python collections namedtuple模块的迷思

Question

赋值覆盖不了的问题

from collections import namedtuple

# namedtuple('名称', [属性list]):
Point = namedtuple('Point', ['x', 'y', 'z'])
p = Point('x', 'y', 'z')
print(p)
Point.x=8
p=Point(4,3,2)
print(p.x, p.y, p.z,p)

结果跟想像的不一样
用Point.x来赋值就把p.x的值锁定住了
再用p=Point()来赋值就覆盖不掉了

Answer 1

看了namedtuple的源码，大概知道是怎么回事了。。。
单看下面这段

    for index, name in enumerate(field_names):
        try:
            itemgetter_object, doc = cache[index]
        except KeyError:
            itemgetter_object = _itemgetter(index)
            doc = f'Alias for field number {index}'
            cache[index] = itemgetter_object, doc
        class_namespace[name] = property(itemgetter_object, doc=doc)

    result = type(typename, (tuple,), class_namespace)

题中p = Point(4, 3, 2)创建时，p.x, p.y和p.z并非实例属性，而是Point中修饰过的property object
大概意思就是求p.x时，该方法继承于Point，调用Point.x，即返回传入p的第0个对象，当Point.x被赋值为8后，p.x的调用仍按此路线走，所以返回了8，造成类属性覆盖实例属性的现象