# 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 `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 `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

void PyErr_Clear()
  Part of the Stable ABI.

  Clear the error indicator. If the error indicator is not set, there is no effect.

void PyErr_PrintEx(int set_sys_last_vars)
  Part of the Stable ABI.

  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 variable `sys.last_exc` is set to the printed exception. For backwards compatibility, the deprecated variables `sys.last_type`, `sys.last_value` and `sys.last_traceback` are also set to the type, value and traceback of this exception, respectively.

  Changed in version 3.12: The setting of `sys.last_exc` was added.

void PyErr_Print()
  Part of the Stable ABI.

  Alias for `PyErr_PrintEx(1)`.

void PyErr_WriteUnraisable(PyObject *obj)
  Part of the Stable ABI.

  Call `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 `__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. If obj is `NULL`, only the traceback is printed.

  An exception must be set when calling this function.

  Changed in version 3.4: Print a traceback. Print only traceback if obj is `NULL`.

  Changed in version 3.8: Use `sys.unraisablehook()`.

void PyErr_DisplayException(PyObject *exc)
  Part of the Stable ABI since version 3.12.

  Print the standard traceback display of `exc` to `sys.stderr`, including chained exceptions and notes.

  New in version 3.12.

## 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.

void PyErr_SetString(PyObject *type, const char *message)
  Part of the Stable ABI.

  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 create a new strong reference to it (e.g. with `Py_INCREF()`). The second argument is an error message; it is decoded from `'utf-8'`.

void PyErr_SetObject(PyObject *type, PyObject *value)
  Part of the Stable ABI.

  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. Part of the Stable ABI.

  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 `PyUnicode_FromFormat()`. format is an ASCII-encoded string.

PyObject *PyErr_FormatV(PyObject *exception, const char *format, va_list vargs)
  Return value: Always NULL. Part of the Stable ABI since version 3.5.

  Same as `PyErr_Format()`, but taking a `va_list` argument rather than a variable number of arguments.

  New in version 3.5.

void PyErr_SetNone(PyObject *type)
  Part of the Stable ABI.

  This is a shorthand for `PyErr_SetObject(type, Py_None)`.

int PyErr_BadArgument()
  Part of the Stable ABI.

  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. Part of the Stable ABI.

  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. Part of the Stable ABI.

  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)
  Return value: Always NULL. Part of the Stable ABI.

  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 `OSError` exception, this is used to define the `filename` attribute of the exception instance.

PyObject *PyErr_SetFromErrnoWithFilenameObjects(PyObject *type, PyObject *filenameObject, PyObject *filenameObject2)
  Return value: Always NULL. Part of the Stable ABI since version 3.7.

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

  New in version 3.4.

PyObject *PyErr_SetFromErrnoWithFilename(PyObject *type, const char *filename)
  Return value: Always NULL. Part of the Stable ABI.

  Similar to `PyErr_SetFromErrnoWithFilenameObject()`, but the filename is given as a C string. filename is decoded from the filesystem encoding and error handler.

PyObject *PyErr_SetFromWindowsErr(int ierr)
  Return value: Always NULL. Part of the Stable ABI on Windows since version 3.7.

  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. Part of the Stable ABI on Windows since version 3.7.

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

  Availability: Windows.

PyObject *PyErr_SetFromWindowsErrWithFilename(int ierr, const char *filename)
  Return value: Always NULL. Part of the Stable ABI on Windows since version 3.7.

  Similar to `PyErr_SetFromWindowsErr()`, with the additional behavior that if filename is not `NULL`, it is decoded from the filesystem encoding (`os.fsdecode()`) and passed to the constructor of `OSError` as a third parameter to be used to define the `filename` attribute of the exception instance.

  Availability: Windows.

PyObject *PyErr_SetExcFromWindowsErrWithFilenameObject(PyObject *type, int ierr, PyObject *filename)
  Return value: Always NULL. Part of the Stable ABI on Windows since version 3.7.

  Similar to `PyErr_SetExcFromWindowsErr()`, with the additional behavior that if filename is not `NULL`, it is passed to the constructor of `OSError` as a third parameter to be used to define the `filename` attribute of the exception instance.

  Availability: Windows.

PyObject *PyErr_SetExcFromWindowsErrWithFilenameObjects(PyObject *type, int ierr, PyObject *filename, PyObject *filename2)
  Return value: Always NULL. Part of the Stable ABI on Windows since version 3.7.

  Similar to `PyErr_SetExcFromWindowsErrWithFilenameObject()`, but accepts a second filename object.

  Availability: Windows.

  New in version 3.4.

PyObject *PyErr_SetExcFromWindowsErrWithFilename(PyObject *type, int ierr, const char *filename)
  Return value: Always NULL. Part of the Stable ABI on Windows since version 3.7.

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

  Availability: Windows.

PyObject *PyErr_SetImportError(PyObject *msg, PyObject *name, PyObject *path)
  Return value: Always NULL. Part of the Stable ABI since version 3.7.

  This is a convenience function to raise `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 `ImportError`’s respective `name` and `path` attributes.

  New in version 3.3.

PyObject *PyErr_SetImportErrorSubclass(PyObject *exception, PyObject *msg, PyObject *name, PyObject *path)
  Return value: Always NULL. Part of the Stable ABI since version 3.6.

  Much like `PyErr_SetImportError()` but this function allows for specifying a subclass of `ImportError` to raise.

  New in version 3.6.

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 `SyntaxError`, then it sets additional attributes, which make the exception printing subsystem think the exception is a `SyntaxError`.

  New in version 3.4.

void PyErr_SyntaxLocationEx(const char *filename, int lineno, int col_offset)
  Part of the Stable ABI since version 3.7.

  Like `PyErr_SyntaxLocationObject()`, but filename is a byte string decoded from the filesystem encoding and error handler.

  New in version 3.2.

void PyErr_SyntaxLocation(const char *filename, int lineno)
  Part of the Stable ABI.

  Like `PyErr_SyntaxLocationEx()`, but the col_offset parameter is omitted.

void PyErr_BadInternalCall()
  Part of the Stable ABI.

  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 `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, `Py_DECREF()` owned references and return an error value).

int PyErr_WarnEx(PyObject *category, const char *message, Py_ssize_t stack_level)
  Part of the Stable ABI.

  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 `PyErr_WarnEx()`, 2 is the function above that, and so forth.

  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_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 `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.

  New in version 3.4.

int PyErr_WarnExplicit(PyObject *category, const char *message, const char *filename, int lineno, const char *module, PyObject *registry)
  Part of the Stable ABI.

  Similar to `PyErr_WarnExplicitObject()` except that message and module are UTF-8 encoded strings, and filename is decoded from the filesystem encoding and error handler.

int PyErr_WarnFormat(PyObject *category, Py_ssize_t stack_level, const char *format, ...)
  Part of the Stable ABI.

  Function similar to `PyErr_WarnEx()`, but use `PyUnicode_FromFormat()` to format the warning message. format is an ASCII-encoded string.

  New in version 3.2.

int PyErr_ResourceWarning(PyObject *source, Py_ssize_t stack_level, const char *format, ...)
  Part of the Stable ABI since version 3.6.

  Function similar to `PyErr_WarnFormat()`, but category is `ResourceWarning` and it passes source to `warnings.WarningMessage`.

  New in version 3.6.

## Querying the error indicator

PyObject *PyErr_Occurred()
  Return value: Borrowed reference. Part of the Stable ABI.

  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.

  The caller must hold the GIL.

  Note

  Do not compare the return value to a specific exception; use `PyErr_ExceptionMatches()` instead, shown below. (The comparison could easily fail since the exception may be an instance instead of a class, in the case of a class exception, or it may be a subclass of the expected exception.)

int PyErr_ExceptionMatches(PyObject *exc)
  Part of the Stable ABI.

  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)
  Part of the Stable ABI.

  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.

PyObject *PyErr_GetRaisedException(void)
  Return value: New reference. Part of the Stable ABI since version 3.12.

  Return the exception currently being raised, clearing the error indicator at the same time. Return `NULL` if the error indicator is not set.

  This function is used by code that needs to catch exceptions, or code that needs to save and restore the error indicator temporarily.

  For example:

      {
         PyObject *exc = PyErr_GetRaisedException();

         /* ... code that might produce other errors ... */

         PyErr_SetRaisedException(exc);
      }

  See also

  `PyErr_GetHandledException()`, to save the exception currently being handled.

  New in version 3.12.

void PyErr_SetRaisedException(PyObject *exc)
  Part of the Stable ABI since version 3.12.

  Set exc as the exception currently being raised, clearing the existing exception if one is set.

  Warning

  This call steals a reference to exc, which must be a valid exception.

  New in version 3.12.

void PyErr_Fetch(PyObject **ptype, PyObject **pvalue, PyObject **ptraceback)
  Part of the Stable ABI.

  Deprecated since version 3.12: Use `PyErr_GetRaisedException()` instead.

  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 legacy code that needs to catch exceptions or save and restore the error indicator temporarily.

  For example:

      {
         PyObject *type, *value, *traceback;
         PyErr_Fetch(&type, &value, &traceback);

         /* ... code that might produce other errors ... */

         PyErr_Restore(type, value, traceback);
      }

void PyErr_Restore(PyObject *type, PyObject *value, PyObject *traceback)
  Part of the Stable ABI.

  Deprecated since version 3.12: Use `PyErr_SetRaisedException()` instead.

  Set the error indicator from the three objects, type, value, and traceback, clearing the existing exception if one is set. 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 legacy code that needs to save and restore the error indicator temporarily. Use `PyErr_Fetch()` to save the current error indicator.

void PyErr_NormalizeException(PyObject **exc, PyObject **val, PyObject **tb)
  Part of the Stable ABI.

  Deprecated since version 3.12: Use `PyErr_GetRaisedException()` instead, to avoid any possible de-normalization.

  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.

  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);
      }

PyObject *PyErr_GetHandledException(void)
  Part of the Stable ABI since version 3.11.

  Retrieve the active exception instance, as would be returned by `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 `PyErr_SetHandledException()` to restore or clear the exception state.

  New in version 3.11.

void PyErr_SetHandledException(PyObject *exc)
  Part of the Stable ABI since version 3.11.

  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 `PyErr_GetHandledException()` to get the exception state.

  New in version 3.11.

void PyErr_GetExcInfo(PyObject **ptype, PyObject **pvalue, PyObject **ptraceback)
  Part of the Stable ABI since version 3.7.

  Retrieve the old-style representation of 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. This function is kept for backwards compatibility. Prefer using `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 `PyErr_SetExcInfo()` to restore or clear the exception state.

  New in version 3.3.

void PyErr_SetExcInfo(PyObject *type, PyObject *value, PyObject *traceback)
  Part of the Stable ABI since version 3.7.

  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 `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 `PyErr_GetExcInfo()` to read the exception state.

  New in version 3.3.

  Changed in version 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

int PyErr_CheckSignals()
  Part of the Stable ABI.

  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 `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 `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 `SIGINT` raises the `KeyboardInterrupt` exception.

void PyErr_SetInterrupt()
  Part of the Stable ABI.

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

  Note

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

int PyErr_SetInterruptEx(int signum)
  Part of the Stable ABI since version 3.10.

  Simulate the effect of a signal arriving. The next time `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 `signal.SIG_DFL` or `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 GIL and from a C signal handler.

  New in version 3.10.

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 `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.

  Changed in version 3.5: On Windows, the function now also supports socket handles.

## Exception Classes

PyObject *PyErr_NewException(const char *name, PyObject *base, PyObject *dict)
  Return value: New reference. Part of the Stable ABI.

  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(const char *name, const char *doc, PyObject *base, PyObject *dict)
  Return value: New reference. Part of the Stable ABI.

  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 3.2.

## Exception Objects

PyObject *PyException_GetTraceback(PyObject *ex)
  Return value: New reference. Part of the Stable ABI.

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

int PyException_SetTraceback(PyObject *ex, PyObject *tb)
  Part of the Stable ABI.

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

PyObject *PyException_GetContext(PyObject *ex)
  Return value: New reference. Part of the Stable ABI.

  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 the `__context__` attribute. If there is no context associated, this returns `NULL`.

void PyException_SetContext(PyObject *ex, PyObject *ctx)
  Part of the Stable ABI.

  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.

PyObject *PyException_GetCause(PyObject *ex)
  Return value: New reference. Part of the Stable ABI.

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

void PyException_SetCause(PyObject *ex, PyObject *cause)
  Part of the Stable ABI.

  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 `None`. This steals a reference to cause.

  The `__suppress_context__` attribute is implicitly set to `True` by this function.

PyObject *PyException_GetArgs(PyObject *ex)
  Return value: New reference. Part of the Stable ABI since version 3.12.

  Return `args` of exception ex.

void PyException_SetArgs(PyObject *ex, PyObject *args)
  Part of the Stable ABI since version 3.12.

  Set `args` of exception ex to args.

PyObject *PyUnstable_Exc_PrepReraiseStar(PyObject *orig, PyObject *excs)
  This is Unstable API. It may change without warning in minor releases.

  Implement part of the interpreter’s implementation of `except*`. orig is the original exception that was caught, and excs is the list of the exceptions that need to be raised. This list contains the unhandled part of orig, if any, as well as the exceptions that were raised from the `except*` clauses (so they have a different traceback from orig) and those that were reraised (and have the same traceback as orig). Return the `ExceptionGroup` that needs to be reraised in the end, or `None` if there is nothing to reraise.

  New in version 3.12.

## 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)
  Return value: New reference. Part of the Stable ABI.

  Create a `UnicodeDecodeError` object with the attributes encoding, object, length, start, end and reason. encoding and reason are UTF-8 encoded strings.

PyObject *PyUnicodeDecodeError_GetEncoding(PyObject *exc)
PyObject *PyUnicodeEncodeError_GetEncoding(PyObject *exc)
  Return value: New reference. Part of the Stable ABI.

  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 value: New reference. Part of the Stable ABI.

  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)
  Part of the Stable ABI.

  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)
  Part of the Stable ABI.

  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)
  Part of the Stable ABI.

  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)
  Part of the Stable ABI.

  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 value: New reference. Part of the Stable ABI.

  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)
  Part of the Stable ABI.

  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). They are also not needed for tp_call implementations because the call protocol takes care of recursion handling.

int Py_EnterRecursiveCall(const char *where)
  Part of the Stable ABI since version 3.9.

  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()`. If 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 `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 `RecursionError` message caused by the recursion depth limit.

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

void Py_LeaveRecursiveCall(void)
  Part of the Stable ABI since version 3.9.

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

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

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

int Py_ReprEnter(PyObject *object)
  Part of the Stable ABI.

  Called at the beginning of the `tp_repr` implementation to detect cycles.

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

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

  Otherwise, the function returns zero and the `tp_repr` implementation can continue normally.

void Py_ReprLeave(PyObject *object)
  Part of the Stable ABI.

  Ends a `Py_ReprEnter()`. Must be called once for each invocation of `Py_ReprEnter()` that returns zero.

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

New in version 3.3: `PyExc_BlockingIOError`, `PyExc_BrokenPipeError`, `PyExc_ChildProcessError`, `PyExc_ConnectionError`, `PyExc_ConnectionAbortedError`, `PyExc_ConnectionRefusedError`, `PyExc_ConnectionResetError`, `PyExc_FileExistsError`, `PyExc_FileNotFoundError`, `PyExc_InterruptedError`, `PyExc_IsADirectoryError`, `PyExc_NotADirectoryError`, `PyExc_PermissionError`, `PyExc_ProcessLookupError` and `PyExc_TimeoutError` were introduced following PEP 3151.

New in version 3.5: `PyExc_StopAsyncIteration` and `PyExc_RecursionError`.

New in version 3.6: `PyExc_ModuleNotFoundError`.

These are compatibility aliases to `PyExc_OSError`:

C Name | Notes
--- | ---
`PyExc_EnvironmentError` |
`PyExc_IOError` |
`PyExc_WindowsError` | [2]

Changed in version 3.3: These aliases used to be separate exception types.

Notes:

[1] (1,2,3,4,5)

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.

## 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 Name | Python Name | Notes
--- | --- | ---
`PyExc_Warning` | `Warning` | [3]
`PyExc_BytesWarning` | `BytesWarning` |
`PyExc_DeprecationWarning` | `DeprecationWarning` |
`PyExc_FutureWarning` | `FutureWarning` |
`PyExc_ImportWarning` | `ImportWarning` |
`PyExc_PendingDeprecationWarning` | `PendingDeprecationWarning` |
`PyExc_ResourceWarning` | `ResourceWarning` |
`PyExc_RuntimeWarning` | `RuntimeWarning` |
`PyExc_SyntaxWarning` | `SyntaxWarning` |
`PyExc_UnicodeWarning` | `UnicodeWarning` |
`PyExc_UserWarning` | `UserWarning` |

New in version 3.2: `PyExc_ResourceWarning`.

Notes:

[3]

This is a base class for other standard warning categories.