python中的魔法方法
__init__:类实例化会触发
class Student():
def __init__(self):
print('类实例化会触发__init__方法')
Student() # 类实例化会触发__init__方法
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__str__:打印对象会触发
class Student:
def __init__(self,name,age):
self.name = name
self.age = age
# def __str__(self):
# return '%s的年龄是：%d' % (self.name,self.age)
yuan = Student('yuan',20)
print(yuan)
# 不定义__str__的结果：<__main__.Student object at 0x000002045FC47C40>
# 定义__str__的结果：yuan的年龄是：20
# 调用时机
使用print打印对象（类的实例化得到的对象）的时候会使用到__str__方法
1.如果没有定义__str__方法，print(对象)则会打印对象的引用地址
2.如果自己定义了__str__方法，print(对象)则会打印方法的返回值
# 注意事项
必须使用return且返回字符串类型
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__call__:对象()触发，类也是对象 类(),类的实例化过程调用元类的__call__
# 作用：将实力换对象变成一个可调用对象（判断一个对象是否是可调用对象：通过内置函数callable判断）
print(callable(对象)) # True
class People(object):
def __init__(self,name):
self.name=name
def __call__(self,friend):
print(self.name)
a = People('yuan')
a.__call__('yue') # 调用方法一
a('yue') # 调用方法二 a('yue')等价与a.__call__('yue')
# 当没有定义__call__时，TypeError: 'People' object is not callable
# 当定义了__call__时，则执行__call__方法
__new__:在类实例化会触发，它比__init__早
实例1：先调用__new__()方法再调用__init__()方法
class Person(object):
def __new__(cls):
print("__new__ called")
return super().__new__(cls)
def __init__(self):
print("__init__ called")
a = Person()
# 结果：
__new__ called
__init__ called
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实例2：__new__()方法构造一个类实例，并将该实例传递给自身的__init__()方法，即__init__()方法的self参数
class Person(object):
def __new__(cls):
print("__new__ called")
instance = super().__new__(cls)
print(type(instance))
print(instance)
print(id(instance))
return instance
def __init__(self):
print("__init__ called")
print(id(self))
b = Person()
# 结果：
__new__ called
<class '__main__.Person'>
<__main__.Person object at 0x1093c1580>
4449899904
__init__ called
4449899904
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实例3：如果__new__()方法不返回任何实例的话，init()方法将不会被调用
class Person(object):
def __new__(cls):
print("__new__ called")
def __init__(self):
print("__init__ called")
c = Person()
＃ 结果：
__new__ called
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实例4：如果__new__()方法返回一个其他类的实例的话，那它自身的__init__()方法将不会被调用。而且，new()方法将会初始化一个其他类的对象
class Animal(object):
def __init__(self):
print("animal __init__")
class Person(object):
def __new__(cls):
print("__new__ called")
return Animal()
def __init__(self):
print("__init__ called")
d = Person()
print(type(d))
print(d)
# 结果：
__new__ called
animal __init__
<class '__main__.Animal'>
<__main__.Animal object at 0x000002CEF4DBF010>
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实例5：如果重写__new__()方法时，除了cls参数外不再设置其他参数的话，将无法用__init__()方法来设置初始化参数
class Person(object):
def __new__(cls):
print("__new__ called")
instance = super().__new__(cls)
return instance
def __init__(self, name):
print("__init__ called")
self.name = name
e = Person("Eric")
print(e.name)
# 结果：
Traceback (most recent call last):
File "example.py", line 102, in <module>
e = Person("Eric")
TypeError: __new__() takes 1 positional argument but 2 were given
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实例6：在重写__new__()方法时，需要在参数中加入*args,**kwargs，或者显式地加入对应的参数，才能通过__init__()方法初始化参数
class Person(object):
def __new__(cls, *args,**kwargs): # Or def __new__(cls, name)
print("__new__ called")
instance = super().__new__(cls)
return instance
def __init__(self, name):
print("__init__ called")
self.name = name
e = Person("Eric")
print(e.name)
# 结果：
__new__ called
__init__ called
Eric
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__del__:del 对象和对象回收的时候触发
class Animal(object):
def __init__(self):
print("animal __init__")
class Person:
def __del__(self):
print("销毁对象：{0}".format(self))
p1 = Person() # 5.销毁对象：<__main__.Person object at 0x000001DFCD279FC8>
print(id(p1)) # 1. 2060731260872
p2 = Person() # 3. 销毁对象：<__main__.Person object at 0x000001DFCD284088>
print(id(p2)) # 2. 2060731302024
del p2
print("over") # 4. over
# 2063785424400
# 2063785422432
# 销毁对象：<__main__.Person object at 0x000001E083326E60>
# over
# 销毁对象：<__main__.Person object at 0x000001E083327610>
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__setattr__，__getattr__:（.拦截方法），当对象.属性-->赋值会调用__setattr__方法，如果取值会调用__getattr__方法
标签：__,暗室逢灯,self,拉萨,init,艰苦,print,new,def From： https://www.cnblogs.com/yuanxiaojiang/p/17403417.html