Python3中的单例模式 hasattr() getattr() serattr() delattr()

#python中单例模式
#1.使用__new__ class Singleton: def __new__(cls,*args,**kwargs): if not hasattr(cls,'_instance'): cls._instance=super().__new__(cls) return cls._instance s0=Singleton() s1=Singleton() print(id(s0)) print(id(s1)) 

In [10]:

#hasattr(object,name)函数用于判断是否包含对应的属性
#参数： object------对象 name------字符串，属性名 #返回值： 如果对象有该属性返回True,否则返回False class People: country="china" def __init__(self,name): self.name=name def people_info(self): print("%s is xxx"%(self.name)) obj=People('aaa') print(obj.name) print(hasattr(People,'country')) print(hasattr(obj,'name')) print(hasattr(People,'name')) print(hasattr(People,'people_info')) print(hasattr(obj,'people_info')) print(People.__dict__) print(obj.__dict__) 

 

aaa
True
True
False
True
True
{'__module__': '__main__', 'country': 'china', '__init__': <function People.__init__ at 0x0000024A642A7A60>, 'people_info': <function People.people_info at 0x0000024A642A78C8>, '__dict__': <attribute '__dict__' of 'People' objects>, '__weakref__': <attribute '__weakref__' of 'People' objects>, '__doc__': None}
{'name': 'aaa'}

In [14]:

#2.使用装饰器
from functools import wraps def singlenton(cls): instances={} @wraps(cls) def getinstance(*args,**kwargs): if cls not in instances: instances[cls]=cls(*args,**kwargs) return instances return getinstance @singlenton class Bar: pass b0=Bar() b1=Bar() print(id(b0)) print(id(b1)) 

 

2518531933456
2518531933456

 

装饰器的用法（小应用）

def outer(func): def inner(): print("before func") ret=func() return ret+1 return inner #返回inner函数

@outer #解释器执行 foo=outer(foo) def foo(): return 1

print(foo) foo()

 

这个过程中执行了下面几步

1.函数 foo 作为 装饰器 outer 的参数被传入 2.函数 inner 对 func 进行调用，然后装饰器 outer 返回 inner 3.原来的函数名 foo 关联到 inner，如上面的foo 所示，调用 foo时间上是在调用 inner

In [ ]:

#装饰器的应用小例子
#将对象作为函数的参数，去判断对象参数 def wrapper(func): def checker(a,b): if a.x<0 or a.y <0: a=Coordinate(a.x if a.x>0 else 0,a.y if a.y>0 else 0) if b.x<0 or b.y<0: b=Coordinate(b.x if b.x>0 else 0,b.y if b.y>0 else 0) ret=func(a,b) if ret.x<0 or ret.y<0: ret = Coordinate(ret.x if ret.x>0 else 0,re.y if ret.y>0 else 0) return ret return checker class Coordinate(object): def __init__(self,x,y): self.x=x self.y=y def __repr__(self): return "Coord:"+str(self.__dict__) @wrapper def add(a,b): return Coordinate(a.x+b.x,a.y+b.y) @wrapper def sub(a,b): return Coordinate(a.x-b.x,a.y-b.y) one=Coordinate(100,200) two=Coordinate(300,400) three=Coordinate(-100,-100) sub(one,three) add(one,three) sub(one,two) # print(one) 

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def hello(cls): cls.hello=staticmethod(lambda:"hello") return cls @hello class World(object): pass World.hello 

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#使用元类，可以控制类的创建过程
class Singleton(type): '''元类继承type''' _instance={} def __call__(cls,*args,**kwargs): if cls not in cls._instance: cls._instance[cls]=super().__call__(*args,**kwargs) return cls._instance class Bar(metaclass=Singleton): pass b0=Bar() b1=Bar() print(id(b0)) print(id(b1)) ''' 输出结果： 2473667780824 2473667780824 '''

#装饰器还可以为类增加额外的成员

def hello(cls): cls.hello=staticmethod(lambda:"hello") return cls @hello class World(object): pass World.hello #<function __main__.hello.<locals>.<lambda>()> World.hello() 

Out[3]:

'hello'

In [6]:

#functools.wraps 则可以将原函数对象的指定属性复制给包装函数对象, 默认有 __module__、__name__、__doc__,或者通过参数选择
from functools import wraps def logged(func): @wraps(func) def with_logging(*args,**kwargs): print(func.__name__+'was called') return func(*args,**kwargs) return with_logging @logged def f(x): '''does some math''' return x+x*x def f(x): '''does some math''' return x+x*x f=logged(f) f.__name__ #'f' f.__doc__ 

Out[6]:

'does some math'

In [15]:

def logged(func): def with_logging(*args,**kwargs): print(func.__name__+'was called') return func(*args,**kwargs) return with_logging @logged def f(x): '''does some math''' return x+x*x def f(x): '''does some math''' return x+x*x f=logged(f) f.__name__ #'with_logging' print(f.__doc__) 

 

None

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Python装饰器（decorator）在实现的时候，被装饰后的函数其实已经是另外一个函数了（函数名等函数属性会发生改变）， 为了不影响，Python的functools包中提供了一个叫wraps的decorator来消除这样的副作用。写一个decorator的时候， 最好在实现之前加上functools的wrap，它能保留原有函数的名称和docstring。 

In [17]:

#使用元类，可以控制类的创建过程
class Singleton(type): '''元类继承type''' _instance={} def __call__(cls,*args,**kwargs): if cls not in cls._instance: cls._instance[cls]=super().__call__(*args,**kwargs) return cls._instance class Bar(metaclass=Singleton): pass b0=Bar() b1=Bar() print(id(b0)) print(id(b1)) 

 

2473667780824
2473667780824

In [ ]:

#getattr()函数
#用于返回一个对象的属性值 #getattr(object,name,default) ''' 参数 ： object-----对象 name------字符串，对象属性 default ----默认返回值，如果不提供该参数，在没有对象属性时，将触发AttributeError 返回值： 放回对象属性值 ''' 

In [23]:

class People: country='chaina' def __init__(self,name): self.name=name def people_info(self): print('%s is xxx'%(self.name)) obj=getattr(People,'country') print(obj) ''' obj1=getattr(People,'countryjjjjj') print(obj1)#没有属性的话，又没有默认属性参数的话，就会报错 ''' obj=getattr(People,'conutryjjjjj',None) print(obj) 

 

chaina
None

In [24]:

#setattr()函数
''' setattr函数，用于设置属性值，该属性必须存在 语法：setattr(object,name,value) 参数： object----对象 name------字符串，对象属性 value-----属性值 返回值： 无 ''' class People: country='china' def __init__(self,name): self.name=name def people_info(self): print('%s is xxx'%(self.name)) obj=People('aaa') setattr(People,'x',111)#等同于People.x=111 print(People.x) print(obj.__dict__) print(People.__dict__) 

 

111
{'name': 'aaa'}
{'__module__': '__main__', 'country': 'china', '__init__': <function People.__init__ at 0x0000023FF21688C8>, 'people_info': <function People.people_info at 0x0000023FF21686A8>, '__dict__': <attribute '__dict__' of 'People' objects>, '__weakref__': <attribute '__weakref__' of 'People' objects>, '__doc__': None, 'x': 111}

In [25]:

#delattr()函数
''' delattr函数用于删除属性 delattr(x,'foobar')相当于del x.doobar 语法： setattr(object,name) 参数： object ---对象 name--必须是对象的属性 返回值： 无 ''' class People: country='China' def __init__(self,name): self.name=name def people_info(self): print('%s is xxx' %(self.name)) delattr(People,'country') #等同于del People.country print(People.__dict__) 

 

{'__module__': '__main__', '__init__': <function People.__init__ at 0x0000023FF2168D08>, 'people_info': <function People.people_info at 0x0000023FF2168AE8>, '__dict__': <attribute '__dict__' of 'People' objects>, '__weakref__': <attribute '__weakref__' of 'People' objects>, '__doc__': None}

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class Foo: def run(self): while True: cmd=input('cmd>>:'.strip()) if hasattr(self,cmd): func=getattr(self,cmd) func() def download(self): print('download.....') def upload(self): print('upload...') obj=Foo() obj.run() 

 

cmd>>:download
download.....