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()

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

void PyErr_PrintEx(int set_sys_last_vars)

   Print a standard traceback to "sys.stderr" and clear the error
   indicator. Call this function only when the error indicator is set.
   (Otherwise it will cause a fatal error!)

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

void PyErr_Print()

   Alias for "PyErr_PrintEx(1)".

void PyErr_WriteUnraisable(PyObject *obj)

   This utility function prints a warning message to "sys.stderr" when
   an exception has been set but it is impossible for the interpreter
   to actually raise the exception.  It is used, for example, when an
   exception occurs in an "__del__()" method.

   The function is called with a single argument *obj* that identifies
   the context in which the unraisable exception occurred. If
   possible, the repr of *obj* will be printed in the warning message.


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)

   This is the most common way to set the error indicator.  The first
   argument specifies the exception type; it is normally one of the
   standard exceptions, e.g. "PyExc_RuntimeError".  You need not
   increment its reference count. The second argument is an error
   message; it is decoded from "'utf-8"’.

void PyErr_SetObject(PyObject *type, PyObject *value)

   This function is similar to "PyErr_SetString()" but lets you
   specify an arbitrary Python object for the “value” of the
   exception.

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

   This function sets the error indicator and returns *NULL*.
   *exception* should be a Python exception class.  The *format* and
   subsequent parameters help format the error message; they have the
   same meaning and values as in "PyUnicode_FromFormat()". *format* is
   an ASCII-encoded string.

PyObject* PyErr_FormatV(PyObject *exception, const char *format, va_list vargs)
    *Return value: Always NULL.*

   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)

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

int PyErr_BadArgument()

   This is a shorthand for "PyErr_SetString(PyExc_TypeError,
   message)", where *message* indicates that a built-in operation was
   invoked with an illegal argument.  It is mostly for internal use.

PyObject* PyErr_NoMemory()
    *Return value: Always NULL.*

   This is a shorthand for "PyErr_SetNone(PyExc_MemoryError)"; it
   returns *NULL* so an object allocation function can write "return
   PyErr_NoMemory();" when it runs out of memory.

PyObject* PyErr_SetFromErrno(PyObject *type)
    *Return value: Always NULL.*

   This is a convenience function to raise an exception when a C
   library function has returned an error and set the C variable
   "errno".  It constructs a tuple object whose first item is the
   integer "errno" value and whose second item is the corresponding
   error message (gotten from "strerror()"), and then calls
   "PyErr_SetObject(type, object)".  On Unix, when the "errno" value
   is "EINTR", indicating an interrupted system call, this calls
   "PyErr_CheckSignals()", and if that set the error indicator, leaves
   it set to that.  The function always returns *NULL*, so a wrapper
   function around a system call can write "return
   PyErr_SetFromErrno(type);" when the system call returns an error.

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

   Similar to "PyErr_SetFromErrno()", with the additional behavior
   that if *filenameObject* is not *NULL*, it is passed to the
   constructor of *type* as a third parameter.  In the case of
   "OSError" exception, this is used to define the "filename"
   attribute of the exception instance.

PyObject* PyErr_SetFromErrnoWithFilenameObjects(PyObject *type, PyObject *filenameObject, PyObject *filenameObject2)

   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.*

   Similar to "PyErr_SetFromErrnoWithFilenameObject()", but the
   filename is given as a C string.  *filename* is decoded from the
   filesystem encoding ("os.fsdecode()").

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

   This is a convenience function to raise "WindowsError". If called
   with *ierr* of "0", the error code returned by a call to
   "GetLastError()" is used instead.  It calls the Win32 function
   "FormatMessage()" to retrieve the Windows description of error code
   given by *ierr* or "GetLastError()", then it constructs a tuple
   object whose first item is the *ierr* value and whose second item
   is the corresponding error message (gotten from "FormatMessage()"),
   and then calls "PyErr_SetObject(PyExc_WindowsError, object)". This
   function always returns *NULL*. Availability: Windows.

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

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

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

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

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

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

PyObject* PyErr_SetExcFromWindowsErrWithFilenameObjects(PyObject *type, int ierr, PyObject *filename, PyObject *filename2)

   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.*

   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)

   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.

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)

   Like "PyErr_SyntaxLocationObject()", but *filename* is a byte
   string decoded from the filesystem encoding ("os.fsdecode()").

   New in version 3.2.

void PyErr_SyntaxLocation(const char *filename, int lineno)

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

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 "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)

   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)

   Similar to "PyErr_WarnExplicitObject()" except that *message* and
   *module* are UTF-8 encoded strings, and *filename* is decoded from
   the filesystem encoding ("os.fsdecode()").

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

   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.


Querying the error indicator
============================

PyObject* PyErr_Occurred()
    *Return value: Borrowed reference.*

   Test whether the error indicator is set.  If set, return the
   exception *type* (the first argument to the last call to one of the
   "PyErr_Set*()" functions or to "PyErr_Restore()").  If not set,
   return *NULL*.  You do not own a reference to the return value, so
   you do not need to "Py_DECREF()" it.

   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)

   Equivalent to "PyErr_GivenExceptionMatches(PyErr_Occurred(), exc)".
   This should only be called when an exception is actually set; a
   memory access violation will occur if no exception has been raised.

int PyErr_GivenExceptionMatches(PyObject *given, PyObject *exc)

   Return true if the *given* exception matches the exception 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.

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);
        }

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
     "PyErr_Fetch()" to save the current error indicator.

void PyErr_NormalizeException(PyObject**exc, PyObject**val, PyObject**tb)

   Under certain circumstances, the values returned by "PyErr_Fetch()"
   below can be “unnormalized”, meaning that "*exc" is a class object
   but "*val" is not an instance of the  same class.  This function
   can be used to instantiate the class in that case.  If the values
   are already normalized, nothing happens. The delayed normalization
   is implemented to improve performance.

   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);
        }

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
     "PyErr_SetExcInfo()" to restore or clear the exception state.

   New in version 3.3.

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

   New in version 3.3.


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

int PyErr_CheckSignals()

   This function interacts with Python’s signal handling.  It checks
   whether a signal has been sent to the processes and if so, invokes
   the corresponding signal handler.  If the "signal" module is
   supported, this can invoke a signal handler written in Python.  In
   all cases, the default effect for "SIGINT" is to raise the
   "KeyboardInterrupt" exception.  If an exception is raised the error
   indicator is set and the function returns "-1"; otherwise the
   function returns "0".  The error indicator may or may not be
   cleared if it was previously set.

void PyErr_SetInterrupt()

   This function simulates the effect of a "SIGINT" signal arriving —
   the next time "PyErr_CheckSignals()" is called,
   "KeyboardInterrupt" will be raised.  It may be called without
   holding the interpreter lock.

int PySignal_SetWakeupFd(int fd)

   This utility function specifies a file descriptor to which 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.*

   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.*

   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.*

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

int PyException_SetTraceback(PyObject *ex, PyObject *tb)

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

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 "__context__".  If
   there is no context associated, this returns *NULL*.

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*.

PyObject* PyException_GetCause(PyObject *ex)

   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 "__cause__".

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 "None".  This steals a reference to
   *cause*.

   "__suppress_context__" is implicitly set to "True" by this
   function.


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

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

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

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

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

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

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

   Create a "UnicodeTranslateError" object with the attributes
   *object*, *length*, *start*, *end* and *reason*. *reason* is a
   UTF-8 encoded string.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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


Recursion Control
=================

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

int Py_EnterRecursiveCall(const char *where)

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

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

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

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

void Py_LeaveRecursiveCall()

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

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)

   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)

   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_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"                  | (2)        |
+-------------------------------------------+-----------------------------------+------------+
| "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".

These are compatibility aliases to "PyExc_OSError":

+---------------------------------------+------------+
| C Name                                | Notes      |
+=======================================+============+
| "PyExc_EnvironmentError"              |            |
+---------------------------------------+------------+
| "PyExc_IOError"                       |            |
+---------------------------------------+------------+
| "PyExc_WindowsError"                  | (3)        |
+---------------------------------------+------------+

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

Notes:

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

2. This is the same as "weakref.ReferenceError".

3. Only defined on Windows; protect code that uses this by testing
   that the preprocessor macro "MS_WINDOWS" is defined.


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"                         | (1)        |
+--------------------------------------------+-----------------------------------+------------+
| "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:

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