Two ways to create a class (create a class through a metaclass)

First of all we need to be clear: in python, everything is an object

class Student:

　　pass

zhangsan = Student()

What are the characteristics of the object:

1. Can be cited

a = zhangsan

2. Can be entered as a function parameter

func(zhangsan)

3. Can be used as the return value of a function

def func():

　　return zhangsan

4. Can be used as a container element

lis = ["hello, world", 1, zhangsan]

And the classes we define also use these features, so we can replace the above zhangsan with Student, and draw the conclusion:

Classes are objects too! ! !

Since a class is an object, it must be instantiated by a class, and this class is the metaclass

a = 5

print(type(a))

get a is of type int

print(type(type(a)))

Get the type type, that is, the class of int is the type class, and this type is the metaclass we are looking for!

The class of a class is a metaclass, the type class

 Now that we know that classes are also instantiated through metaclasses, can we also create classes through metaclasses in the future?

of course can

Method 1: Create a class directly

class People:
    country = "China"

    def __init__(self, name, age):
        self.name = name
        self.age = age

    def tell_info(self):
        print(f"{self.name} is from {self.country}, and he is {self.age} years old")


chen = People("chenjun", 21)
chen.tell_info()

Method 2: Create a class through a metaclass
First, we need the three elements that make up the class as parameters of the metaclass

Parameter 1: class name, such as People above

Parameter 2: The base class (parent class) of the class, in python3, if the inherited class is not specified, the object class is inherited by default

Parameter 3: Namespace

Then the definition code is as follows:

class_name = "People"
class_bases = (object,)
class_dic = {}
class_body = """
country = "China"

def __init__(self, name, age):
    self.name = name
    self.age = age

def tell_info(self):
    print(f"{self.name} is from {self.country}, and he is {self.age} years old")
"""
exec(class_body, globals(), class_dic)
People = type(class_name, class_bases, class_dic)
chen = People("chenjun", 21)
chen.tell_info()