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December 12, 2016 12:46
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Differents singletons implementation in python. From http://stackoverflow.com/a/6798042 & http://stackoverflow.com/a/7346105
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class Singleton(object): | |
_instances = {} | |
def __new__(class_, *args, **kwargs): | |
if class_ not in class_._instances: | |
class_._instances[class_] = super(Singleton, class_).__new__(class_, *args, **kwargs) | |
return class_._instances[class_] | |
class MyClass(Singleton): | |
pass | |
c = MyClass() |
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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) | |
@Singleton | |
class Foo: | |
def __init__(self): | |
print 'Foo created' | |
f = Foo() # Error, this isn't how you get the instance of a singleton | |
f = Foo.Instance() # Good. Being explicit is in line with the Python Zen | |
g = Foo.Instance() # Returns already created instance | |
print f is g # True |
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