Python full stack built-in functions and double-click method

Two built-in functions:

There are many built-in functions, two are documented here.

1. isinstance () Determine the type of object, including inheritance
2. issubclass () to determine the inheritance relationship between classes

class A:
	pass
class B(A):
	pass
b = B()
print(isinstance(b,B))
print(isinstance(b,A))
#两个都返回True

class A:pass
class B(A):pass
print(issubclass(B,A)) #返回True,B是A的子类
print(issubclass(A,B)) #返回False

___first name__:

There are four
ways to call it: it is a special method in the class,
double down method,
magic method,
built-in method

_call_:

class A:
	def __call__(self,*args,**kwargs):
		print("执行call方法")
a = A()
a() #对象名() 相当于调用__call__方法
#或者
a = A()()

___len ___:

class Mylst:
	def __init__(self):
		self.lst = [1,2,3,4,5,6]
		self.name = 'alex'
	def __len__(self):
		print("执行__len__了")
		return len(self.lst)
l = Mylst()
print(len(l)）

len (obj) is equivalent to calling the obj's __len__ method
_ len_ method return value is the return value of the len function
If an obj object does not have a __len__ method, then the len function will report an error

___new ___: Constructor

class Single:
    def __new__(cls, *args, **kwargs):
        obj = object.__new__(cls)
        print("在new方法里",obj)
        return obj

    def __init__(self):
        print("在init方法里",self)
s = Single()

1. Open a space that belongs to the object
2. Pass the space of the object to self, execute init
3. Return the space of this object to the caller