Python3中的單例模式 hasattr() getattr() serattr() delattr()

#python中單例模式
#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)函數用於判斷是否包含對應的屬性
#參數： object------對象 name------字符串，屬性名 #返回值： 如果對象有該屬性返回True,否則返回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.使用裝飾器
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

 

裝飾器的用法（小應用）

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

@outer #解釋器執行 foo=outer(foo) def foo(): return 1

print(foo) foo()

 

這個過程中執行了下面幾步

1.函數 foo 作為 裝飾器 outer 的參數被傳入 2.函數 inner 對 func 進行調用，然后裝飾器 outer 返回 inner 3.原來的函數名 foo 關聯到 inner，如上面的foo 所示，調用 foo時間上是在調用 inner

In [ ]:

#裝飾器的應用小例子
#將對象作為函數的參數，去判斷對象參數 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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#使用元類，可以控制類的創建過程
class Singleton(type): '''元類繼承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)) ''' 輸出結果： 2473667780824 2473667780824 '''

#裝飾器還可以為類增加額外的成員

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 則可以將原函數對象的指定屬性復制給包裝函數對象, 默認有 __module__、__name__、__doc__,或者通過參數選擇
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裝飾器（decorator）在實現的時候，被裝飾后的函數其實已經是另外一個函數了（函數名等函數屬性會發生改變）， 為了不影響，Python的functools包中提供了一個叫wraps的decorator來消除這樣的副作用。寫一個decorator的時候， 最好在實現之前加上functools的wrap，它能保留原有函數的名稱和docstring。 

In [17]:

#使用元類，可以控制類的創建過程
class Singleton(type): '''元類繼承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()函數
#用於返回一個對象的屬性值 #getattr(object,name,default) ''' 參數 ： object-----對象 name------字符串，對象屬性 default ----默認返回值，如果不提供該參數，在沒有對象屬性時，將觸發AttributeError 返回值： 放回對象屬性值 ''' 

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)#沒有屬性的話，又沒有默認屬性參數的話，就會報錯 ''' obj=getattr(People,'conutryjjjjj',None) print(obj) 

 

chaina
None

In [24]:

#setattr()函數
''' setattr函數，用於設置屬性值，該屬性必須存在 語法：setattr(object,name,value) 參數： object----對象 name------字符串，對象屬性 value-----屬性值 返回值： 無 ''' 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)#等同於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()函數
''' delattr函數用於刪除屬性 delattr(x,'foobar')相當於del x.doobar 語法： setattr(object,name) 參數： object ---對象 name--必須是對象的屬性 返回值： 無 ''' class People: country='China' def __init__(self,name): self.name=name def people_info(self): print('%s is xxx' %(self.name)) delattr(People,'country') #等同於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.....