概述

运行时，区别于编译时，指的是程序被加载到内存中执行的时候 反射，reflection，指的是运行时获取类型定义信息 一个对象能够在运行时，像照镜子一样，反射处其类型信息 简单说，在python中，能够通过一个对象，找出其type、class、attribute或method的能力，称为反射或者自省 具有反射能力的函数有type()、isinstance()、callable()、dir()、getattr()等 反射相关的函数和方法 需求 有一个Point类，查看它实例的属性，并修改它。动态为实例增加属性

class Point: def __init__(self, x, y): self.x = x self.y = y def __str__(self): return "Point({}, {})".format(self.x, self.y) def show(self): print(self.x, self.y) p = Point(4, 5) print(p) # Point(4, 5) print(p.__dict__) # {'x': 4, 'y': 5} p.__dict__['y'] = 16 # {'x': 4, 'y': 16} print(p.__dict__) # {'x': 4, 'y': 16, 'z': 10} p.z = 10 print(p.__dict__) print(dir(p)) print(p.__dir__())

上例通过属性字典__dict__来访问对象的属性，本质上也是利用的反射的能力。 Python提供了内置的函数

内建函数意义getattr(object,name[,default])通过name返回object的属性值。当属性不存在，将使用default返回，如果没有default，则抛出AttributeError。name必须为字符串setattr(object,name,value)object的属性存在，则覆盖，不存在，新增hasattr(object,name)判断对象是否有这个名字的属性，name必须为字符串 class Point: def __init__(self, x, y): self.x = x self.y = y def __str__(self): return "Point({}, {})".format(self.x, self.y) def show(self): print(self) p1 = Point(4, 5) p2 = Point(10, 10) print(repr(p1), repr(p2)) print(p1.__dict__) # {'x': 4, 'y': 5} setattr(p1, 'y', 16) setattr(p1, 'z', 16) print(getattr(p1, '__dict__')) # {'x': 4, 'y': 16, 'z': 16} # 动态调用方法 if hasattr(p1, 'show'): getattr(p1, 'show')() # Point(4, 16) # 动态增加方法 # 为类增加方法 if not hasattr(Point, 'add'): setattr(Point, 'add', lambda self,other: Point(self.x +other.x, self.y + other.y)) print(Point.add) # <function <lambda> at 0x000002018E6E51E0> print(p1.add) # <bound method <lambda> of <__main__.Point object at 0x000001EC6C2A86A0>> print(p1.add(p2)) # 绑定 # Point(14, 26) # 为实例增加方法，未绑定 if not hasattr(p1, 'sub'): setattr(p1, 'sub', lambda self, other: Point(self.x - other.x, self.y - other.y)) print(p1.sub(p1, p1)) # Point(0, 0) print(p1.sub) # <function <lambda> at 0x0000018C324E5620> # add在谁里面， sub在谁里面 print(p1.__dict__) # {'x': 4, 'y': 16, 'z': 16, 'sub': <function <lambda> at 0x00000180472D5620>} print(Point.__dict__) # {'__module__': '__main__', '__init__': <function Point.__init__ at 0x00000180472D5158>,\ # '__str__': <function Point.__str__ at 0x00000180472D5268>, 'show': <function Point.show at 0x00000180472D5598>, \ # '__dict__': <attribute '__dict__' of 'Point' objects>, '__weakref__': <attribute '__weakref__' of 'Point' objects>,\ # '__doc__': None, 'add': <function <lambda> at 0x00000180472D51E0>}

思考

这种动态增加属性的方式和装饰器修饰一个类、Mixin方式的差异在哪里

这种动态增删属性的方式是运行时改变类或者实例的方式，但是装饰器或Mixin都是定义时就决定了，因此反射能力具有更大的灵活性

练习

命令分发器，通过名称找到对应的函数执行

思路：名称找对象的方法

class Dispatcher: def __init__(self): pass def reg(self, name, fn): setattr(self, name, fn) def run(self): while True: cmd = input('>>>').strip() if cmd == 'quit': break getattr(self, cmd, lambda :print('Unknowm Cmd {}'.format(cmd)))() dis = Dispatcher() dis.reg('ls', lambda :print('ls')) dis.run()

上例中使用getattr方法找到对象的属性的方式，比自己维护一个字典来建立名词和函数之间的关系的方式好多了

反射相关的魔术方法

__getattr__()、__setattrr__()、__delattr__()

__getattr__()

class Base: n = 0 class Point(Base): z = 6 def __init__(self, x, y): self.x = x self.y = y def show(self): print(self.x, self.y) def __getattr__(self, item): return "missing {}".format(item) p1 = Point(4,5) print(p1.x) # 4 print(p1.z) # 6 print(p1.n) # 0 print(p1.y) # 5

实例属性会按照继承关系找，如果找不到，就会执行__getattr__()方法，如果没有这个方法，就会抛出AttrbuteError异常表示找不到属性

查找属性的顺序： instance.__dict__ --> instance.__class__.__dict__ --> 继承的祖先类(直到object)的__dict --> 找不到调用__getattr__()

__setattrr__()

class Base: n = 0 class Point(Base): z = 6 def __init__(self, x, y): self.x = x self.y = y def show(self): print(self.x, self.y) def __getattr__(self, item): return "missing {}".format(item) def __setattr__(self, key, value): print("setattr {}={}".format(key,value)) p1 = Point(4,5) print(1, p1.x) print(2, p1.z) print(3, p1.n) print(4, p1.t) p1.x = 50 print(5, p1.x) print(6, p1.__dict__) p1.__dict__['x'] = 60 print(7, p1.__dict__) print(8, p1.x) 执行结果 setattr x=4 setattr y=5 1 missing x 2 6 3 0 4 missing t setattr x=50 5 missing x 6 {} 7 {'x': 60} 8 60

实例通过.点号设置属性，例如self.x = x属性赋值，就会调用__setattr_-()，属性要加到实例的__dict__中，就需要自己完成

class Point(Base): z = 6 def __init__(self, x, y): self.x = x self.y = y def show(self): print(self.x, self.y) def __getattr__(self, item): return "missing {}".format(item) def __setattr__(self, key, value): print("setattr {}={}".format(key,value)) self.__dict__[key] = value # 操作字典

__setattr__()方法，可以拦截对实例属性的增加、修改操作，如果要设置生效，需要自己操作实例的__dict__

例子

__getattr__和__setattr__综合使用

class B: b = 200 class A(B): z = 100 d = {} def __init__(self, x, y): self.x = x setattr(self, 'y', y) self.__dict__['a'] = 5 def __getattr__(self, item): print('------', item) return self.d[item] def __setattr__(self, key, value): print('setattr key ', key) print('setattr key ', value) self.d[key] = value def __delattr__(self, item): print('can not del {}'.format(item)) a = A(4, 5) print(a.__dict__) print(A.__dict__) print(a.x , a.y) print(a.a) # 执行结果 setattr key x setattr key 4 setattr key y setattr key 5 {'a': 5} {'__module__': '__main__', 'z': 100, 'd': {'x': 4, 'y': 5},\ '__init__': <function A.__init__ at 0x000001062E6C5158>,\ '__getattr__': <function A.__getattr__ at 0x000001062E6C5268>, \ '__setattr__': <function A.__setattr__ at 0x000001062E6C5598>, \ '__delattr__': <function A.__delattr__ at 0x000001062E6C5620>, \ '__doc__': None} ------ x ------ y 4 5 5

__delattr__()

class Point: Z = 5 def __init__(self, x, y): self.x = x self.y = y def __delattr__(self, item): print('Can not del {}'.format(item)) p = Point(4, 5) del p.x p.z = 15 del p.z del p.Z print(Point.__dict__) print(p.__dict__) del Point.Z print(Point.__dict__)

执行结果

Can not del x Can not del z Can not del Z {'__module__': '__main__', 'Z': 5, '__init__': <function Point.__init__ at 0x0000029FBF8B5158>, \ '__delattr__': <function Point.__delattr__ at 0x0000029FBF8B5268>, '__dict__': <attribute '__dict__' of 'Point' objects>, \ '__weakref__': <attribute '__weakref__' of 'Point' objects>, '__doc__': None} {'x': 4, 'y': 5, 'z': 15} {'__module__': '__main__', '__init__': <function Point.__init__ at 0x0000029FBF8B5158>, \ '__delattr__': <function Point.__delattr__ at 0x0000029FBF8B5268>, '__dict__': <attribute '__dict__' of 'Point' objects>,\ '__weakref__': <attribute '__weakref__' of 'Point' objects>, '__doc__': None}

可以阻止通过实例来删除属性的操作。但是通过类依然可以删除属性

__getattribute__()

class Base: n = 0 class Point(Base): z = 6 def __init__(self, x , y): self.x = x self.y = y def __getattr__(self, item): return "missing {}".format(item) def __getattribute__(self, item): return item p1 = Point(4, 5) print(p1.__dict__) # __dict__ print(p1.x) # x print(p1.z) # z print(p1.n) # n print(p1.t) # t print(Point.__dict__) # {'__module__': '__main__', 'z': 6, '__init__': <function Point.__init__ at 0x0000016930285158>,\ '__getattr__': <function Point.__getattr__ at 0x0000016930285268>, \ '__getattribute__': <function Point.__getattribute__ at 0x0000016930285598>, '__doc__': None} print(Point.z) # 6

实例的所有的属性访问，第一个都会调用__getattribute__方法，它阻止了属性的查找，该方法应该返回（计算后的）值或者抛出一个AttributeError异常

它的return值将作为属性查找的结果如果抛出AttributeError异常，则会直接调用__getattr__方法，表示属性没有找到 class Base: n = 0 class Point(Base): z = 6 def __init__(self, x , y): self.x = x self.y = y def __getattr__(self, item): return "missing {}".format(item) def __getattribute__(self, item): # raise AttributeError("Not Found") # pass # return self.__dict__[item] # RecursionError return object.__getattribute__(self, item) p1 = Point(4, 5) print(p1.__dict__) # {'x': 4, 'y': 5} print(p1.x) # 4 print(p1.z) # 6 print(p1.n) # 0 print(p1.t) # missing t print(Point.__dict__) # {'__module__': '__main__', 'z': 6, '__init__': <function Point.__init__ at 0x0000029AF7765158>, \ '__getattr__': <function Point.__getattr__ at 0x0000029AF7765268>, \ '__getattribute__': <function Point.__getattribute__ at 0x0000029AF7765598>, '__doc__': None} print(Point.z) # 6 __getattribute__方法中为了避免在该方法中无限的递归，它的实现应该永远调用基类的同名方法打以访问需要的任何属性，例如object.__getattribute__(self, item)除非明确地知道__getattribute__方法用来做什么，否则不要使用它

总结

魔术方法意义__getattr__()当通过搜索实例、实例的类及祖先类查不到属性，就会调用此方法__setattr__()通过.点访问实例属性，进行增加、修改都要调用它__delattr__()当通过实例来删除属性时调用此方法__getattribute__()实例所有的属性调用都从这个方法开始 属性查找顺序： 实例调用__getattribute__() --> instance.__dict__ --> instance.__class__.__dict__ --> 继承的祖先类(直到object)的__dict__–>调用__getattr__()