使用模块

其实，**Python 的模块就是天然的单例模式**，因为模块在第一次导入时，会生成 `.pyc` 文件，当第二次导入时，就会直接加载 `.pyc` 文件，而不会再次执行模块代码。
因此，我们只需把相关的函数和数据定义在一个模块中，就可以获得一个单例对象了。如果我们真的想要一个单例类，可以考虑这样做：

mysingleton.py

class Singleton(object):
    def foo(self):
        pass
singleton = Singleton()

将上面的代码保存在文件 mysingleton.py 中，要使用时，直接在其他文件中导入此文件中的对象，这个对象即是单例模式的对象

from a import singleton

使用装饰

def Singleton(cls):
    instance = None

    def _singleton(*args, **kargs):
        nonlocal instance
        if not instance:
            instance = cls(*args, **kargs)
        return instance

    return _singleton


@Singleton
class A(object):
    def __init__(self, x=0):
        self.x = x


a1 = A(2)
a2 = A(3)
print(a1.x)
print(a2.x)

print(a1 is a2)

适用类方法

class Singleton(object):
    _instance=None
    def __init__(self):
        pass
    @classmethod
    def instance(cls, *args, **kwargs):
        if not cls._instance:
            cls._instance=cls(*args, **kwargs)
        return cls._instance

a1=Singleton.instance()
a2=Singleton().instance()

print(a1 is a2)

基于new方法实现

class Singleton(object):
    _instance=None
    def __init__(self):
        pass


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

obj1 = Singleton()
obj2 = Singleton()
print(obj1 is obj2)

基于metclass方式实现

class SingletonType(type):
    _instance=None
    def __call__(cls, *args, **kwargs):
        if not cls._instance:
            # cls._instance = super().__call__(*args, **kwargs)
            cls._instance = object.__new__(cls)
            cls._instance.__init__(*args, **kwargs)
        return cls._instance

class Foo(metaclass=SingletonType):
    def __init__(self,name):
        self.name = name


obj1 = Foo('name')
obj2 = Foo('name')
print(obj1.name)
print(obj1 is obj2)

补充问题

from django.contrib import admin
from django.urls import path, include
from home import views  # pycharm报错，但实际上不报错 ，只需要把加入到环境变量的路径都做成source root即可
from django.views.static import serve
from django.conf import settings
import os
from django.shortcuts import HttpResponse

class Foo():
    url = 'http://www.baidu.com'

    def demo(self,request):
        # self 就不是Foo的对象了，它是request对象
        print(type(self)) # django.core.handlers.wsgi.WSGIRequest
        print(self.url) # 'WSGIRequest' object has no attribute 'url'
        return HttpResponse('sdfa')


urlpatterns = [


    path('demo/', Foo().demo),  # Foo.demo(request)
]