class Singleton(object[,...]):
staticVar1 = None
staticVar2 = None
def __init__(self):
if self.__class__.staticVar1==None :
# create class instance variable for instantiation of class
# assign class instance variable values to class static variables
else:
# assign class static variable values to class instance variables
class NothingSpecial:
pass
_the_one_and_only = None
def TheOneAndOnly():
global _the_one_and_only
if not _the_one_and_only:
_the_one_and_only = NothingSpecial()
return _the_one_and_only
或者如果在模块第一次导入时实例化没有问题:
class NothingSpecial:
pass
THE_ONE_AND_ONLY = NothingSpecial()
@Singleton
class Foo:
def __init__(self):
print 'Foo created'
f = Foo() # Error, this isn't how you get the instance of a singleton
f = Foo.instance() # Good. Being explicit is in line with the Python Zen
g = Foo.instance() # Returns already created instance
print f is g # True
下面是代码:
class Singleton:
"""
A non-thread-safe helper class to ease implementing singletons.
This should be used as a decorator -- not a metaclass -- to the
class that should be a singleton.
The decorated class can define one `__init__` function that
takes only the `self` argument. Also, the decorated class cannot be
inherited from. Other than that, there are no restrictions that apply
to the decorated class.
To get the singleton instance, use the `instance` method. Trying
to use `__call__` will result in a `TypeError` being raised.
"""
def __init__(self, decorated):
self._decorated = decorated
def instance(self):
"""
Returns the singleton instance. Upon its first call, it creates a
new instance of the decorated class and calls its `__init__` method.
On all subsequent calls, the already created instance is returned.
"""
try:
return self._instance
except AttributeError:
self._instance = self._decorated()
return self._instance
def __call__(self):
raise TypeError('Singletons must be accessed through `instance()`.')
def __instancecheck__(self, inst):
return isinstance(inst, self._decorated)
class Singleton(object):
def __new__(cls, *args, **kwds):
it = cls.__dict__.get("__it__")
if it is not None:
return it
cls.__it__ = it = object.__new__(cls)
it.init(*args, **kwds)
return it
def init(self, *args, **kwds):
pass
我可能会重写成这样:
class Singleton(object):
"""Use to create a singleton"""
def __new__(cls, *args, **kwds):
"""
>>> s = Singleton()
>>> p = Singleton()
>>> id(s) == id(p)
True
"""
it_id = "__it__"
# getattr will dip into base classes, so __dict__ must be used
it = cls.__dict__.get(it_id, None)
if it is not None:
return it
it = object.__new__(cls)
setattr(cls, it_id, it)
it.init(*args, **kwds)
return it
def init(self, *args, **kwds):
pass
class A(Singleton):
pass
class B(Singleton):
pass
class C(A):
pass
assert A() is A()
assert B() is B()
assert C() is C()
assert A() is not B()
assert C() is not B()
assert C() is not A()
