Singleton and multi Embodiment Mode

Singleton design pattern and multi Embodiment Mode

What is a design pattern

Design patterns are solutions to common problems faced by developers in the development process

These solutions are numerous developers through trial and error for a long period of time summary out

Singleton

Only a single class object is created

Multi-pattern

A class can have multiple objects are created

## 平时我们使用的都是多例模式
class custom():
    pass

c1 = custom()
c2 = custom()
print(c1)   # <__main__.custom object at 0x0000020305FEA248>
print(c2)   # <__main__.custom object at 0x0000020305FEA188>
print(c1 is c2)   # False
## 我们可以通过他们的内存地址来判断是否是同一个实例也可以使用is判断

Create a singleton instance

1. The method of binding by class

   class custom():
   
       # 记录实例对象
       _is_create = None
       def __init__(self, name):
           self.name = name
   
       # 通过类方法创建实例
       @classmethod
       def create_class(cls, name):
           # 如果没有对象
           if not cls._is_create:
               # 创建对象并将对象赋值给_is_create
               cls._is_create = cls(name)
           return cls._is_create
   
   # 通过类方法来创建类的实例化对象
   c1 = custom.create_class('张三')
   c2 = custom.create_class('李四')
   
   print(c1 is c2)   # False

2. By decorator

   def wer(func):
       # 记录多个类的实例化对象
       _instance = {}
   
       def inner(*args, **kwargs):
           # 如果没有对象
           if func not in _instance:
               # 创建对象并将对象赋值给_instance
               _instance[func] = func(*args, **kwargs)
           return _instance[func]
       return inner
   
   @wer
   class Myclass(object):
       pass
   
   m1 = Myclass()
   m2 = Myclass()
   
   print(m1 is m2)   # True

3. By yuan class

   class Mytype(type):
   
       # 当实例化类的时候会调用元类的__call__方法
       def __call__(self, *args, **kwargs):
           # 如果实例化的类中没有_instance属性就创建一个实例化对象
           if not getattr(self, '_instance'):
               # 创建”空“对象赋值给_instance
               self._instance = self.__new__(self)
               # 为“空”对象初始化
               self._instance.__init__(*args, **kwargs)
   
           return self._instance
   
   class Foo(metaclass=Mytype):
       _instance = None
   
   f1 = Foo()
   f2 = Foo()
   print(f1 is f2)

4. Import in python code module is an object, but the object is to import only once, whether you are importing a few times.

   In Python module is a special single-mode embodiment.