Python __getattr__ __getattribute__ __setattr__

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__getattr__

如果某类定义了__getattr__，同时系统在该类对象的实例字典中又找不到待查询的属性，系统就会调用这个方法,这种行为非常适合无结构数据的按需访问。

class A:def __init__(self):self.name = 'Tom'def __getattr__(self,name):setattr(self, name, name)return "can not find %s attr" %name
>>> a = A()
>>> a.name									# name属性本身在实例字典里，所以绝对不会触发__getattr__
'Tom'
>>> a.age									# age 会触发__getattr__，又调用setattr方法放入属性字典
'can not find age attr'
>>> 
>>> a = A()
>>> a.__dict__
{'name': 'Tom'}
>>> a.name
'Tom'
>>> a.age
'can not find age attr'
>>> a.__dict__
{'name': 'Tom', 'age': 'age'}

__getattribute__

程序每次访问对象属性时，Python系统都会调用这个特殊的方法，即使属性字典里面已经有了改属性，也依然会触发__getattribute__。

class B(object):def __init__(self):self.name = 'Frank'def __getattribute__(self, name):print('Called __getattribute__(%s)' % name)try:return super().__getattribute__(name)except AttributeError:setattr(self, name, name)return "can not find %s attr" %name
>>> b = B()
>>> b.name
Called __getattribute__(name)
'Frank'
>>> b.age
Called __getattribute__(age)
'can not find age attr'
>>> b.age
Called __getattribute__(age)
'age'

如果类还定义了__getattr__方法,除非__getattribute__调用它或者出现AttributeError错误，否则__getattr__方法不会被调用。

class B(object):def __init__(self):self.name = 'Frank'def __getattribute__(self, name):print('Called __getattribute__(%s)' % name)super().__getattribute__(name)def __getattr__(self, name):print('Called __getattr__(%s)' % name)
>>> b = B()
>>> b.a
Called __getattribute__(a)
Called __getattr__(a)

__getattribute__方法下应该避免使用self.attr访问属性，否则会出现无限递归。如要要访问实例属性，应该直接通过super()来做。

class B(object):def __init__(self):self.name = 'Frank'def __getattribute__(self, name):
#        return self.__dict__['name']				#导致无限递归return super().__getattribute__(name)		#正常
>>> b = B()
>>> b.name

__setattr__ 
只要对实例属性赋值，无论是直接赋值，还是通过内置的setattr赋值，都会触发__setattr__方法。__setattr__下还有self.attr的赋值操作就会出现无线递归的调用__setattr__的情况。一般情况都还是继承

class C:def __setattr__(self, name, value):print('Called __setattr__(%s, %r)' % (name, value))super().__setattr__(name, value)
>>> c = C()
>>> c.a = 1
Called __setattr__(a, 1)
>>> c.a
1
>>> c.b = 2
Called __setattr__(b, 2)
>>> setattr(c,'d',2)
>>> c.__dict__
{'a': 1, 'd': 2, 'b': 2}

hasattr函数来判断对象是否拥有相关的属性，并用内置的getattr函数来获取属性值。这些函数会先在实例字典中搜索待查询的属性。然后再调用__getattr__

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