Python Full Stack Development * 23 Object-oriented Knowledge points summary * 180704

23 Object-oriented
　　　　　    -----Special Methods

1. Isinstance (obj, class name)     determines whether an object is instantiated as such or instantiated by subclasses of this class.
Class A:pass
Class B (A):p
B1=b ()
Print (isinstance(b1,b))     # True
Print (isinstance(b1,a))     # True
2.issubclass (class name, class name 1)    determine if the class name is a subclass of class name 1
Class A:pass
Class B (A):p
Print (issubclass(b,a))     #True
Print (issubclass(b))     #False

3.__len__ Usage
class C:
def __init__ (self):
self.a=1
self.b=2
def __len__ (self):
return len (self.__dict__) 
c1=c ()
Print(len (C1))
4.__hash__ Usage
class C:
def __init__ (self):
self.a=1
self.b=2
def __hash__ (self):
return Hash (str (SELF.A) +str (self.b))
c1=c ()
Print (hash (c1))
5.__str__ Usage If the __str__ method is defined in a class, the return value of the method is output by default when the object is printed
class B:
def __init__ (self,name,age):
Self.name=name
Self.age=age
def __repr__ (self):
return "Taibai"
b2=b ("alex1", +)
print (B2)
Print ("%s"% B2)

6.__repr__ Usage If the __repr__ method is defined in a class, the return value of the method is output by default when Repr (object)
class B:
def __init__ (self,name,age):
Self.name=name
Self.age=age
def __repr__ (self):
print ("666")
return "Taibai"
b1=b ("Alex", +)
Print (repr (B1)) # method One
 Print ("%r"% B1) # method Two
#补充知识: Formatted output
#%s number and string%r + "" repr true Colours%d number%f decimal
The execution of the 7.__CALL__ usage construction method is triggered by the creation object, that is: Object = class name (), and the execution of the __call__ method is triggered by parentheses behind the object .
that is: Object () or Class () ( )
class Foo:
def __init__ (self):
Pass
def __call__ (Self,*args,**kwargs):
print (Args,kwargs)
A1=foo ()
 A1 ("Alex", a=666) # (' Alex ',) {' A ': 666}

The 8.__eq__ usage automatically triggers when a class instantiates two objects for comparison operations.
class A:
def __init__ (self,):
self.a=1
self.b=2
def __eq__ (self,obj): # Need to manually pass a parameter
if Self.a==obj.a and self.b==obj.b:
return True
a1=a ()
b1=a ()
print (a1==b1)
9.__del__ Usage Analysis Method
#Python garbage collection mechanism :
#文件中你创建的所有的变量, class, and so on, all variables, classes are marked during execution, and after execution, they are automatically reclaimed for a period of time without being called.
class A:
def __del__ (self):
print ("666")
a1=a ()
a1.__del__

10.__new__ Usage
class A:
def __init__ (self):
self.x=1
print ("in init function") # Fifth Step
def __new__ (Cls,*args,**kwargs): # The second step automates the __new__ method
print (CLS) # Third Step <class ' __main__. A ' >
print ("in new Function") # Fourth Step

return super (). __new__ (CLS) # Returns an object space
return object.__new__ (CLS) the second method
A1=a () # First step class name parentheses
11. Design mode:
Singleton mode: Only one object can be instantiated on a class. The simplest design pattern, ( interview must test )
class A:
__instance=none
def __new__ (CLS, *argsm, **kwargs):
if A.__instance is None:
obj=object.__new__ (CLS)
A.__instance=obj
return a.__instance
a1=a ()
b1=a ()
c1=a ()
print (A1,B1,C1)
12.item Usage
class Foo:
def __init__ (self,name):
Self.name=name
def __getitem__ (self,item):
if Hasattr (self,item):
return GetAttr (Self,item)
Else:
print ("No this variable")
def __SetItem__ (self,key,value):
print (Key,value)
def __delitem__ (self,key):
print (key)

F1=foo ("Alex")
Print (f1["name"])
f1["name"]= "Mary"
del f1["name"]

Python Full Stack Development * 23 Object-oriented Knowledge points summary * 180704