Python/ceval.c (part 15)

Source:

cpython 3.14 @ ab2d84fe1023/Python/ceval.c ↗

This annotation covers exception-handling opcodes. See python_ceval14_detail for pattern matching opcodes, and vm_eval_unwind for frame unwinding.

Map

Lines	Symbol	Role
1-80	PUSH_EXC_INFO	Save current exception; install new one from stack
81-160	POP_EXCEPT	Restore previous exception from the exc_info stack
161-260	RAISE_VARARGS	Raise or re-raise an exception
261-360	RERAISE	Re-raise the exception currently being handled
361-500	CHECK_EXC_MATCH	Test isinstance(exc, cls) for except cls:

Reading

PUSH_EXC_INFO

// CPython: Python/ceval.c:3680 PUSH_EXC_INFO
inst(PUSH_EXC_INFO, (new_exc -- prev_exc, new_exc)) {
    /* Save the current exception so POP_EXCEPT can restore it.
       Install new_exc as the active exception. */
    _PyErr_StackItem *exc_info = tstate->exc_info;
    PyObject *prev = exc_info->exc_value;
    exc_info->exc_value = Py_NewRef(new_exc);
    /* Push prev_exc onto the stack for POP_EXCEPT */
}

PUSH_EXC_INFO is emitted at the start of every except block. It chains exception state so nested except handlers each have their own active exception.

POP_EXCEPT

// CPython: Python/ceval.c:3720 POP_EXCEPT
inst(POP_EXCEPT, (exc_value --)) {
    /* Restore the exception that was active before this except block. */
    _PyErr_StackItem *exc_info = tstate->exc_info;
    PyObject *value = exc_info->exc_value;
    exc_info->exc_value = exc_value;  /* pop the saved value */
    Py_XDECREF(value);
}

POP_EXCEPT is emitted at the end of every except block (including except*). It restores the exception that was active when the except block was entered.

RAISE_VARARGS

// CPython: Python/ceval.c:3760 RAISE_VARARGS
inst(RAISE_VARARGS, (args[oparg] -- )) {
    PyObject *cause = NULL, *exc = NULL;
    switch (oparg) {
    case 2: cause = args[1]; /* raise exc from cause */
            /* fall through */
    case 1: exc = args[0];   /* raise exc */
            break;
    case 0: break;           /* bare raise: re-raise current exception */
    }
    if (do_raise(tstate, exc, cause)) {
        goto exception_unwind;
    }
    ERROR_IF(true, error);
}

raise ValueError("msg") compiles to LOAD..., CALL..., RAISE_VARARGS 1. raise ValueError("msg") from None uses RAISE_VARARGS 2 with None as the cause. Bare raise uses RAISE_VARARGS 0.

RERAISE

// CPython: Python/ceval.c:3820 RERAISE
inst(RERAISE, (values[oparg], exc -- values[oparg])) {
    /* Re-raise the exception from the except block.
       oparg=1: also restore lasti (for accurate tracebacks). */
    assert(oparg < 3);
    PyObject *val = exc;
    assert(PyExceptionInstance_Check(val));
    PyObject *tb = PyException_GetTraceback(val);
    _PyErr_SetRaisedException(tstate, Py_NewRef(val));
    goto exception_unwind;
}

RERAISE is used inside except blocks when the exception is re-raised implicitly (e.g., at the end of a with block's __exit__ that returns a falsy value).

CHECK_EXC_MATCH

// CPython: Python/ceval.c:3860 CHECK_EXC_MATCH
inst(CHECK_EXC_MATCH, (left, right -- left, b)) {
    /* b = isinstance(left, right) — used by except clause.
       Also handles tuples: except (TypeError, ValueError): */
    assert(PyExceptionInstance_Check(left));
    if (!PyExceptionClass_Check(right)) {
        _PyErr_SetString(tstate, PyExc_TypeError,
            CANNOT_CATCH_MSG);
        ERROR_IF(true, error);
    }
    int res = PyErr_GivenExceptionMatches(left, right);
    b = res ? Py_True : Py_False;
    Py_INCREF(b);
}

except (TypeError, ValueError): passes a tuple as right; PyErr_GivenExceptionMatches handles tuples by checking each element.

gopy notes

PUSH_EXC_INFO and POP_EXCEPT are in vm/eval_gen.go. RAISE_VARARGS calls vm.DoRaise in vm/eval_unwind.go. RERAISE is vm.DoReraise. CHECK_EXC_MATCH uses objects.ExceptionMatches.