2.1.2 Python面向對象之反射以及內置方法

點擊跳轉Python筆記總目錄

閱讀目錄

1,isinstance和issubclass

2,反射

• setattr
• delattr
• getattr
• hasattr

3,__str__和__repr__

4,__del__

5,item系列

• __getitem__
• __setitem__
• __delitem__

6,__new__

7,__call__

8,__len__

9,__hash__

10,__eq__

---

1，isinstance和issubclass

isinstance(obj,cls)檢查是否obj是不是類 cls 的對象

class Foo(object):
     pass
  
obj = Foo()
  
isinstance(obj, Foo)
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issubclass(sub, super)檢查sub類是不是 super 類的派生類

class Foo(object):
    pass
 
class Bar(Foo):
    pass
 
issubclass(Bar, Foo)
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2,反射

1 什麼是反射 反射的概念是由Smith在1982年首次提出的，主要是指程序能夠訪問、檢測和修改它自己狀態或行爲的一種能力（自省）。這一律唸的提出很快引起了計算機科學領域關於應用反射性的研究。它首先被程序語言的設計領域所採用,並在Lisp和麪向對象方面取得了成績。 2 python面向對象中的反射： 經過字符串的形式操做對象相關的屬性。python中的一切事物都是對象（均可以使用反射） 四個能夠實現自省的函數 下列方法適用於類和對象(一切皆對象，類自己也是一個對象) hasattr

def hasattr(*args, **kwargs): ## real signature unknown
    """ Return whether the object has an attribute with the given name. This is done by calling getattr(obj, name) and catching AttributeError. """
    pass
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getattr

def getattr(object, name, default=None): ## known special case of getattr
    """ getattr(object, name[, default]) -> value Get a named attribute from an object; getattr(x, 'y') is equivalent to x.y. When a default argument is given, it is returned when the attribute doesn't exist; without it, an exception is raised in that case. """
    pass
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setattr

def setattr(x, y, v): ## real signature unknown; restored from __doc__
    """ Sets the named attribute on the given object to the specified value. setattr(x, 'y', v) is equivalent to ``x.y = v'' """
    pass
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delattr

def delattr(x, y): ## real signature unknown; restored from __doc__
    """ Deletes the named attribute from the given object. delattr(x, 'y') is equivalent to ``del x.y'' """
    pass
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四個方法的使用演示

class Foo:
    f = '類的靜態變量'
    def __init__(self,name,age):
        self.name=name
        self.age=age

    def say_hi(self):
        print('hi,%s'%self.name)

obj=Foo('egon',73)

#檢測是否含有某屬性
print(hasattr(obj,'name'))
print(hasattr(obj,'say_hi'))

#獲取屬性
n=getattr(obj,'name')
print(n)
func=getattr(obj,'say_hi')
func()

print(getattr(obj,'aaaaaaaa','不存在啊')) #報錯

#設置屬性
setattr(obj,'sb',True)
setattr(obj,'show_name',lambda self:self.name+'sb')
print(obj.__dict__)
print(obj.show_name(obj))

#刪除屬性
delattr(obj,'age')
delattr(obj,'show_name')
delattr(obj,'show_name111')#不存在,則報錯

print(obj.__dict__)
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3，str__和__repr

改變對象的字符串顯示__str__,__repr__ 自定製格式化字符串__format__

#_*_coding:utf-8_*_

format_dict={
    'nat':'{obj.name}-{obj.addr}-{obj.type}',#學校名-學校地址-學校類型
    'tna':'{obj.type}:{obj.name}:{obj.addr}',#學校類型:學校名:學校地址
    'tan':'{obj.type}/{obj.addr}/{obj.name}',#學校類型/學校地址/學校名
}
class School:
    def __init__(self,name,addr,type):
        self.name=name
        self.addr=addr
        self.type=type

    def __repr__(self):
        return 'School(%s,%s)' %(self.name,self.addr)
    def __str__(self):
        return '(%s,%s)' %(self.name,self.addr)

    def __format__(self, format_spec):
        ## if format_spec
        if not format_spec or format_spec not in format_dict:
            format_spec='nat'
        fmt=format_dict[format_spec]
        return fmt.format(obj=self)

s1=School('oldboy1','北京','私立')
print('from repr: ',repr(s1))
print('from str: ',str(s1))
print(s1)

''' str函數或者print函數--->obj.__str__() repr或者交互式解釋器--->obj.__repr__() 若是__str__沒有被定義,那麼就會使用__repr__來代替輸出 注意:這倆方法的返回值必須是字符串,不然拋出異常 '''
print(format(s1,'nat'))
print(format(s1,'tna'))
print(format(s1,'tan'))
print(format(s1,'asfdasdffd'))
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%s和%r

class B:

     def __str__(self):
         return 'str : class B'

     def __repr__(self):
         return 'repr : class B'


b=B()
print('%s'%b)
print('%r'%b)


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4，del

析構方法，當對象在內存中被釋放時，自動觸發執行。

注：此方法通常無須定義，由於Python是一門高級語言，程序員在使用時無需關心內存的分配和釋放，由於此工做都是交給Python解釋器來執行，因此，析構函數的調用是由解釋器在進行垃圾回收時自動觸發執行的。 簡單示範

class Foo:

    def __del__(self):
        print('執行我啦')

f1=Foo()
del f1
print('------->')

#輸出結果
執行我啦
------->


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5，item系列

__getitem__ __setitem__ __delitem__

class Foo:
    def __init__(self,name):
        self.name=name

    def __getitem__(self, item):
        print(self.__dict__[item])

    def __setitem__(self, key, value):
        self.__dict__[key]=value
    def __delitem__(self, key):
        print('del obj[key]時,我執行')
        self.__dict__.pop(key)
    def __delattr__(self, item):
        print('del obj.key時,我執行')
        self.__dict__.pop(item)

f1=Foo('sb')
f1['age']=18
f1['age1']=19
del f1.age1
del f1['age']
f1['name']='alex'
print(f1.__dict__)
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6,__new__

class A:
    def __init__(self):
        self.x = 1
        print('in init function')
    def __new__(cls, *args, **kwargs):
        print('in new function')
        return object.__new__(A, *args, **kwargs)

a = A()
print(a.x)
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單例模式

class Singleton:
    def __new__(cls, *args, **kw):
        if not hasattr(cls, '_instance'):
            cls._instance = object.__new__(cls, *args, **kw)
        return cls._instance

one = Singleton()
two = Singleton()

two.a = 3
print(one.a)
## 3
## one和two徹底相同,能夠用id(), , is檢測
print(id(one))
## 29097904
print(id(two))
## 29097904
print(one  two)
## True
print(one is two)
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7,__call__

對象後面加括號，觸發執行。 注：構造方法的執行是由建立對象觸發的，即：對象 = 類名() ；而對於 call 方法的執行是由對象後加括號觸發的，即：對象() 或者 類()()

class Foo:

    def __init__(self):
        pass
    
    def __call__(self, *args, **kwargs):

        print('__call__')


obj = Foo() ## 執行 __init__
obj()       ## 執行 __call__
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8,__len__

class A:
    def __init__(self):
        self.a = 1
        self.b = 2

    def __len__(self):
        return len(self.__dict__)
a = A()
print(len(a))
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9,__hash__

class A:
    def __init__(self):
        self.a = 1
        self.b = 2

    def __hash__(self):
        return hash(str(self.a)+str(self.b))
a = A()
print(hash(a))
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10,__eq__

class A:
    def __init__(self):
        self.a = 1
        self.b = 2

    def __eq__(self,obj):
        if  self.a  obj.a and self.b  obj.b:
            return True
a = A()
b = A()
print(a  b)
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紙牌遊戲

class FranchDeck:
    ranks = [str(n) for n in range(2,11)] + list('JQKA')
    suits = ['紅心','方板','梅花','黑桃']

    def __init__(self):
        self._cards = [Card(rank,suit) for rank in FranchDeck.ranks
                                        for suit in FranchDeck.suits]

    def __len__(self):
        return len(self._cards)

    def __getitem__(self, item):
        return self._cards[item]

deck = FranchDeck()
print(deck[0])
from random import choice
print(choice(deck))
print(choice(deck))
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紙牌遊戲2

class FranchDeck:
    ranks = [str(n) for n in range(2,11)] + list('JQKA')
    suits = ['紅心','方板','梅花','黑桃']

    def __init__(self):
        self._cards = [Card(rank,suit) for rank in FranchDeck.ranks
                                        for suit in FranchDeck.suits]

    def __len__(self):
        return len(self._cards)

    def __getitem__(self, item):
        return self._cards[item]

    def __setitem__(self, key, value):
        self._cards[key] = value

deck = FranchDeck()
print(deck[0])
from random import choice
print(choice(deck))
print(choice(deck))

from random import shuffle
shuffle(deck)
print(deck[:5])
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一道面試題

class Person:
    def __init__(self,name,age,sex):
        self.name = name
        self.age = age
        self.sex = sex

    def __hash__(self):
        return hash(self.name+self.sex)

    def __eq__(self, other):
        if self.name  other.name and self.sex  other.sex:return True


p_lst = []
for i in range(84):
    p_lst.append(Person('egon',i,'male'))

print(p_lst)
print(set(p_lst))
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