python 屬性搜索優先級和descriptor

屬性搜索優先級和descriptor

########################
# 定義
########################

# 數據描述符
class DataDesc(object):
    def __init__(self, *args, **kwargs):
        pass

    def __get__(self, instance, cls):
        print(instance is None)
        return 'DataDesc'

    def __set__(self, instance, value):
        return None

    def __delete__(self, instance):
        return None


# 數據描述符
class DataDesc2(object):
    def __init__(self, *args, **kwargs):
        pass

    def __get__(self, instance, cls):
        return 'DataDesc2'

    def __set__(self, instance, value):
        return None


# 非數據描述符
class NonDataDesc(object):
    def __init__(self, *args, **kwargs):
        pass

    def __get__(self, instance, cls):
        print(instance is None)
        return 'NonDataDesc'


class Foo(object):
    test_attr = DataDesc()
    test_attr2 = DataDesc()
    test_attr3 = NonDataDesc()
    test_attr4 = 'test_attr4'

    def __getattr__(self, item):
        return 'hehe__getattr__'


########################
# 一般情況
########################
foo = Foo()
'''
1. foo.__dict__['test_attr4']
2. for cls in Foo.__mro__:
       cls.__dict__['test_attr4']
3. foo.__getattr__('test_attr4')
'''
print(foo.test_attr4)
foo.__dict__['test_attr4'] = 666
print(foo.test_attr4)
print(id(foo.test_attr4) == id(foo.__dict__['test_attr4']))
foo.__dict__.pop('test_attr4')
print(foo.test_attr4)
print(foo.test_attr5)
print(foo.__getattr__('test_attr5'))

########################
# 數據型描述符
########################
foo = Foo()
'''
1. type(foo).__dict__['test_attr'].__get__(foo, type(foo))
2. foo.__dict__['test_attr4']
3. for cls in Foo.__mro__:
       cls.__dict__['test_attr4']
4. foo.__getattr__('test_attr4')
'''
print(foo.test_attr)
foo.__dict__['test_attr'] = 666
print(foo.test_attr)
print(id(foo.test_attr) == id(foo.__dict__['test_attr']))
delattr(Foo, 'test_attr')
print(foo.test_attr)
print(id(foo.test_attr) == id(foo.__dict__['test_attr']))

########################
# 非數據型描述符
########################
foo = Foo()
'''
1. foo.__dict__['test_attr4']
2. type(foo).__dict__['test_attr'].__get__(foo, type(foo))
3. for cls in Foo.__mro__:
       cls.__dict__['test_attr4']
4. foo.__getattr__('test_attr4')
'''
print(foo.test_attr3)
foo.__dict__['test_attr3'] = 666
print(foo.test_attr3)
print(id(foo.test_attr3) == id(foo.__dict__['test_attr3']))
del foo.test_attr3
print(foo.test_attr3)

########################
# 類調用描述符
########################
'''
FOO.__dict__['test_attr'].__get__(None, FOO)
'''
print(Foo.test_attr2)
print(Foo.test_attr3)

########################
# __getattribute__
########################
'''
如果被重寫,上面代碼介紹的東西會被覆蓋掉
除非函數內部調用object.__getattribute__
否則別這麼幹
'''


class Foo2(object):
    test_attr = DataDesc()
    test_attr2 = DataDesc()
    test_attr3 = NonDataDesc()
    test_attr4 = 'test_attr4'

    def __getattribute__(self, item):
        return 'in __getattribute__'


foo = Foo2()
print(foo.test_attr3)

########################
# 例子
########################
import time


class LazyProperty(object):
    def __init__(self, func):
        self.func = func

    def __get__(self, instance, cls):
        if instance is not None:
            val = self.func(instance)
            setattr(instance, self.func.__name__, val)
            return val


class Foo3(object):
    @LazyProperty
    def method(self):
        time.sleep(5)
        return 666


foo3 = Foo3()
time1 = time.time()
print(foo3.method)
time2 = time.time()
print(time2 - time1)
print(foo3.method)
print(time.time() - time2)