One 、 The main function

from module_a import Cat

# __init__()  Constructors 
cat = Cat("Tom", 15)

# # __del__()  Destructor   Call explicitly or automatically at the end of the program 
# del cat
# # print(cat)  Not printable 

# __doc__ __modules__ __class__
print(cat.__doc__)  #  Print class description document 
print(cat.__module__)  #  Print the module name of the class 
print(cat.__class__)  #  Print the location of the class 

# __call__()
cat(1, 2)  #  Make the class be called like a function 
print(callable(cat))

# __dict__
print(cat.__dict__)  #  Print common and private member properties in dictionary form 
print(cat._Cat__private_name)  #  Access private member properties 

# __str__()
print(cat)  #  If the print object returns a property   Not the access address 
print(str(cat))

# __len__()
print(len(cat))  #  You must define how to measure class length 

# __iter__()
#  Must return iterators 
for i in cat:
    print(i)

# __getitem__() __setitem__() __delitem__()
print(cat["name"])  #  Make the class subscript accessible 
cat["name"] = "Tim"
print(cat["name"])  #  Make the class subscript accessible 
print(cat.name)
del cat["name"]
# print(cat.name)  Deleted class member variable   Can't print 

#  Mathematical operations   If no function is defined, an error is reported   If the addition type is not defined, return none
# __add__() +
# __sub__() -
# __mul__() *
# __div__() /
# __mod__() %
# __pow__() **
cat_another = Cat("Tom_another", 20)
print(cat + cat_another)
cat2 = cat + cat_another
cat3 = cat_another + cat2
# cat3 = cat2 + cat_another  The order of transmission must be met 
print(cat3)

Two 、 class

class Cat:
    """  This is a cat  """
    def __init__(self, name, tail_length=10):
        self.name = name
        self.__private_name = name
        self.tail_length = tail_length
        print(" I'm a cat , My name is %s" % self.name)

    def __del__(self):
        print(" I was recycled by the system ")

    def __call__(self, *args, **kwargs):
        print("Cat", args[0]+args[1])

    def __str__(self):
        return " I am a %s" % self.name

    def __len__(self):
        return self.tail_length

    def __iter__(self):
        return iter([1, 2, 3, 4])

    def __getitem__(self, item):
        if item =="name":
            return self.name
        else:
            return None

    def __setitem__(self, key, value):
        if key == "name":
            self.name = value

    def __delitem__(self, key):
        if key == "name":
            del self.name

    def __add__(self, other):
        if isinstance(other, Cat):
            return [self, other]
        elif isinstance(other, list):
            other.append(self)
            return other