class Bar(object):
    def __init__(self, a):
        self.a = a


class BarSlotted(object):
    __slots__ = "a",

    def __init__(self, a):
        self.a = a


# create class instance
bar = Bar(1)
bar_slotted = BarSlotted(1)

print(set(dir(bar)) - set(dir(bar_slotted)))
# {'__dict__', '__weakref__'}   
'''
使用 __slots__ 后, 類里面會減少 __dict__  __weakref__ 兩個方法。
__weakref__  --- 弱引用  詳情鏈接 https://docs.python.org/zh-cn/3/library/weakref.html
'''

class BarSlotted(object):
    __slots__ = "a",'__dict__'

    def __init__(self, a):
        self.a = a
        
bar_slotted = BarSlotted(1)
bar_slotted.b = "111"

class Parent(object):
    __slots__ = "x", "y"


class Child(Parent):
    __slots__ = "z",
    # 重復(fù)的 x, y 可以不寫,
    # __slots__ = "x", "y", "z"


child = Child()
print(dir(child)) 
# [..., 'x', 'y', 'z']

class ParentA(object):
    __slots__ = "x",


class ParentB(object):
    __slots__ = "y",


class Child(ParentA, ParentB):
    pass

'''
Traceback (most recent call last):
  File "C:/Users/15284/PycharmProjects/pythonProject/test.py", line 69, in <module>
    class Child(ParentA, ParentB):
TypeError: multiple bases have instance lay-out conflict
'''

class AbstractA(object):
  __slots__ = ()

class AbstractB(object):
  __slots__ = "x"

class Child(AbstractA, AbstractB):
  __slots__ = "x", "y"