Python 单例模式讲解和代码示例
单例是一种创建型设计模式， 让你能够保证一个类只有一个实例， 并提供一个访问该实例的全局节点。
单例拥有与全局变量相同的优缺点。 尽管它们非常有用， 但却会破坏代码的模块化特性。
在某些其他上下文中， 你不能使用依赖于单例的类。 你也将必须使用单例类。 绝大多数情况下， 该限制会在创建单元测试时出现。
复杂度：
流行度：
使用示例： 许多开发者将单例模式视为一种反模式。 因此它在 Python 代码中的使用频率正在逐步减少。
识别方法： 单例可以通过返回相同缓存对象的静态构建方法来识别。
基础单例
实现一个粗糙的单例非常简单。 你仅需隐藏构造函数并实现一个静态的构建方法即可。
相同的类在多线程环境中会出错。 多线程可能会同时调用构建方法并获取多个单例类的实例。
main.py: 概念示例
class SingletonMeta(type):
"""
The Singleton class can be implemented in different ways in Python. Some
possible methods include: base class, decorator, metaclass. We will use the
metaclass because it is best suited for this purpose.
"""
_instances = {}
def __call__(cls, *args, **kwargs):
"""
Possible changes to the value of the `__init__` argument do not affect
the returned instance.
"""
if cls not in cls._instances:
instance = super().__call__(*args, **kwargs)
cls._instances[cls] = instance
return cls._instances[cls]
class Singleton(metaclass=SingletonMeta):
def some_business_logic(self):
"""
Finally, any singleton should define some business logic, which can be
executed on its instance.
"""
# ...
if __name__ == "__main__":
# The client code.
s1 = Singleton()
s2 = Singleton()
if id(s1) == id(s2):
print("Singleton works, both variables contain the same instance.")
else:
print("Singleton failed, variables contain different instances.")
Output.txt: 执行结果
Singleton works, both variables contain the same instance.
线程安全单例
为了解决这个问题， 你必须在创建首个单例对象时对线程进行同步。
main.py: 概念示例
from threading import Lock, Thread
class SingletonMeta(type):
"""
This is a thread-safe implementation of Singleton.
"""
_instances = {}
_lock: Lock = Lock()
"""
We now have a lock object that will be used to synchronize threads during
first access to the Singleton.
"""
def __call__(cls, *args, **kwargs):
"""
Possible changes to the value of the `__init__` argument do not affect
the returned instance.
"""
# Now, imagine that the program has just been launched. Since there's no
# Singleton instance yet, multiple threads can simultaneously pass the
# previous conditional and reach this point almost at the same time. The
# first of them will acquire lock and will proceed further, while the
# rest will wait here.
with cls._lock:
# The first thread to acquire the lock, reaches this conditional,
# goes inside and creates the Singleton instance. Once it leaves the
# lock block, a thread that might have been waiting for the lock
# release may then enter this section. But since the Singleton field
# is already initialized, the thread won't create a new object.
if cls not in cls._instances:
instance = super().__call__(*args, **kwargs)
cls._instances[cls] = instance
return cls._instances[cls]
class Singleton(metaclass=SingletonMeta):
value: str = None
"""
We'll use this property to prove that our Singleton really works.
"""
def __init__(self, value: str) -> None:
self.value = value
def some_business_logic(self):
"""
Finally, any singleton should define some business logic, which can be
executed on its instance.
"""
def test_singleton(value: str) -> None:
singleton = Singleton(value)
print(singleton.value)
if __name__ == "__main__":
# The client code.
print("If you see the same value, then singleton was reused (yay!)\n"
"If you see different values, "
"then 2 singletons were created (booo!!)\n\n"
"RESULT:\n")
process1 = Thread(target=test_singleton, args=("FOO",))
process2 = Thread(target=test_singleton, args=("BAR",))
process1.start()
process2.start()
Output.txt: 执行结果
If you see the same value, then singleton was reused (yay!)
If you see different values, then 2 singletons were created (booo!!)
RESULT:
FOO
FOO
