Python 3.10.9 (main, Jan 11 2023, 09:18:18) [Clang 14.0.6 ] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import dis
>>> dis.dis("a.attr")
  1           0 LOAD_NAME                0 (a)
              2 LOAD_ATTR                1 (attr)
              4 RETURN_VALUE
>>>

TARGET(LOAD_ATTR) {
    PyObject *name = GETITEM(names, oparg);
    PyObject *owner = TOP();
    PyObject *res = PyObject_GetAttr(owner, name);
    Py_DECREF(owner);
    SET_TOP(res);
    if (res == NULL)
        goto error;
    DISPATCH();
}

PyObject *
PyObject_GetAttr(PyObject *v, PyObject *name)
{
    // 首先獲取對(duì)象 v 的類型 ，對(duì)應(yīng)上面的代碼的話就是找到對(duì)象 a 的類型
    PyTypeObject *tp = Py_TYPE(v);

    if (!PyUnicode_Check(name)) {
        PyErr_Format(PyExc_TypeError,
                     "attribute name must be string, not '%.200s'",
                     name->ob_type->tp_name);
        return NULL;
    }
    // 獲取對(duì)象的 tp_getattro 函數(shù) 這個(gè)函數(shù)就是負(fù)責(zé)屬性查找的函數(shù) 我們一般使用的這個(gè)屬性查找函數(shù)都是
    // object 這個(gè)基類的屬性查找函數(shù)
    if (tp->tp_getattro != NULL)
        return (*tp->tp_getattro)(v, name);
    if (tp->tp_getattr != NULL) {
        const char *name_str = PyUnicode_AsUTF8(name);
        if (name_str == NULL)
            return NULL;
        return (*tp->tp_getattr)(v, (char *)name_str);
    }
    PyErr_Format(PyExc_AttributeError,
                 "'%.50s' object has no attribute '%U'",
                 tp->tp_name, name);
    return NULL;
}

PyTypeObject PyBaseObject_Type = {
    PyVarObject_HEAD_INIT(&PyType_Type, 0)
    "object",                                   /* tp_name */
    sizeof(PyObject),                           /* tp_basicsize */
    0,                                          /* tp_itemsize */
    object_dealloc,                             /* tp_dealloc */
    0,                                          /* tp_print */
    0,                                          /* tp_getattr */
    0,                                          /* tp_setattr */
    0,                                          /* tp_reserved */
    object_repr,                                /* tp_repr */
    0,                                          /* tp_as_number */
    0,                                          /* tp_as_sequence */
    0,                                          /* tp_as_mapping */
    (hashfunc)_Py_HashPointer,                  /* tp_hash */
    0,                                          /* tp_call */
    object_str,                                 /* tp_str */
    // 這個(gè)就是真正的屬性查找函數(shù)
    PyObject_GenericGetAttr,                    /* tp_getattro */
    PyObject_GenericSetAttr,                    /* tp_setattro */
    0,                                          /* tp_as_buffer */
    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE,   /* tp_flags */
    PyDoc_STR("object()\n--\n\nThe most base type"),  /* tp_doc */
    0,                                          /* tp_traverse */
    0,                                          /* tp_clear */
    object_richcompare,                         /* tp_richcompare */
    0,                                          /* tp_weaklistoffset */
    0,                                          /* tp_iter */
    0,                                          /* tp_iternext */
    object_methods,                             /* tp_methods */
    0,                                          /* tp_members */
    object_getsets,                             /* tp_getset */
    0,                                          /* tp_base */
    0,                                          /* tp_dict */
    0,                                          /* tp_descr_get */
    0,                                          /* tp_descr_set */
    0,                                          /* tp_dictoffset */
    object_init,                                /* tp_init */
    PyType_GenericAlloc,                        /* tp_alloc */
    object_new,                                 /* tp_new */
    PyObject_Del,                               /* tp_free */
};

// 從上面的 object 定義可以看到真正的查找函數(shù)為 PyObject_GenericGetAttr 其函數(shù)內(nèi)容如下所示：
PyObject *
PyObject_GenericGetAttr(PyObject *obj, PyObject *name)
{
    return _PyObject_GenericGetAttrWithDict(obj, name, NULL, 0);
}

/* Generic GetAttr functions - put these in your tp_[gs]etattro slot. */

PyObject *
_PyObject_GenericGetAttrWithDict(PyObject *obj, PyObject *name,
                                 PyObject *dict, int suppress)
{
    /* Make sure the logic of _PyObject_GetMethod is in sync with
       this method.

       When suppress=1, this function suppress AttributeError.
    */
    // 首先獲取對(duì)象的類型 針對(duì)于上面的源代碼來(lái)說(shuō)就是找到對(duì)象 a 的類型
    PyTypeObject *tp = Py_TYPE(obj);
    PyObject *descr = NULL;
    PyObject *res = NULL;
    descrgetfunc f;
    Py_ssize_t dictoffset;
    PyObject **dictptr;

    if (!PyUnicode_Check(name)){
        PyErr_Format(PyExc_TypeError,
                     "attribute name must be string, not '%.200s'",
                     name->ob_type->tp_name);
        return NULL;
    }
    Py_INCREF(name);

    if (tp->tp_dict == NULL) {
        if (PyType_Ready(tp) < 0)
            goto done;
    }
    // 這個(gè)是從所有的基類當(dāng)中找到一個(gè)名字為 name 的對(duì)象 如果沒(méi)有就返回 NULL
    // 這里的過(guò)程還是比較復(fù)雜 需要從類的 mro 序列當(dāng)中進(jìn)行查找
    descr = _PyType_Lookup(tp, name);

    f = NULL;
    // 如果找到的類對(duì)象不為空 也就是在類本身或者基類當(dāng)中找到一個(gè)名為 name 的對(duì)象
    if (descr != NULL) {
        Py_INCREF(descr);
        // 得到類對(duì)象的 __get__ 函數(shù)
        f = descr->ob_type->tp_descr_get;
        // 如果對(duì)象有 __get__ 函數(shù)則進(jìn)行進(jìn)一步判斷
        if (f != NULL && PyDescr_IsData(descr)) { // PyDescr_IsData(descr) 這個(gè)宏是查看對(duì)象是否有 __set__ 函數(shù)
            // 如果是類對(duì)象又有 __get__ 函數(shù) 又有 __set__ 函數(shù) 則直接調(diào)用對(duì)象的 __get__ 函數(shù) 并且將結(jié)果返回
            // 這里需要注意一下優(yōu)先級(jí) 這個(gè)優(yōu)先級(jí)是最高的 如果一個(gè)類對(duì)象定義了 __set__ 和 __get__ 函數(shù)，那么
            // 就會(huì)直接調(diào)用類對(duì)象的 __get__ 函數(shù)并且將這個(gè)函數(shù)的返回值返回
            res = f(descr, obj, (PyObject *)obj->ob_type);
            if (res == NULL && suppress &&
                    PyErr_ExceptionMatches(PyExc_AttributeError)) {
                PyErr_Clear();
            }
            goto done;
        }
    }
    // 如果沒(méi)有名為 name 的類對(duì)象 或者雖然有名為 name 的對(duì)象 但是只要沒(méi)有同時(shí)定義 __get__ 和 __set__ 函數(shù)就需要
    // 繼續(xù)往下執(zhí)行 從對(duì)象本省的 dict 當(dāng)中尋找
    if (dict == NULL) {
        /* Inline _PyObject_GetDictPtr */
        // 這部分代碼就是從對(duì)象 obj 當(dāng)中找到對(duì)象的 __dict__ 字段
        dictoffset = tp->tp_dictoffset;
        if (dictoffset != 0) {
            if (dictoffset < 0) {
                Py_ssize_t tsize;
                size_t size;

                tsize = ((PyVarObject *)obj)->ob_size;
                if (tsize < 0)
                    tsize = -tsize;
                size = _PyObject_VAR_SIZE(tp, tsize);
                assert(size <= PY_SSIZE_T_MAX);

                dictoffset += (Py_ssize_t)size;
                assert(dictoffset > 0);
                assert(dictoffset % SIZEOF_VOID_P == 0);
            }
            dictptr = (PyObject **) ((char *)obj + dictoffset);
            dict = *dictptr;
        }
    }
    // 如果對(duì)象 obj 存在 __dict__ 字段 那么就返回 __dict__ 字段當(dāng)中名字等于 name 的對(duì)象
    if (dict != NULL) {
        Py_INCREF(dict);
        res = PyDict_GetItem(dict, name);
        if (res != NULL) {
            Py_INCREF(res);
            Py_DECREF(dict);
            goto done;
        }
        Py_DECREF(dict);
    }
    // 如果類對(duì)象定義了 __get__ 函數(shù)沒(méi)有定義 __set__ 函數(shù)而且在 dict 當(dāng)中沒(méi)有找到名為 name 的對(duì)象的話
    // 那么久調(diào)用類對(duì)象的 __get__ 函數(shù)
    if (f != NULL) {
        res = f(descr, obj, (PyObject *)Py_TYPE(obj));
        if (res == NULL && suppress &&
                PyErr_ExceptionMatches(PyExc_AttributeError)) {
            PyErr_Clear();
        }
        goto done;
    }
    // 如果類對(duì)象沒(méi)有定義 __get__ 函數(shù)那么就直接將這個(gè)類對(duì)象返回
    if (descr != NULL) {
        res = descr;
        descr = NULL;
        goto done;
    }

    if (!suppress) {
        PyErr_Format(PyExc_AttributeError,
                     "'%.50s' object has no attribute '%U'",
                     tp->tp_name, name);
    }
  done:
    Py_XDECREF(descr);
    Py_DECREF(name);
    return res;
}

def find_name_in_mro(cls, name, default):
    "Emulate _PyType_Lookup() in Objects/typeobject.c"
    for base in cls.__mro__:
        if name in vars(base):
            return vars(base)[name]
    return default

def object_getattribute(obj, name):
    "Emulate PyObject_GenericGetAttr() in Objects/object.c"
    null = object()
    objtype = type(obj)
    cls_var = find_name_in_mro(objtype, name, null)
    descr_get = getattr(type(cls_var), '__get__', null)
    if descr_get is not null:
        if (hasattr(type(cls_var), '__set__')
            or hasattr(type(cls_var), '__delete__')):
            return descr_get(cls_var, obj, objtype)     # data descriptor
    if hasattr(obj, '__dict__') and name in vars(obj):
        return vars(obj)[name]                          # instance variable
    if descr_get is not null:
        return descr_get(cls_var, obj, objtype)         # non-data descriptor
    if cls_var is not null:
        return cls_var                                  # class variable
    raise AttributeError(name)