#!/usr/bin/python -tt
# -*- coding: utf-8 -*-
'''
Copyright 2014-2015 Teppo Perä
Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at
http://www.apache.org/licenses/LICENSE-2.0
Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
'''
from pytraits.support.errors import SingletonError
[docs]class Singleton(type):
""" Turn the class to immutable singleton.
>>> class Example(object, metaclass=Singleton):
... pass
...
>>> a = Example()
>>> b = Example()
>>> id(a) == id(b)
True
Having your instance as a singleton is faster than creating from scratch
>>> import timeit
>>> class MySingleton(object, metaclass=Singleton):
... def __init__(self):
... self._store = dict(one=1, two=2, three=3, four=4)
...
>>> class NonSingleton:
... def __init__(self):
... self._store = dict(one=1, two=2, three=3, four=4)
...
>>> #timeit.timeit(NonSingleton) > timeit.timeit(MySingleton)
After creating a singleton, data it is holding should not be changed.
There is a small enforcement done for these singletons to prevent modifying
the contents. With little effort it is doable, but don't do it. :)
>>> MySingleton().new_item = False
Traceback (most recent call last):
...
pytraits.support.errors.SingletonError: Singletons are immutable!
"""
def __call__(self, *args, **kwargs):
try:
return self.__instance
except AttributeError:
def immutable_object(*args):
raise SingletonError()
self.__instance = super().__call__(*args, **kwargs)
self.__setitem__ = immutable_object
self.__setattr__ = immutable_object
return self.__instance
if __name__ == "__main__":
import doctest
doctest.testmod()