class SingletonMeta(type):
"""
A Singleton meta class.
A class that derives from this class will have only 1 instance of that type for the process.
"""
_instances = {}
def __call__(cls, *args, **kwargs):
if cls not in cls._instances:
cls._instances[cls] = super(SingletonMeta, cls).__call__(*args, **kwargs)
return cls._instances[cls]
class _SingletonWrapper:
"""
A singleton wrapper class. Its instances would be created
for each decorated class.
"""
def __init__(self, cls):
self.__wrapped__ = cls
self._instance = None
def __call__(self, *args, **kwargs):
"""Returns a single instance of decorated class"""
if self._instance is None:
self._instance = self.__wrapped__(*args, **kwargs)
return self._instance
[docs]def singleton(cls):
"""
A singleton decorator. Returns a wrapper objects. A call on that object
returns a single instance object of decorated class. Use the __wrapped__
attribute to access decorated class directly in unit tests
"""
return _SingletonWrapper(cls)