Can we have an int return value so that we don't need to call PyErr_Occurred()? IMHO, it's an API anti-pattern to require double checking what NULL means.

PyAPI_FUNC(int) _PyObject_GetAttrIdWithoutError(
    PyObject *, struct _Py_Identifier *, PyObject **);

• -1 error
• 0 not found
• 1 found

I'd also do this for _PyObject_GetAttrWithoutError which we committed today. /cc @methane

Make sense. Go ahead.

For usability, third argument should be filled with NULL when returning 1.
By it, caller can use if (val == NULL) instead of assigning return value to local variable.

if (_PyObject_GetAttrIdWithoutError((PyObject *)deque, &PyId___dict__, &func) < 0) {
    // when error happens
    return NULL;
}
if (func == NULL) {
    // when `__dict__` not found.
    Py_RETURN_NONE;
}

Yeah, I agree, it's even better.

Then I think It would be better to rename the function to _PyObject_LookupAttrId or _PyObject_FindAttrId. _PyObject_GetAttrIdWithoutError looks misleading bacause despite its name it can raise an error.

I agree. "find" is far better than "without error".

I like LookupAttrId. It sounds better.

BTW, I'd also remove the leading underscore from _PyObject_GetAttrWithoutError, making it PyObject_LookupAttr (and we keep the underscore for _PyObject_LookupAttrId).

Hm, "lookup" make sense too because there are _PyType_Lookup and _PyType_LookupId
and they doesn't set exception.
And +1 to make it public API.