Python/ceval.c (part 59)

Source:

cpython 3.14 @ ab2d84fe1023/Python/ceval.c ↗

This annotation covers exception context management opcodes. See python_ceval58_detail for RAISE_VARARGS, RERAISE, and CHECK_EXC_MATCH.

Map

Lines	Symbol	Role
1-80	PUSH_EXC_INFO	Save current exception, push new one
81-160	POP_EXCEPT	Restore saved exception, pop handler frame
161-260	WITH_EXCEPT_START	Call __exit__ with active exception
261-380	CLEANUP_THROW	Handle throw() into a generator
381-500	Exception stack layout	How exc_info interacts with the value stack

Reading

PUSH_EXC_INFO

// CPython: Python/ceval.c:3820 PUSH_EXC_INFO
inst(PUSH_EXC_INFO, (new_exc -- prev_exc, new_exc)) {
    _PyErr_StackItem *exc_info = tstate->exc_info;
    if (exc_info->previous_item == NULL) {
        assert(exc_info == &tstate->exc_state);
    }
    prev_exc = exc_info->exc_value;
    DEAD(prev_exc);  /* Will be restored by POP_EXCEPT */
    exc_info->exc_value = Py_NewRef(new_exc);
}

PUSH_EXC_INFO saves the current active exception (prev_exc) onto the value stack and installs new_exc as the thread-state active exception. This enables except clauses to call sys.exc_info() and get the current exception.

POP_EXCEPT

// CPython: Python/ceval.c:3848 POP_EXCEPT
inst(POP_EXCEPT, (exc_value --)) {
    _PyErr_StackItem *exc_info = tstate->exc_info;
    PyObject *value = exc_info->exc_value;
    exc_info->exc_value = exc_value;
    Py_XDECREF(value);
}

POP_EXCEPT restores the previously active exception from the value stack. After the except block finishes, the exception that was active before the try is reinstated. This maintains correct sys.exc_info() behavior during nested exception handlers.

WITH_EXCEPT_START

// CPython: Python/ceval.c:3880 WITH_EXCEPT_START
inst(WITH_EXCEPT_START, (exit_self, lasti, exc --  exit_self, lasti, exc, res)) {
    /* Call exit(exc_type, exc_val, exc_tb) */
    PyObject *exc_type = Py_TYPE(exc);
    PyObject *exc_tb = PyException_GetTraceback(exc);
    PyObject *stack[4] = {NULL, exc_type, exc, exc_tb};
    res = PyObject_Vectorcall(exit_self, stack + 1,
                              3 | PY_VECTORCALL_ARGUMENTS_OFFSET, NULL);
    Py_XDECREF(exc_tb);
    ERROR_IF(res == NULL, error);
}

__exit__ receives (exc_type, exc_value, exc_traceback). If it returns a truthy value, the exception is suppressed. lasti is the offset of the WITH_EXCEPT_START instruction, used to resume after the with block on suppression.

CLEANUP_THROW

// CPython: Python/ceval.c:3940 CLEANUP_THROW
inst(CLEANUP_THROW, (exc_value, last_sent_val, tb --  out0, out1)) {
    /* Called when throw() raises StopIteration — unwrap it */
    if (PyErr_GivenExceptionMatches(exc_value, PyExc_StopIteration)) {
        out0 = Py_NewRef(Py_None);
        out1 = Py_NewRef(((PyStopIterationObject *)exc_value)->value);
    } else {
        /* Re-raise */
        Py_INCREF(exc_value);
        PyErr_SetRaisedException(exc_value);
        ERROR_IF(true, error);
    }
}

CLEANUP_THROW handles the case where generator.throw(exc) causes a StopIteration inside the generator. The StopIteration.value becomes the result of the yield from or await expression.

gopy notes

PUSH_EXC_INFO is in vm/eval_unwind.go; it updates vm.Frame.ExcInfo. POP_EXCEPT restores the saved exception value. WITH_EXCEPT_START calls objects.CallVectorcall with the exit function. CLEANUP_THROW is in vm/eval_gen.go and unwraps objects.StopIteration.