Why is the Borg pattern better than the Singleton pattern in Python
Why is the Borg pattern better than the Singleton pattern?
I ask because I don't see them resulting in anything different.
Borg:
class Borg:
__shared_state = {}
# init internal state variables here
__register = {}
def __init__(self):
self.__dict__ = self.__shared_state
if not self.__register:
self._init_default_register()
Singleton:
class Singleton:
def __init__(self):
# init internal state variables here
self.__register = {}
self._init_default_register()
# singleton mechanics external to class, for example this in the module
Singleton = Singleton()
What I want to display here is that the service object, whether implemented as Borg or Singleton, has a nontrivial internal state (it provides some service based on it) (I mean it has to be something useful it's not a Singleton/Borg just for fun).
And this state has to be inited. Here the Singleton implementation is more straightforward, since we treat init as the set-up of the global state. I find it awkward that the Borg object has to query its internal state to see if it should update itself.
It becomes worse the more internal state you have. For example, if the object has to listen to the Application's teardown signal to save its register to disk, that registration should only be done once as well, and this is easier with a Singleton.
The real reason that borg is different comes down to subclassing.
If you subclass a borg, the subclass' objects have the same state as their parents classes objects, unless you explicitly override the shared state in that subclass. Each subclass of the singleton pattern has its own state and therefore will produce different objects.
Also in the singleton pattern the objects are actually the same, not just the state (even though the state is the only thing that really matters).
In python if you want a unique "object" that you can access from anywhere just create a class Unique that only contains static attributes, @staticmethod s, and @classmethod s; you could call it the Unique Pattern. Here I implement and compare the 3 patterns:
Unique
#Unique Pattern
class Unique:
#Define some static variables here
x = 1
@classmethod
def init(cls):
#Define any computation performed when assigning to a "new" object
return cls
Singleton
#Singleton Pattern
class Singleton:
__single = None
def __init__(self):
if not Singleton.__single:
#Your definitions here
self.x = 1
else:
raise RuntimeError('A Singleton already exists')
@classmethod
def getInstance(cls):
if not cls.__single:
cls.__single = Singleton()
return cls.__single
Borg
#Borg Pattern
class Borg:
__monostate = None
def __init__(self):
if not Borg.__monostate:
Borg.__monostate = self.__dict__
#Your definitions here
self.x = 1
else:
self.__dict__ = Borg.__monostate
Test
#SINGLETON
print "nSINGLETONn"
A = Singleton.getInstance()
B = Singleton.getInstance()
print "At first B.x = {} and A.x = {}".format(B.x,A.x)
A.x = 2
print "After A.x = 2"
print "Now both B.x = {} and A.x = {}n".format(B.x,A.x)
print "Are A and B the same object? Answer: {}".format(id(A)==id(B))
#BORG
print "nBORGn"
A = Borg()
B = Borg()
print "At first B.x = {} and A.x = {}".format(B.x,A.x)
A.x = 2
print "After A.x = 2"
print "Now both B.x = {} and A.x = {}n".format(B.x,A.x)
print "Are A and B the same object? Answer: {}".format(id(A)==id(B))
#UNIQUE
print "nUNIQUEn"
A = Unique.init()
B = Unique.init()
print "At first B.x = {} and A.x = {}".format(B.x,A.x)
A.x = 2
print "After A.x = 2"
print "Now both B.x = {} and A.x = {}n".format(B.x,A.x)
print "Are A and B the same object? Answer: {}".format(id(A)==id(B))
Output:
SINGLETON
At first B.x = 1 and A.x = 1
After A.x = 2
Now both B.x = 2 and A.x = 2
Are A and B the same object? Answer: True
BORG
At first B.x = 1 and A.x = 1
After A.x = 2
Now both B.x = 2 and A.x = 2
Are A and B the same object? Answer: False
UNIQUE
At first B.x = 1 and A.x = 1
After A.x = 2
Now both B.x = 2 and A.x = 2
Are A and B the same object? Answer: True
In my opinion, Unique implementation is the easiest, then Borg and finally Singleton with an ugly number of two functions needed for its definition.
It is not. What is generally not recommended is a pattern like this in python:
class Singleton(object):
_instance = None
def __init__(self, ...):
...
@classmethod
def instance(cls):
if cls._instance is None:
cls._instance = cls(...)
return cls._instance
where you use a class method to get the instance instead of the constructor. Python's metaprogramming allows much better methods, eg the one on Wikipedia:
class Singleton(type):
def __init__(cls, name, bases, dict):
super(Singleton, cls).__init__(name, bases, dict)
cls.instance = None
def __call__(cls, *args, **kw):
if cls.instance is None:
cls.instance = super(Singleton, cls).__call__(*args, **kw)
return cls.instance
class MyClass(object):
__metaclass__ = Singleton
print MyClass()
print MyClass()
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