假设我们在一个与数据库交互的系统中，需要进行数据库操作，那么我们就有必要了解单例模式，单例模式使得我们不用每次进行数据库的操作时都去链接数据库。
我将循环渐进，由浅入深的写一下单例模式示例。
实例1：
db.py
class Foo(object):
def __init__(self):
self.conn = "连接数据库"
def get(self):
return self.conn
obj = Foo()
views.py
import db print(db.obj)
run.py
import db import views print(db.obj)
执行run.py,打印结果如下，这就是单例模式
<db.Foo object at 0x000001C1D8FC5A58> <db.Foo object at 0x000001C1D8FC5A58>
实例二：
注意这里使用了锁来解决线程安全问题
import threading
import time
class Foo(object):
instance = None
lock = threading.Lock()
def __init__(self):
self.a1 = 1
self.a2 = 2
import time
import random
time.sleep(2)
@classmethod
def get_instance(cls,*args,**kwargs):
if not cls.instance:
with cls.lock:
if not cls.instance:
obj = cls(*args,**kwargs)
cls.instance = obj
return cls.instance
return cls.instance
def task():
obj = Foo.get_instance()
print(obj)
import threading
for i in range(5):
t = threading.Thread(target=task,)
t.start()
time.sleep(10)
Foo.get_instance()
实例三：
import threading
class Foo(object):
instance = None
lock = threading.Lock()
def __init__(self):
self.a1 = 1
self.a2 = 2
import time
time.sleep(2)
def __new__(cls, *args, **kwargs):
if not cls.instance:
with cls.lock:
if not cls.instance:
obj = super(Foo, cls).__new__(cls, *args, **kwargs)
cls.instance = obj
return cls.instance
return cls.instance
def task():
obj = Foo()
print(obj)
import threading
for i in range(5):
t = threading.Thread(target=task, )
t.start()
实例四：
import threading
lock = threading.Lock()
class Singleton(type):
def __call__(cls, *args, **kwargs):
if not hasattr(cls, ‘instance‘):
with lock:
if not hasattr(cls, ‘instance‘):
obj = cls.__new__(cls, *args, **kwargs)
obj.__init__(*args, **kwargs)
setattr(cls, ‘instance‘, obj)
return getattr(cls, ‘instance‘)
return getattr(cls, ‘instance‘)
class Foo(object, metaclass=Singleton):
def __init__(self):
self.name = ‘alex‘
obj1 = Foo()
obj2 = Foo()
print(obj1, obj2)
