從原始碼看Python的descriptor
摘要：
descriptor是Python初學者比較不好懂的概念之一，再加上官網文件說的也不是很清楚，
就更容易讓人誤解了。但是原始碼總不會說不清楚，所以我們從原始碼看清楚descriptor到底
在幹啥。
PyObject *
_PyObject_GenericGetAttrWithDict(P...
descriptor是Python初學者比較不好懂的概念之一，再加上官網文件說的也不是很清楚， 就更容易讓人誤解了。但是原始碼總不會說不清楚，所以我們從原始碼看清楚descriptor到底 在幹啥。
PyObject *
_PyObject_GenericGetAttrWithDict(PyObject *obj, PyObject *name, PyObject *dict)
{
/* Make sure the logic of _PyObject_GetMethod is in sync with
this method.
*/
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;
}
descr = _PyType_Lookup(tp, name);// 從自身包括繼承關係中找descriptor
f = NULL;
if (descr != NULL) {
Py_INCREF(descr);
f = descr->ob_type->tp_descr_get;
if (f != NULL && PyDescr_IsData(descr)) {
// 如果有descriptor並且是data descriptor，那麼就呼叫並返回
res = f(descr, obj, (PyObject *)obj->ob_type);
goto done;
}
}
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);
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;
}
}
if (dict != NULL) {
Py_INCREF(dict);
// 從__dict__裡查詢屬性並返回
res = PyDict_GetItem(dict, name);
if (res != NULL) {
Py_INCREF(res);
Py_DECREF(dict);
goto done;
}
Py_DECREF(dict);
}
if (f != NULL) {
// 非data descriptor，返回
res = f(descr, obj, (PyObject *)Py_TYPE(obj));
goto done;
}
if (descr != NULL) {
res = descr;
descr = NULL;
goto done;
}
PyErr_Format(PyExc_AttributeError,
"'%.50s' object has no attribute '%U'",
tp->tp_name, name);
done:
Py_XDECREF(descr);
Py_DECREF(name);
return res;
}
PyObject *
PyObject_GenericGetAttr(PyObject *obj, PyObject *name)
{
// 呼叫此函式，往上看
return _PyObject_GenericGetAttrWithDict(obj, name, NULL);
}
所以可以得知：
- descriptor是作為屬性時才會生效，如下demo中，Foo是作為屬性時，才會生效 - 屬性查詢順序為 data descriptor -> __dict__ -> descriptor
In [1]: class Foo:
...:def __get__(self, obj, type):
...:print("__get__")
...:def __set__(self, obj, val):
...:print("__set__")
...:
In [2]: class Test:
...:bar = Foo()
...:
In [3]: t = Test()
In [4]: t.bar
__get__
In [5]: foo = Foo()
In [6]: foo.bar
---------------------------------------------------------------------------
AttributeErrorTraceback (most recent call last)
<ipython-input-6-673afadb7b8e> in <module>()
----> 1 foo.bar
AttributeError: 'Foo' object has no attribute 'bar'