单例模式（一）

### 1. 通过模块导入
```
from student import student
```
### 2. 使用new方法

```
class Student:
    _instance = None

def __new__(cls, *args, **kwargs):
        if not cls._instance:
            cls._instance = super().__new__(cls)
        return cls._instance

def __init__(self, name, age):
        print("init")
        self.name = name
        self.age = age
		
s = Student("xiaoming01", 15)
s = Student("xiaoming02", 16)
```
但是这个方法有个问题，就是每次获取实例的时候都会执行初始化方法`__init__`

### 3. 使用函数装饰器
```
def singleton(cls):
    instances = {}

def _singleton(*args, **kwargs):
        if cls not in instances:
            instances[cls] = cls(*args, **kwargs)
        return instances[cls]
    return _singleton

@singleton
class Student2(object):
    def __init__(self, name, age):
        print("init")
        self.name = name
        self.age = age
```
这个方法比较好一点，初始化方法只会执行一次。
但是这个方法容易导致内存的增长，要注意清理`instances`存储的对象。

### 4. 使用类的装饰器
```
class SingleTon:
    _instances = {}

def __init__(self, cls_name):
        self.cls_name = cls_name

def __call__(self, *args, **kwargs):
        if self.cls_name not in SingleTon._instances:
            SingleTon._instances[self.cls_name] = self.cls_name(*args, **kwargs)
        return SingleTon._instances.get(self.cls_name)

@SingleTon
class Student2(object):
    def __init__(self, name, age):
        print("init")
        self.name = name
        self.age = age
```
类的装饰器和函数装饰器的效果是一样的

### 5. 使用函数来创建对象
```
_instances = {}

def create_student(name, age):
    key = f"{name}_{age}"
    if key in _instances:
        return _instances[key]
    stu = Student(name, age)
    _instances[key] = stu
    return stu
```
这种方法其实跟函数装饰器是一样的，装饰器算隐式调用，这个算显示调用
### 参考
> [6 种方法实现单例模式](https://www.cnblogs.com/wozijisun/p/16635365.html)