Python/errors.c (part 3)

Source:

cpython 3.14 @ ab2d84fe1023/Python/errors.c ↗

This annotation covers unraisable exceptions and exception chaining machinery. See python_errors_detail and python_errors2_detail for PyErr_SetString, PyErr_Fetch, and PyErr_NormalizeException.

Map

Lines	Symbol	Role
1-80	_PyErr_WriteUnraisableMsg	Print an unraisable exception to sys.stderr
81-180	PyErr_FormatUnraisable	Raise an unraisable exception with context
181-300	Exception chaining	__cause__, __context__, __suppress_context__
301-450	sys.excepthook / sys.unraisablehook	Per-interpreter hooks for unhandled exceptions
451-600	_PyErr_Display	Format and print a full traceback to stderr

Reading

_PyErr_WriteUnraisableMsg

// CPython: Python/errors.c:82 _PyErr_WriteUnraisableMsg
void
_PyErr_WriteUnraisableMsg(const char *err_msg, PyObject *obj)
{
    /* Called when an exception occurs in a destructor, __del__,
       weakref callback, or finalizer — contexts where it cannot propagate. */
    PyThreadState *tstate = _PyThreadState_GET();
    PyObject *exc = _PyErr_GetTopmostException(tstate)->exc_value;
    if (!PyErr_GivenExceptionMatches(exc, PyExc_Exception)) {
        /* Only Exception subclasses are printed */
        _PyErr_Clear(tstate);
        return;
    }
    /* Call sys.unraisablehook(UnraisableHookArgs) */
    ...
}

PyErr_FormatUnraisable

// CPython: Python/errors.c:130 PyErr_FormatUnraisable
void
PyErr_FormatUnraisable(const char *format, ...)
{
    /* 1. Build the context message from format string. */
    /* 2. Call _PyErr_WriteUnraisableMsg with the current exception. */
    /* After the call, the exception is cleared. */
}

Used in __del__ finalizers, weak reference callbacks, and thread cleanup.

Exception chaining

// CPython: Python/errors.c:220 _PyException_ChainExceptions
void
_PyException_ChainExceptions(PyObject *typ, PyObject *val, PyObject *tb)
{
    /* Implicit chaining (__context__):
       when an exception is raised inside an except block,
       the original exception becomes __context__. */
    PyObject *current_exc = tstate->exc_info->exc_value;
    if (current_exc != NULL && current_exc != Py_None) {
        /* val.__context__ = current exception */
        PyException_SetContext(val, Py_NewRef(current_exc));
    }
}

raise X from Y sets X.__cause__ = Y and X.__suppress_context__ = True (explicit chaining). raise X inside except E sets X.__context__ = E implicitly.

sys.excepthook

// CPython: Python/errors.c:380 _PyErr_Display
void
_PyErr_Display(PyObject *file, PyObject *exc)
{
    /* Walk exc.__context__ / exc.__cause__ chain.
       Print traceback for each, with "During handling..." or "The above...".
       Then print the final exception. */
    PyObject *hook = _PySys_GetAttr(tstate, &_Py_ID(excepthook));
    if (hook != NULL)
        PyObject_CallOneArg(hook, exc);
}

sys.excepthook is called with the uncaught exception. Replacing it redirects all uncaught exception output (used by IDEs and crash reporters).

sys.unraisablehook

// CPython: Python/errors.c:420 sys.unraisablehook
/* sys.unraisablehook(unraisablehook_args) is called by
   _PyErr_WriteUnraisableMsg.  Default handler prints to sys.stderr.
   Custom hooks can log to a file or suppress output entirely. */

gopy notes

_PyErr_WriteUnraisableMsg is vm.WriteUnraisable in vm/exceptions.go. Exception chaining is set in vm/eval_unwind.go when entering PUSH_EXC_INFO. sys.excepthook and sys.unraisablehook are stored in interp.SysDict and called from vm.HandleUncaughtException.