61 lines
No EOL
2.1 KiB
Python
61 lines
No EOL
2.1 KiB
Python
"""
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The Singleton classes.
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Copyright (c) 2011-2013 by Johannes Findeisen and Rafael Timmerberg
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This file is part of Linspector (http://linspector.org).
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Linspector is free software: you can redistribute it and/or modify
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it under the terms of the GNU Affero General Public License as
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published by the Free Software Foundation, either version 3 of the
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License, or (at your option) any later version.
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This program is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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GNU Affero General Public License for more details.
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You should have received a copy of the GNU Affero General Public License
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along with this program. If not, see <http://www.gnu.org/licenses/>.
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"""
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#see http://stackoverflow.com/questions/42558/python-and-the-singleton-pattern
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class Singleton:
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"""
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A non-thread-safe helper class to ease implementing singletons.
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This should be used as a decorator -- not a metaclass -- to the
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class that should be a singleton.
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The decorated class can define one `__init__` function that
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takes only the `self` argument. Other than that, there are
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no restrictions that apply to the decorated class.
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To get the singleton instance, use the `Instance` method. Trying
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to use `__call__` will result in a `TypeError` being raised.
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Limitations: The decorated class cannot be inherited from.
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"""
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def __init__(self, decorated):
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self._decorated = decorated
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def Instance(self):
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"""
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Returns the singleton instance. Upon its first call, it creates a
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new instance of the decorated class and calls its `__init__` method.
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On all subsequent calls, the already created instance is returned.
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"""
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try:
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return self._instance
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except AttributeError:
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self._instance = self._decorated()
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return self._instance
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def __call__(self):
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raise TypeError('Singletons must be accessed through `Instance()`.')
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def __instancecheck__(self, inst):
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return isinstance(inst, self._decorated) |