單例模式可能是最簡單的設計模式，單例是非常通用的對象。讓你能夠保證一個類只有一個實例， 並提供一個訪問該實例的全局節點。
我們可以將船的船長視為單例模式的現實生活中的例子。在船上，他是負責人。他負責重要的決定，由於這一責任，他收到了一些請求。
如前所述，單例模式的一個用例是創建一個維護程序全局狀態的單個對象。其他可能的用例如下：
class Singleton:
"""Definition of a Singleton object."""
singleton_instance = None
def __init__( self):
"""
Override the initialization
mechanism, returning only the single instance.
"""
...
@ staticmethod
def get_singleton():
"""
Method for fetching the Singleton instance.
Is static so that it can be accessed everywhere.
"""
...
@ staticmethod
def update_singleton( val):
"""
Method for setting value of Singleton instance.
Is static so that it can be accessed everywhere.
"""
...
存儲在 Singleton 實例中的數據是任意的。重要的是，無論數據，呼叫者和范圍如何，Singleton 對象都會返回同一實例。這使得單元在實現諸如全局設置或運行配置之類的內容時有用。
使用下面的代碼片段來播放 Active Singleton 實現。嘗試用數據結構（例如字典）替換可變Singleton_instance，並查看 Getter 和 Setter 的實現如何更改。嘗試編寫一些共享 Singleton 實例的功能。
class Singleton:
"""Definition of a Singleton object."""
# Maintain state of Singleton
singleton_instance = None
def __init__( self):
"""Override the initialization mechanism."""
if Singleton. singleton_instance is None:
Singleton. singleton_instance = self
@ staticmethod
def get_singleton():
"""
Method for fetching the Singleton instance.
Is static so that it can be accessed everywhere.
"""
if Singleton. singleton_instance is None:
Singleton() # Call __init__ to initialize instance
return Singleton. singleton_instance
@ staticmethod
def update_singleton( val):
"""
Method for setting value of Singleton instance.
Is static so that it can be accessed everywhere.
"""
if Singleton. singleton_instance is None:
Singleton() # Call __init__ to initialize instance
Singleton. singleton_instance = val
Singleton. update_singleton( "Michael")
print( "Value in Singleton instance is: " + Singleton. get_singleton())
Singleton() # Try to create a new Singleton instance
print( "Value in Singleton instance is STILL: " + Singleton. get_singleton())
單例模式也可以通過使單例類使用元類（其類型，具有先前定義的元類）來實現。根據需要，元類的 __call__() 方法保存的代碼可確保只能創建類的一個實例：
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.")
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