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| 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) | |