Jusene's Blog

python 反射

python
字数统计: 636阅读时长: 3 min
2018/06/17 Share

反射

反射就是通过字符串的形式，导入模块；通过字符串的形式，去模块寻找指定函数，并执行。利用字符串的形式去对象（模块）中操作（查找/获取/删除/添加）成员，一种基于字符串的事件驱动！

反射可以在运行的时候获取类的信息，如:

1
class Point:
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    def __init__(self, x, y):
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        self.x = x
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        self.y = y
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    def print(self):
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        return self.x + self.y
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p=Point(1,2)
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p.__dict__
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{'x': 1, 'y': 2}

* getattr setattr hasattr *

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getattr(p, 'print')()
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3
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setattr(p,'example','python')
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p.example
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python
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hasattr(p,'example')
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True

setattr对象是实例，给函数动态增加方法需要先把函数转化为方法，转化方法types.MethodType

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def mm(self):
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    print(self.x)
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import types
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setattr(p,'mm',types.MethodType(mm,p))
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p.mm()
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1

通过getattr获取本身的函数执行:

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class Command:
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    def cmd1(self):
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        print('cmd1')
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    def cmd2(self):
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        print('cmd2')
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    def run(self):
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        while True;
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            cmd = input('>>>')
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            if cmd == 'quit':
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                return 
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            getattr(self, cmd, lambda x:print('not found cmd {}'.format(cmd)))()

* getattr setattr delattr *

  • getattr 
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class A:
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    def __init__(self,x):
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        self.x = x
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    def __getattr__(self,name):
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        return 'can not found {} property'.format(name)
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a=A(1)
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a.x
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1
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a.y
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can not found y property

当一个类定义了__getattr__方法时，如果访问不存在的成员，会调用__getattr__方法

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a=A(1)
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a.__dict__
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{'x': 1}
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a.__class__.__dict__
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mappingproxy({'__dict__': <attribute '__dict__' of 'A' objects>,
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              '__doc__': None,
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              '__getattr__': <function __main__.A.__getattr__>,
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              '__init__': <function __main__.A.__init__>,
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              '__module__': '__main__',
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              '__weakref__': <attribute '__weakref__' of 'A' objects>})
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属性查找:
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__dict__ > __class__.__dict__ > getattr
  • setattr
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class A:
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    def __init__(self,n):
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        self.x = n
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    def __setattr__(self,name,value):
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        print('set {} to {}'.format(name,value))
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a = A(1)
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set x to 1
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a.x
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AttributeError: 'A' object has no attribute 'x'
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a.__dict__
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{}
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class A:
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    def __init__(self,n):
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        self.x = n
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    def __setattr__(self,name,value):
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        print('set {} to {}'.format(name,value))
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        self.__dict__[name] = value
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a = A(1)
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set x to 1
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a.x
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1
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a.__dict__
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{'x': 1}

当一个类实现__setattr__时，任何地方对这个类的对象增加属性，或者对现有属性赋值，都会调用__setattr__

当需要对实例属性修改，做一些额外操作的时候，可以使用__setattr__

  • delattr
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class A:
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    def __init__(self):
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        self.x = 3
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    def __delattr__(self, name):
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        print('can not remove property {}'.format(name))
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a=A()
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del a.x
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can not remove property x
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a.y = 2
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del a.y
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can not remove property y

当一个类实现__delattr__时，任何地方对这个类的对象删除属性，都会调用__delattr__

  • getattribute
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class A:
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    def __init__(self):
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        self.x = 3
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    def __getattribute__(self,name):
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        return 'haha'
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a = A()
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a.x
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haha
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__getattribute__ > __dict__ > __class__.__dict__ > __getattr__

原文作者：Zhang Jusene

原文链接：http://jusene.github.io/2018/06/17/python-reflex/

发表日期：June 17th 2018, 1:59:35 pm

更新日期：November 30th 2019, 1:54:24 am

版权声明：本文采用知识共享署名-非商业性使用 4.0 国际许可协议进行许可

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