我试图覆盖Python类的__setattr__
方法,因为每次实例属性更改其值时我都想调用另一个函数。但是,我不希望在__init__
方法中出现这种情况,因为在此初始化期间,我设置了一些将在以后使用的属性:
到目前为止,我有这个解决方案,但没有在运行时覆盖__setattr__
:
class Foo(object):
def __init__(self, a, host):
object.__setattr__(self, 'a', a)
object.__setattr__(self, 'b', b)
result = self.process(a)
for key, value in result.items():
object.__setattr__(self, key, value)
def __setattr__(self, name, value):
print(self.b) # Call to a function using self.b
object.__setattr__(self, name, value)
但是,我想避免这些object.__setattr__(...)
并在__setattr__
方法的末尾覆盖__init__
:
class Foo(object):
def __init__(self, a, b):
self.a = a
self.b = b
result = self.process(a)
for key, value in result.items():
setattr(self, key, value)
# override self.__setattr__ here
def aux(self, name, value):
print(self.b)
object.__setattr__(self, name, value)
我尝试了self.__dict__['__setitem__'] = self.aux
和object.__setitem__['__setitem__'] = self.aux
,但这些尝试都没有效果。我已阅读this section of the data model reference,但看起来自己的__setattr__
的分配有点棘手。
如何才能在__setattr__
结束时覆盖__init__
,或者至少有一个pythonic解决方案,只在构造函数中以正常方式调用__setattr__
?
答案 0 :(得分:23)
不幸的是,没有办法“在init之后覆盖”python特殊方法;作为查找工作方式的副作用。问题的症结在于python实际上并没有查看实例;除了上课;在它开始查找特殊方法之前;所以没有办法让对象的状态影响查找哪个方法。
如果您不喜欢__init__
中的特殊行为,则可以重构代码,将特殊知识放在__setattr__
中。类似的东西:
class Foo(object):
__initialized = False
def __init__(self, a, b):
try:
self.a = a
self.b = b
# ...
finally:
self.__initialized = True
def __setattr__(self, attr, value):
if self.__initialzed:
print(self.b)
super(Foo, self).__setattr__(attr, value)
编辑:实际上,有一种方法可以改变查找哪种特殊方法,只要你在初始化后更改其类即可。这种方法会让你深入到元类的杂草中,所以没有进一步的解释,这就是它的外观:
class AssignableSetattr(type):
def __new__(mcls, name, bases, attrs):
def __setattr__(self, attr, value):
object.__setattr__(self, attr, value)
init_attrs = dict(attrs)
init_attrs['__setattr__'] = __setattr__
init_cls = super(AssignableSetattr, mcls).__new__(mcls, name, bases, init_attrs)
real_cls = super(AssignableSetattr, mcls).__new__(mcls, name, (init_cls,), attrs)
init_cls.__real_cls = real_cls
return init_cls
def __call__(cls, *args, **kwargs):
self = super(AssignableSetattr, cls).__call__(*args, **kwargs)
print "Created", self
real_cls = cls.__real_cls
self.__class__ = real_cls
return self
class Foo(object):
__metaclass__ = AssignableSetattr
def __init__(self, a, b):
self.a = a
self.b = b
for key, value in process(a).items():
setattr(self, key, value)
def __setattr__(self, attr, value):
frob(self.b)
super(Foo, self).__setattr__(attr, value)
def process(a):
print "processing"
return {'c': 3 * a}
def frob(x):
print "frobbing", x
myfoo = Foo(1, 2)
myfoo.d = myfoo.c + 1
答案 1 :(得分:5)
@ SingleNegationElimination的答案很棒,但它不能用于继承,因为子类的__mro__
存储是超类的原始类。灵感来自他的答案,几乎没有变化,
这个想法很简单,在__setattr__
之前切换__init__
,并在__init__
完成后将其恢复。
class CleanSetAttrMeta(type):
def __call__(cls, *args, **kwargs):
real_setattr = cls.__setattr__
cls.__setattr__ = object.__setattr__
self = super(CleanSetAttrMeta, cls).__call__(*args, **kwargs)
cls.__setattr__ = real_setattr
return self
class Foo(object):
__metaclass__ = CleanSetAttrMeta
def __init__(self):
super(Foo, self).__init__()
self.a = 1
self.b = 2
def __setattr__(self, key, value):
print 'after __init__', self.b
super(Foo, self).__setattr__(key, value)
class Bar(Foo):
def __init__(self):
super(Bar, self).__init__()
self.c = 3
>>> f = Foo()
>>> f.a = 10
after __init__ 2
>>>
>>> b = Bar()
>>> b.c = 30
after __init__ 2