Source code for bob.bio.base.utils.singleton

# A singleton class decorator, based on http://stackoverflow.com/a/7346105/3301902

[docs]class Singleton(object): """ 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 an arbitrary list of parameters. To get the singleton instance, use the :py:meth:`instance` method. Trying to use `__call__` will result in a `TypeError` being raised. Limitations: * The decorated class cannot be inherited from. * The documentation of the decorated class is replaced with the documentation of this class. """ def __init__(self, decorated): self._decorated = decorated # see: functools.WRAPPER_ASSIGNMENTS: self.__doc__ = decorated.__doc__ self.__name__ = decorated.__name__ self.__module__ = decorated.__module__ self.__mro__ = decorated.__mro__ self.__bases__ = [] self._instance = None
[docs] def create(self, *args, **kwargs): """Creates the singleton instance, by passing the given parameters to the class' constructor.""" self._instance = self._decorated(*args, **kwargs)
[docs] def instance(self): """Returns the singleton instance. The function :py:meth:`create` must have been called before.""" if self._instance is None: raise RuntimeError("The class has not yet been instantiated using the 'create' method.") return self._instance
def __call__(self): raise TypeError('Singletons must be accessed through the `instance()` method.') def __instancecheck__(self, inst): return isinstance(inst, self._decorated)