Python/ceval.c (part 100)

Source:

cpython 3.14 @ ab2d84fe1023/Python/ceval.c ↗

This annotation covers exception raising and the exception stack. See python_ceval99_detail for RESUME, COPY_FREE_VARS, and RETURN_VALUE.

Map

Lines	Symbol	Role
1-80	RAISE_VARARGS	Explicit raise statement
81-160	RERAISE	Re-raise current exception (raise with no args)
161-240	PUSH_EXC_INFO	Enter except clause: push exception info
241-340	POP_EXCEPT	Exit except clause: pop exception info
341-500	Exception chaining	__cause__, __context__, __suppress_context__

Reading

RAISE_VARARGS

// CPython: Python/ceval.c:3060 RAISE_VARARGS
inst(RAISE_VARARGS, (args[oparg] --)) {
    /* oparg: 0=reraise, 1=raise exc, 2=raise exc from cause */
    PyObject *cause = NULL, *exc = NULL;
    switch (oparg) {
    case 2:
        cause = args[1];
        /* fall through */
    case 1:
        exc = args[0];
        break;
    }
    if (do_raise(tstate, exc, cause)) {
        assert(_PyErr_Occurred(tstate));
        goto exception_unwind;
    }
    Py_XDECREF(exc);
    Py_XDECREF(cause);
}

raise ValueError("msg") compiles to LOAD_GLOBAL ValueError; LOAD_CONST "msg"; CALL 1; RAISE_VARARGS 1. raise exc from cause uses oparg=2. do_raise normalizes the exception (converts class to instance), sets __cause__ and __suppress_context__, and sets it as the current exception.

RERAISE

// CPython: Python/ceval.c:3120 RERAISE
inst(RERAISE, (values[oparg], exc --)) {
    /* Re-raise exception from the exception stack */
    /* oparg=1: also restore the lasti for traceback */
    if (oparg) {
        PyObject *lasti = values[0];
        frame->f_lasti = PyLong_AsLong(lasti);
    }
    assert(_PyErr_Occurred(tstate));
    Py_INCREF(exc);
    _PyErr_SetRaisedException(tstate, exc);
    goto exception_unwind;
}

RERAISE is emitted for bare raise inside except blocks and for except* clause re-raising. It re-raises the exception that is on top of the exception stack (pushed by PUSH_EXC_INFO). The lasti restoration ensures the traceback points to the original raise site.

PUSH_EXC_INFO

// CPython: Python/ceval.c:3160 PUSH_EXC_INFO
inst(PUSH_EXC_INFO, (new_exc -- prev_exc, new_exc)) {
    _PyErr_StackItem *exc_info = tstate->exc_info;
    /* Push current exception onto the exception stack */
    if (exc_info->previous_item == NULL) {
        _PyErr_StackItem *new_item = ... /* alloc */;
        exc_info->previous_item = new_item;
    }
    prev_exc = exc_info->exc_value;  /* old exception */
    Py_XINCREF(new_exc);
    exc_info->exc_value = new_exc;   /* install new exception */
}

PUSH_EXC_INFO is the entry point into an except clause. It saves the previously active exception and installs the new one. This allows nested except handlers to correctly restore the exception context on exit.

POP_EXCEPT

// CPython: Python/ceval.c:3200 POP_EXCEPT
inst(POP_EXCEPT, (exc --)) {
    _PyErr_StackItem *exc_info = tstate->exc_info;
    PyObject *value = exc_info->exc_value;
    exc_info->exc_value = exc;  /* restore previous exception */
    Py_XDECREF(value);
    Py_DECREF(exc);
}

POP_EXCEPT restores the exception that was active before the except block was entered. At the end of an except clause, the exception that was being handled is discarded and the enclosing exception context is restored.

gopy notes

RAISE_VARARGS is in vm/eval_unwind.go; calls vm.DoRaise which normalizes and sets vm.ThreadState.CurrentException. RERAISE calls vm.Reraise. PUSH_EXC_INFO pushes onto vm.ThreadState.ExcStack. POP_EXCEPT pops and restores vm.ThreadState.ExcStack. Exception chaining sets __cause__ and __context__ on objects.BaseException.