// cpython/Python/ceval.c
PyObject *name = GETITEM(names, oparg);
            PyObject *owner = TOP();
            PyObject *res = PyObject_GetAttr(owner, name);

# cpython/Objects/object.c
PyObject *
PyObject_GetAttr(PyObject *v, PyObject *name)
{
    PyTypeObject *tp = Py_TYPE(v);
    ...
    if (tp->tp_getattro != NULL)
            return (*tp->tp_getattro)(v, name);
    ...

// cpython/Objects/typeobject.c
    TPSLOT("__getattribute__", tp_getattr, NULL, NULL, ""), // 历史遗留
    ...
    TPSLOT("__getattribute__", tp_getattro, slot_tp_getattr_hook, wrap_binaryfunc, // tp_getattro 槽对应__getattribute__ C dispatcher 为 slot_tp_getattr_hook

PyTypeObject PyBaseObject_Type = {
    ...
    PyObject_GenericGetAttr,                    /* tp_getattro */ // object的C结构tp_getattro槽位指向了PyObject_GenericGetAttr
    ...

/* There are two slot dispatch functions for tp_getattro.

   - slot_tp_getattro() is used when __getattribute__ is overridden
     but no __getattr__ hook is present;

   - slot_tp_getattr_hook() is used when a __getattr__ hook is present.

   The code in update_one_slot() always installs slot_tp_getattr_hook(); this
   detects the absence of __getattr__ and then installs the simpler slot if
   necessary. */

static PyObject *
slot_tp_getattro(PyObject *self, PyObject *name)
{
    PyObject *stack[1] = {name};
    return call_method(self, &PyId___getattribute__, stack, 1);
}

...

static PyObject *
slot_tp_getattr_hook(PyObject *self, PyObject *name)
{
    PyTypeObject *tp = Py_TYPE(self);
    PyObject *getattr, *getattribute, *res;
    _Py_IDENTIFIER(__getattr__);

    /* speed hack: we could use lookup_maybe, but that would resolve the
       method fully for each attribute lookup for classes with
       __getattr__, even when the attribute is present. So we use
       _PyType_Lookup and create the method only when needed, with
       call_attribute. */
    getattr = _PyType_LookupId(tp, &PyId___getattr__);
    if (getattr == NULL) {
        /* No __getattr__ hook: use a simpler dispatcher */
        tp->tp_getattro = slot_tp_getattro;
        return slot_tp_getattro(self, name);
    }
    Py_INCREF(getattr);
    /* speed hack: we could use lookup_maybe, but that would resolve the
       method fully for each attribute lookup for classes with
       __getattr__, even when self has the default __getattribute__
       method. So we use _PyType_Lookup and create the method only when
       needed, with call_attribute. */
    getattribute = _PyType_LookupId(tp, &PyId___getattribute__);
    if (getattribute == NULL ||
        (Py_TYPE(getattribute) == &PyWrapperDescr_Type &&
         ((PyWrapperDescrObject *)getattribute)->d_wrapped ==
         (void *)PyObject_GenericGetAttr))
        res = PyObject_GenericGetAttr(self, name);
    else {
        Py_INCREF(getattribute);
        res = call_attribute(self, getattribute, name);
        Py_DECREF(getattribute);
    }
    if (res == NULL && PyErr_ExceptionMatches(PyExc_AttributeError)) {
        PyErr_Clear();
        res = call_attribute(self, getattr, name);
    }
    Py_DECREF(getattr);
    return res;
}

# cpython/Objects/object.c
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.
    */

    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;
    }

    // 按照MRO查找类型及其基类中的**描述符**，如果存在且是**数据描述符**，则调用数据描述符的__get__方法
    descr = _PyType_Lookup(tp, name);

    f = NULL;
    if (descr != NULL) {
        Py_INCREF(descr);
        f = descr->ob_type->tp_descr_get;
        if (f != NULL && PyDescr_IsData(descr)) {
            res = f(descr, obj, (PyObject *)obj->ob_type);
            if (res == NULL && suppress &&
                    PyErr_ExceptionMatches(PyExc_AttributeError)) {
                PyErr_Clear();
            }
            goto done;
        }
    }

    // 查找实例字典，如果存在，直接返回该属性的值
    if (dict == NULL) {
        /* Inline _PyObject_GetDictPtr */
        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);
                _PyObject_ASSERT(obj, size <= PY_SSIZE_T_MAX);

                dictoffset += (Py_ssize_t)size;
                _PyObject_ASSERT(obj, dictoffset > 0);
                _PyObject_ASSERT(obj, dictoffset % SIZEOF_VOID_P == 0);
            }
            dictptr = (PyObject **) ((char *)obj + dictoffset);
            dict = *dictptr;
        }
    }
    if (dict != NULL) {
        Py_INCREF(dict);
        res = PyDict_GetItemWithError(dict, name);
        if (res != NULL) {
            Py_INCREF(res);
            Py_DECREF(dict);
            goto done;
        }
        else {
            Py_DECREF(dict);
            if (PyErr_Occurred()) {
                if (suppress && PyErr_ExceptionMatches(PyExc_AttributeError)) {
                    PyErr_Clear();
                }
                else {
                    goto done;
                }
            }
        }
    }

    // 如果实例字典中没有同名属性，如果存在非数据描述符（仅实现了 __get__ 方法的描述符），则调用描述符的 __get__ 方法
    if (f != NULL) {
        res = f(descr, obj, (PyObject *)Py_TYPE(obj));
        if (res == NULL && suppress &&
                PyErr_ExceptionMatches(PyExc_AttributeError)) {
            PyErr_Clear();
        }
        goto done;
    }

    // 如果上述步骤均未命中，则在类型的 dict 以及其基类的 dict 中按照 MRO查找普通属性（在步骤1 `PyType_Lookup` 已经做了这个事情，只是还未使用，一次类查找，多次优先级判断）

    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;
}

class A:
    pass

a = A()
a.x = 10

print(a.__dict__) # {'x': 10}

class B:
    __slots__ = ("x",)

b = B()
b.x = 1

class C:
    __slots__ = ("x", "__dict__")

c = C()
c.y = 2     # OK

class Descriptor:

    def __get__(self, obj, type):
        # 描述符自己决定怎么处理 obj is None，即通过类的方式访问
        if obj is None:
            print('__get__(): Accessing x from the class', type)
            return self
        
        print('__get__(): Accessing x from the object', obj)
        return 'X from descriptor'

    def __set__(self, obj, value):
        print('__set__(): Setting x on the object', obj)
        obj.__dict__['_x'] = value

class Foo:
    x = Descriptor()

print(Foo.x)
# __get__(): Accessing x from the class <class '__main__.Foo'>
# <__main__.Descriptor object at 0x00000202A1A46F90>

foo = Foo()
print(foo.x)
# __get__(): Accessing x from the object <__main__.Foo object at 0x00000202A1A470E0>
# X from descriptor

// cpython/Objects/descrobject.c
PyTypeObject PyProperty_Type = {
    PyVarObject_HEAD_INIT(&PyType_Type, 0)
    "property",                                 /* tp_name */
    ...
    0,                                          /* tp_dict */
    property_descr_get,                         /* tp_descr_get */
    property_descr_set,                         /* tp_descr_set */
    0,                                          /* tp_dictoffset */
    property_init,                              /* tp_init */
    PyType_GenericAlloc,                        /* tp_alloc */
    PyType_GenericNew,                          /* tp_new */
    PyObject_GC_Del,                            /* tp_free */
};

class A:
  def f(self, x):
    return x

a = A()

print(A.f, a.f) #<function A.f at ...> <bound method A.f of <__main__.A object at ...>>
print(A.f(None, 2))
print(A.__dict__['f']) #<function A.f at ...>

# f 是一个描述符
# 通过A.f访问时，判断self 为None返回正常函数
# 通过a.f实例访问时，返回一个 MethodType 对象
# 变成了绑定方法调用，在调用时会自动将绑定的对象作为第一个参数
print(a.f(2))
bound_method = a.f
print(bound_method(2))

// cpython/Objects/funcobject.c
/* Bind a function to an object */
static PyObject *
func_descr_get(PyObject *func, PyObject *obj, PyObject *type)
{
    if (obj == Py_None || obj == NULL) {
        Py_INCREF(func);
        return func;
    }
    return PyMethod_New(func, obj); // 本质是(function, instance) 的二元包装
}

// cpython/Include/classobject.h
typedef struct {
    PyObject_HEAD
    PyObject *im_func;   /* The callable object implementing the method */
    PyObject *im_self;   /* The instance it is bound to */
    PyObject *im_weakreflist; /* List of weak references */
    vectorcallfunc vectorcall;
} PyMethodObject;

// cpython/Objects/classobject.c

/* Method objects are used for bound instance methods returned by
   instancename.methodname. ClassName.methodname returns an ordinary
   function.
*/

PyObject *
PyMethod_New(PyObject *func, PyObject *self)
{
    PyMethodObject *im;
    if (self == NULL) {
        PyErr_BadInternalCall();
        return NULL;
    }
    im = free_list;
    if (im != NULL) {
        free_list = (PyMethodObject *)(im->im_self);
        (void)PyObject_INIT(im, &PyMethod_Type);
        numfree--;
    }
    else {
        im = PyObject_GC_New(PyMethodObject, &PyMethod_Type);
        if (im == NULL)
            return NULL;
    }
    im->im_weakreflist = NULL;
    Py_INCREF(func);
    im->im_func = func;
    Py_XINCREF(self);
    im->im_self = self;
    im->vectorcall = method_vectorcall;
    _PyObject_GC_TRACK(im);
    return (PyObject *)im;
}

class A:
    def f(self, x):
        return self, x

    @classmethod
    def g(cls, x):
        return cls, x

a = A()

print(A.__dict__['g']) # <classmethod(<function A.g at ...>)>

# a.g也是绑定A
print(A.g, a.g) # <bound method A.g of <class '__main__.A'>> <bound method A.g of <class '__main__.A'>>

PyDoc_STRVAR(classmethod_doc,
"classmethod(function) -> method\n\
\n\
Convert a function to be a class method.\n\
\n\
A class method receives the class as implicit first argument,\n\
just like an instance method receives the instance.\n\
To declare a class method, use this idiom:\n\
\n\
  class C:\n\
      @classmethod\n\
      def f(cls, arg1, arg2, ...):\n\
          ...\n\
\n\
It can be called either on the class (e.g. C.f()) or on an instance\n\
(e.g. C().f()).  The instance is ignored except for its class.\n\
If a class method is called for a derived class, the derived class\n\
object is passed as the implied first argument.\n\
\n\
Class methods are different than C++ or Java static methods.\n\
If you want those, see the staticmethod builtin.");

PyTypeObject PyClassMethod_Type = {
    PyVarObject_HEAD_INIT(&PyType_Type, 0)
    "classmethod",
    ...
    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE | Py_TPFLAGS_HAVE_GC,
    classmethod_doc,                            /* tp_doc */
    (traverseproc)cm_traverse,                  /* tp_traverse */
    (inquiry)cm_clear,                          /* tp_clear */
    0,                                          /* tp_richcompare */
    0,                                          /* tp_weaklistoffset */
    0,                                          /* tp_iter */
    0,                                          /* tp_iternext */
    0,                                          /* tp_methods */
    cm_memberlist,              /* tp_members */
    cm_getsetlist,                              /* tp_getset */
    0,                                          /* tp_base */
    0,                                          /* tp_dict */
    cm_descr_get,                               /* tp_descr_get */
    0,                                          /* tp_descr_set */
    offsetof(classmethod, cm_dict),             /* tp_dictoffset */
    cm_init,                                    /* tp_init */
    PyType_GenericAlloc,                        /* tp_alloc */
    PyType_GenericNew,                          /* tp_new */
    PyObject_GC_Del,                            /* tp_free */
};

static PyObject *
cm_descr_get(PyObject *self, PyObject *obj, PyObject *type)
{
    classmethod *cm = (classmethod *)self;

    if (cm->cm_callable == NULL) {
        PyErr_SetString(PyExc_RuntimeError,
                        "uninitialized classmethod object");
        return NULL;
    }
    if (type == NULL)
        type = (PyObject *)(Py_TYPE(obj));
    return PyMethod_New(cm->cm_callable, type);
}

class E:
    @staticmethod
    def f(x):
        return x * 10

e = E()
print(E.f, e.f) # <function E.f at ...> <function E.f at ...>
print(E.__dict__['f']) # <staticmethod(<function E.f at ...>)>
print(E.__dict__['f'](3)) # 30

/* A static method does not receive an implicit first argument.
   To declare a static method, use this idiom:

     class C:
         @staticmethod
         def f(arg1, arg2, ...):
             ...

   It can be called either on the class (e.g. C.f()) or on an instance
   (e.g. C().f()). Both the class and the instance are ignored, and
   neither is passed implicitly as the first argument to the method.

   Static methods in Python are similar to those found in Java or C++.
   For a more advanced concept, see class methods above.
*/

PyTypeObject PyStaticMethod_Type = {
    PyVarObject_HEAD_INIT(&PyType_Type, 0)
    "staticmethod",
    ...
    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE | Py_TPFLAGS_HAVE_GC,
    staticmethod_doc,                           /* tp_doc */
    (traverseproc)sm_traverse,                  /* tp_traverse */
    (inquiry)sm_clear,                          /* tp_clear */
    0,                                          /* tp_richcompare */
    0,                                          /* tp_weaklistoffset */
    0,                                          /* tp_iter */
    0,                                          /* tp_iternext */
    0,                                          /* tp_methods */
    sm_memberlist,              /* tp_members */
    sm_getsetlist,                              /* tp_getset */
    0,                                          /* tp_base */
    0,                                          /* tp_dict */
    sm_descr_get,                               /* tp_descr_get */
    0,                                          /* tp_descr_set */
    offsetof(staticmethod, sm_dict),            /* tp_dictoffset */
    sm_init,                                    /* tp_init */
    PyType_GenericAlloc,                        /* tp_alloc */
    PyType_GenericNew,                          /* tp_new */
    PyObject_GC_Del,                            /* tp_free */
};

static PyObject *
sm_descr_get(PyObject *self, PyObject *obj, PyObject *type)
{
    staticmethod *sm = (staticmethod *)self;

    if (sm->sm_callable == NULL) {
        PyErr_SetString(PyExc_RuntimeError,
                        "uninitialized staticmethod object");
        return NULL;
    }
    Py_INCREF(sm->sm_callable);
    return sm->sm_callable;
}