckSignals()

   .. index::
      pair: module; signal
      single: SIGINT
      single: KeyboardInterrupt (built-in exception)

   This function interacts with Python's signal handling.

   If the function is called from the main thread and under the main Python
   interpreter, 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.

   The function attempts to handle all pending signals, and then returns ``0``.
   However, if a Python signal handler raises an exception, the error
   indicator is set and the function returns ``-1`` immediately (such that
   other pending signals may not have been handled yet: they will be on the
   next :c:func:`PyErr_CheckSignals()` invocation).

   If the function is called from a non-main thread, or under a non-main
   Python interpreter, it does nothing and returns ``0``.

   This function can be called by long-running C code that wants to
   be interruptible by user requests (such as by pressing Ctrl-C).

   .. note::
      The default Python signal handler for :const:`SIGINT` raises the
      :exc:`KeyboardInterrupt` exception.


.. c:function:: void PyErr_SetInterrupt()

   .. index::
      pair: module; signal
      single: SIGINT
      single: KeyboardInterrupt (built-in exception)

   Simulate the effect of a :const:`SIGINT` signal arriving.
   This is equivalent to ``PyErr_SetInterruptEx(SIGINT)``.

   .. note::
      This function is async-signal-safe.  It can be called without
      the :term:`GIL` and from a C signal handler.


.. c:function:: int PyErr_SetInterruptEx(int signum)

   .. index::
      pair: module; signal
      single: KeyboardInterrupt (built-in exception)

   Simulate the effect of a signal arriving. The next time
   :c:func:`PyErr_CheckSignals` is called,  the Python signal handler for
   the given signal number will be called.

   This function can be called by C code that sets up its own signal handling
   and wants Python signal handlers to be invoked as expected when an
   interruption is requested (for example when the user presses Ctrl-C
   to interrupt an operation).

   If the given signal isn't handled by Python (it was set to
   :data:`signal.SIG_DFL` or :data:`signal.SIG_IGN`), it will be ignored.

   If *signum* is outside of the allowed range of signal numbers, ``-1``
   is returned.  Otherwise, ``0`` is returned.  The error indicator is
   never changed by this function.

   .. note::
      This function is async-signal-safe.  It can be called without
      the :term:`GIL` and from a C signal handler.

   .. versionadded:: 3.10


.. c:function:: int PySignal_SetWakeupFd(int fd)

   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)

   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 :exc:`Exception` (accessible in C as
   :c:data:`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.


.. c:function:: PyObject* PyErr_NewExceptionWithDoc(const char *name, const char *doc, PyObject *base, PyObject *dict)

   Same as :c:func:`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.

   .. versionadded:: 3.2


Exception Objects
=================

.. c:function:: PyObject* PyException_GetTraceback(PyObject *ex)

   Return the traceback associated with the exception as a new reference, as
   accessible from Python through :attr:`__traceback__`.  If there is no
   traceback associated, this returns ``NULL``.


.. c:function:: int PyException_SetTraceback(PyObject *ex, PyObject *tb)

   Set the traceback associated with the exception to *tb*.  Use ``Py_None`` to
   clear it.


.. c:function:: PyObject* PyException_GetContext(PyObject *ex)

   Return the context (another exception instance during whose handling *ex* was
   raised) associated with the exception as a new reference, as accessible from
   Python through :attr:`__context__`.  If there is no context associated, this
   returns ``NULL``.


.. c:function:: void PyException_SetContext(PyObject *ex, PyObject *ctx)

   Set the context associated with the exception to *ctx*.  Use ``NULL`` to clear
   it.  There is no type check to make sure that *ctx* is an exception instance.
   This steals a reference to *ctx*.


.. c:function:: PyObject* PyException_GetCause(PyObject *ex)

   Return the cause (either an exception instance, or :const:`None`,
   set by ``raise ... from ...``) associated with the exception as a new
   reference, as accessible from Python through :attr:`__cause__`.


.. c:function:: void PyException_SetCause(PyObject *ex, PyObject *cause)

   Set the cause associated with the exception to *cause*.  Use ``NULL`` to clear
   it.  There is no type check to make sure that *cause* is either an exception
   instance or :const:`None`.  This steals a reference to *cause*.

   :attr:`__suppress_context__` is implicitly set to ``True`` by this function.


.. _unicodeexceptions:

Unicode Exception Objects
=========================

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

.. c:function:: 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 :class:`UnicodeDecodeError` object with the attributes *encoding*,
   *object*, *length*, *start*, *end* and *reason*. *encoding* and *reason* are
   UTF-8 encoded strings.

.. c:function:: PyObject* PyUnicodeDecodeError_GetEncoding(PyObject *exc)
                PyObject* PyUnicodeEncodeError_GetEncoding(PyObject *exc)

   Return the *encoding* attribute of the given exception object.

.. c:function:: PyObject* PyUnicodeDecodeError_GetObject(PyObject *exc)
                PyObject* PyUnicodeEncodeError_GetObject(PyObject *exc)
                PyObject* PyUnicodeTranslateError_GetObject(PyObject *exc)

   Return the *object* attribute of the given exception object.

.. c:function:: 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.

.. c:function:: 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.

.. c:function:: 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.

.. c:function:: 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.

.. c:function:: PyObject* PyUnicodeDecodeError_GetReason(PyObject *exc)
                PyObject* PyUnicodeEncodeError_GetReason(PyObject *exc)
                PyObject* PyUnicodeTranslateError_GetReason(PyObject *exc)

   Return the *reason* attribute of the given exception object.

.. c:function:: 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:

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).
They are also not needed for *tp_call* implementations
because the :ref:`call protocol <call>` takes care of recursion handling.

.. c:function:: int Py_EnterRecursiveCall(const char *where)

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

   If :const:`USE_STACKCHECK` is defined, this function checks if the OS
   stack overflowed using :c:func:`PyOS_CheckStack`.  In this is the case, it
   sets a :exc:`MemoryError` and returns a nonzero value.

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

   *where* should be a UTF-8 encoded string such as ``" in instance check"`` to
   be concatenated to the :exc:`RecursionError` message caused by the recursion
   depth limit.

   .. versionchanged:: 3.9
      This function is now also available in the limited API.

.. c:function:: void Py_LeaveRecursiveCall(void)

   Ends a :c:func:`Py_EnterRecursiveCall`.  Must be called once for each
   *successful* invocation of :c:func:`Py_EnterRecursiveCall`.

   .. versionchanged:: 3.9
      This function is now also available in the limited API.

Properly implementing :c:member:`~PyTypeObject.tp_repr` for container types requires
special recursion handling.  In addition to protecting the stack,
:c:member:`~PyTypeObject.tp_repr` also needs to track objects to prevent cycles.  The
following two functions facilitate this functionality.  Effectively,
these are the C equivalent to :func:`reprlib.recursive_repr`.

.. c:function:: int Py_ReprEnter(PyObject *object)

   Called at the beginning of the :c:member:`~PyTypeObject.tp_repr` implementation to
   detect cycles.

   If the object has already been processed, the function returns a
   positive integer.  In that case the :c:member:`~PyTypeObject.tp_repr` implementation
   should return a string object indicating a cycle.  As examples,
   :class:`dict` objects return ``{...}`` and :class:`list` objects
   return ``[...]``.

   The function will return a negative integer if the recursion limit
   is reached.  In that case the :c:member:`~PyTypeObject.tp_repr` implementation should
   typically return ``NULL``.

   Otherwise, the function returns zero and the :c:member:`~PyTypeObject.tp_repr`
   implementation can continue normally.

.. c:function:: void Py_ReprLeave(PyObject *object)

   Ends a :c:func:`Py_ReprEnter`.  Must be called once for each
   invocation of :c:func:`Py_ReprEnter` that returns zero.


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

.. index::
   single: PyExc_BaseException
   single: PyExc_Exception
   single: PyExc_ArithmeticError
   single: PyExc_AssertionError
   single: PyExc_AttributeError
   single: PyExc_BlockingIOError
   single: PyExc_BrokenPipeError
   single: PyExc_BufferError
   single: PyExc_ChildProcessError
   single: PyExc_ConnectionAbortedError
   single: PyExc_ConnectionError
   single: PyExc_ConnectionRefusedError
   single: PyExc_ConnectionResetError
   single: PyExc_EOFError
   single: PyExc_FileExistsError
   single: PyExc_FileNotFoundError
   single: PyExc_FloatingPointError
   single: PyExc_GeneratorExit
   single: PyExc_ImportError
   single: PyExc_IndentationError
   single: PyExc_IndexError
   single: PyExc_InterruptedError
   single: PyExc_IsADirectoryError
   single: PyExc_KeyError
   single: PyExc_KeyboardInterrupt
   single: PyExc_LookupError
   single: PyExc_MemoryError
   single: PyExc_ModuleNotFoundError
   single: PyExc_NameError
   single: PyExc_NotADirectoryError
   single: PyExc_NotImplementedError
   single: PyExc_OSError
   single: PyExc_OverflowError
   single: PyExc_PermissionError
   single: PyExc_ProcessLookupError
   single: PyExc_RecursionError
   single: PyExc_ReferenceError
   single: PyExc_RuntimeError
   single: PyExc_StopAsyncIteration
   single: PyExc_StopIteration
   single: PyExc_SyntaxError
   single: PyExc_SystemError
   single: PyExc_SystemExit
   single: PyExc_TabError
   single: PyExc_TimeoutError
   single: PyExc_TypeError
   single: PyExc_UnboundLocalError
   single: PyExc_UnicodeDecodeError
   single: PyExc_UnicodeEncodeError
   single: PyExc_UnicodeError
   single: PyExc_UnicodeTranslateError
   single: PyExc_ValueError
   single: PyExc_ZeroDivisionError

+-----------------------------------------+---------------------------------+----------+
| C Name                                  | Python Name                     | Notes    |
+=========================================+=================================+==========+
| :c:data:`PyExc_BaseException`           | :exc:`BaseException`            | [1]_     |
+-----------------------------------------+---------------------------------+----------+
| :c:data:`PyExc_Exception`               | :exc:`Exception`                | [1]_     |
+-----------------------------------------+---------------------------------+----------+
| :c:data:`PyExc_ArithmeticError`         | :exc:`ArithmeticError`          | [1]_     |
+-----------------------------------------+---------------------------------+----------+
| :c:data:`PyExc_AssertionError`          | :exc:`AssertionError`           |          |
+-----------------------------------------+---------------------------------+----------+
| :c:data:`PyExc_AttributeError`          | :exc:`AttributeError`           |          |
+-----------------------------------------+---------------------------------+----------+
| :c:data:`PyExc_BlockingIOError`         | :exc:`BlockingIOError`          |          |
+-----------------------------------------+---------------------------------+----------+
| :c:data:`PyExc_BrokenPipeError`         | :exc:`BrokenPipeError`          |          |
+-----------------------------------------+---------------------------------+----------+
| :c:data:`PyExc_BufferError`             | :exc:`BufferError`              |          |
+-----------------------------------------+---------------------------------+----------+
| :c:data:`PyExc_ChildProcessError`       | :exc:`ChildProcessError`        |          |
+-----------------------------------------+---------------------------------+----------+
| :c:data:`PyExc_ConnectionAbortedError`  | :exc:`ConnectionAbortedError`   |          |
+-----------------------------------------+---------------------------------+----------+
| :c:data:`PyExc_ConnectionError`         | :exc:`ConnectionError`          |          |
+-----------------------------------------+---------------------------------+----------+
| :c:data:`PyExc_ConnectionRefusedError`  | :exc:`ConnectionRefusedError`   |          |
+-----------------------------------------+---------------------------------+----------+
| :c:data:`PyExc_ConnectionResetError`    | :exc:`ConnectionResetError`     |          |
+-----------------------------------------+---------------------------------+----------+
| :c:data:`PyExc_EOFError`                | :exc:`EOFError`                 |          |
+-----------------------------------------+---------------------------------+----------+
| :c:data:`PyExc_FileExistsError`         | :exc:`FileExistsError`          |          |
+-----------------------------------------+---------------------------------+----------+
| :c:data:`PyExc_FileNotFoundError`       | :exc:`FileNotFoundError`        |          |
+-----------------------------------------+---------------------------------+----------+
| :c:data:`PyExc_FloatingPointError`      | :exc:`FloatingPointError`       |          |
+-----------------------------------------+---------------------------------+----------+
| :c:data:`PyExc_GeneratorExit`           | :exc:`GeneratorExit`            |          |
+-----------------------------------------+---------------------------------+----------+
| :c:data:`PyExc_ImportError`             | :exc:`ImportError`              |          |
+-----------------------------------------+---------------------------------+----------+
| :c:data:`PyExc_IndentationError`        | :exc:`IndentationError`         |          |
+-----------------------------------------+---------------------------------+----------+
| :c:data:`PyExc_IndexError`              | :exc:`IndexError`               |          |
+-----------------------------------------+---------------------------------+----------+
| :c:data:`PyExc_InterruptedError`        | :exc:`InterruptedError`         |          |
+-----------------------------------------+---------------------------------+----------+
| :c:data:`PyExc_IsADirectoryError`       | :exc:`IsADirectoryError`        |          |
+-----------------------------------------+---------------------------------+----------+
| :c:data:`PyExc_KeyError`                | :exc:`KeyError`                 |          |
+-----------------------------------------+---------------------------------+----------+
| :c:data:`PyExc_KeyboardInterrupt`       | :exc:`KeyboardInterrupt`        |          |
+-----------------------------------------+---------------------------------+----------+
| :c:data:`PyExc_LookupError`             | :exc:`LookupError`              | [1]_     |
+-----------------------------------------+---------------------------------+----------+
| :c:data:`PyExc_MemoryError`             | :exc:`MemoryError`              |          |
+-----------------------------------------+---------------------------------+----------+
| :c:data:`PyExc_ModuleNotFoundError`     | :exc:`ModuleNotFoundError`      |          |
+-----------------------------------------+---------------------------------+----------+
| :c:data:`PyExc_NameError`               | :exc:`NameError`                |          |
+-----------------------------------------+---------------------------------+----------+
| :c:data:`PyExc_NotADirectoryError`      | :exc:`NotADirectoryError`       |          |
+-----------------------------------------+---------------------------------+----------+
| :c:data:`PyExc_NotImplementedError`     | :exc:`NotImplementedError`      |          |
+-----------------------------------------+---------------------------------+----------+
| :c:data:`PyExc_OSError`                 | :exc:`OSError`                  | [1]_     |
+-----------------------------------------+---------------------------------+----------+
| :c:data:`PyExc_OverflowError`           | :exc:`OverflowError`            |          |
+-----------------------------------------+---------------------------------+----------+
| :c:data:`PyExc_PermissionError`         | :exc:`PermissionError`          |          |
+-----------------------------------------+---------------------------------+----------+
| :c:data:`PyExc_ProcessLookupError`      | :exc:`ProcessLookupError`       |          |
+-----------------------------------------+---------------------------------+----------+
| :c:data:`PyExc_RecursionError`          | :exc:`RecursionError`           |          |
+-----------------------------------------+---------------------------------+----------+
| :c:data:`PyExc_ReferenceError`          | :exc:`ReferenceError`           |          |
+-----------------------------------------+---------------------------------+----------+
| :c:data:`PyExc_RuntimeError`            | :exc:`RuntimeError`             |          |
+-----------------------------------------+---------------------------------+----------+
| :c:data:`PyExc_StopAsyncIteration`      | :exc:`StopAsyncIteration`       |          |
+-----------------------------------------+---------------------------------+----------+
| :c:data:`PyExc_StopIteration`           | :exc:`StopIteration`            |          |
+-----------------------------------------+---------------------------------+----------+
| :c:data:`PyExc_SyntaxError`             | :exc:`SyntaxError`              |          |
+-----------------------------------------+---------------------------------+----------+
| :c:data:`PyExc_SystemError`             | :exc:`SystemError`              |          |
+-----------------------------------------+---------------------------------+----------+
| :c:data:`PyExc_SystemExit`              | :exc:`SystemExit`               |          |
+-----------------------------------------+---------------------------------+----------+
| :c:data:`PyExc_TabError`                | :exc:`TabError`                 |          |
+-----------------------------------------+---------------------------------+----------+
| :c:data:`PyExc_TimeoutError`            | :exc:`TimeoutError`             |          |
+-----------------------------------------+---------------------------------+----------+
| :c:data:`PyExc_TypeError`               | :exc:`TypeError`                |          |
+-----------------------------------------+---------------------------------+----------+
| :c:data:`PyExc_UnboundLocalError`       | :exc:`UnboundLocalError`        |          |
+-----------------------------------------+---------------------------------+----------+
| :c:data:`PyExc_UnicodeDecodeError`      | :exc:`UnicodeDecodeError`       |          |
+-----------------------------------------+---------------------------------+----------+
| :c:data:`PyExc_UnicodeEncodeError`      | :exc:`UnicodeEncodeError`       |          |
+-----------------------------------------+---------------------------------+----------+
| :c:data:`PyExc_UnicodeError`            | :exc:`UnicodeError`             |          |
+-----------------------------------------+---------------------------------+----------+
| :c:data:`PyExc_UnicodeTranslateError`   | :exc:`UnicodeTranslateError`    |          |
+-----------------------------------------+---------------------------------+----------+
| :c:data:`PyExc_ValueError`              | :exc:`ValueError`               |          |
+-----------------------------------------+---------------------------------+----------+
| :c:data:`PyExc_ZeroDivisionError`       | :exc:`ZeroDivisionError`        |          |
+-----------------------------------------+---------------------------------+----------+

.. versionadded:: 3.3
   :c:data:`PyExc_BlockingIOError`, :c:data:`PyExc_BrokenPipeError`,
   :c:data:`PyExc_ChildProcessError`, :c:data:`PyExc_ConnectionError`,
   :c:data:`PyExc_ConnectionAbortedError`, :c:data:`PyExc_ConnectionRefusedError`,
   :c:data:`PyExc_ConnectionResetError`, :c:data:`PyExc_FileExistsError`,
   :c:data:`PyExc_FileNotFoundError`, :c:data:`PyExc_InterruptedError`,
   :c:data:`PyExc_IsADirectoryError`, :c:data:`PyExc_NotADirectoryError`,
   :c:data:`PyExc_PermissionError`, :c:data:`PyExc_ProcessLookupError`
   and :c:data:`PyExc_TimeoutError` were introduced following :pep:`3151`.

.. versionadded:: 3.5
   :c:data:`PyExc_StopAsyncIteration` and :c:data:`PyExc_RecursionError`.

.. versionadded:: 3.6
   :c:data:`PyExc_ModuleNotFoundError`.

These are compatibility aliases to :c:data:`PyExc_OSError`:

.. index::
   single: PyExc_EnvironmentError
   single: PyExc_IOError
   single: PyExc_WindowsError

+-------------------------------------+----------+
| C Name                              | Notes    |
+=====================================+==========+
| :c:data:`PyExc_EnvironmentError`    |          |
+-------------------------------------+----------+
| :c:data:`PyExc_IOError`             |          |
+-------------------------------------+----------+
| :c:data:`PyExc_WindowsError`        | [2]_     |
+-------------------------------------+----------+

.. versionchanged:: 3.3
   These aliases used to be separate exception types.

Notes:

.. [1]
   This is a base class for other standard exceptions.

.. [2]
   Only defined on Windows; protect code that uses this by testing that the
   preprocessor macro ``MS_WINDOWS`` is defined.

.. _standardwarningcategories:

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

.. index::
   single: PyExc_Warning
   single: PyExc_BytesWarning
   single: PyExc_DeprecationWarning
   single: PyExc_FutureWarning
   single: PyExc_ImportWarning
   single: PyExc_PendingDeprecationWarning
   single: PyExc_ResourceWarning
   single: PyExc_RuntimeWarning
   single: PyExc_SyntaxWarning
   single: PyExc_UnicodeWarning
   single: PyExc_UserWarning

+------------------------------------------+---------------------------------+----------+
| C Name                                   | Python Name                     | Notes    |
+==========================================+=================================+==========+
| :c:data:`PyExc_Warning`                  | :exc:`Warning`                  | [3]_     |
+------------------------------------------+---------------------------------+----------+
| :c:data:`PyExc_BytesWarning`             | :exc:`BytesWarning`             |          |
+------------------------------------------+---------------------------------+----------+
| :c:data:`PyExc_DeprecationWarning`       | :exc:`DeprecationWarning`       |          |
+------------------------------------------+---------------------------------+----------+
| :c:data:`PyExc_FutureWarning`            | :exc:`FutureWarning`            |          |
+------------------------------------------+---------------------------------+----------+
| :c:data:`PyExc_ImportWarning`            | :exc:`ImportWarning`            |          |
+------------------------------------------+---------------------------------+----------+
| :c:data:`PyExc_PendingDeprecationWarning`| :exc:`PendingDeprecationWarning`|          |
+------------------------------------------+---------------------------------+----------+
| :c:data:`PyExc_ResourceWarning`          | :exc:`ResourceWarning`          |          |
+------------------------------------------+---------------------------------+----------+
| :c:data:`PyExc_RuntimeWarning`           | :exc:`RuntimeWarning`           |          |
+------------------------------------------+---------------------------------+----------+
| :c:data:`PyExc_SyntaxWarning`            | :exc:`SyntaxWarning`            |          |
+------------------------------------------+---------------------------------+----------+
| :c:data:`PyExc_UnicodeWarning`           | :exc:`UnicodeWarning`           |          |
+------------------------------------------+---------------------------------+----------+
| :c:data:`PyExc_UserWarning`              | :exc:`UserWarning`              |          |
+------------------------------------------+---------------------------------+----------+

.. versionadded:: 3.2
   :c:data:`PyExc_ResourceWarning`.

Notes:

.. [3]
   This is a base class for other standard warning categories.