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class SingletonClass(object):
def __new__(cls):
if not hasattr(cls, 'instance'):
cls.instance = super(SingletonClass, cls).__new__(cls)
return cls.instance
singleton = SingletonClass()
new_singleton = SingletonClass()
print(singleton is new_singleton)
singleton.singl_variable = "Singleton Variable"
print(new_singleton.singl_variable)
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class Singleton:
__instance = None
@staticmethod
def get_instance():
""" Static access method. """
if Singleton.__instance == None:
Singleton()
return Singleton.__instance
def __init__(self):
""" Virtually private constructor. """
if Singleton.__instance != None:
raise Exception("This class is a singleton!")
else:
Singleton.__instance = self
# usage:
singleton = Singleton()
singleton2 = Singleton() # this will raise Exception
singleton3 = Singleton.get_instance()
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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)
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@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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class Singleton:
_instance = None
def __new__(cls, *args, **kwargs):
if not cls._instance:
cls._instance = super().__new__(cls, *args, **kwargs)
return cls._instance
# Example usage
obj1 = Singleton()
obj2 = Singleton()
print(obj1 is obj2) # Output: True
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class Logger(object):
def __new__(cls, *args, **kwargs):
if not hasattr(cls, '_logger'):
cls._logger = super(Logger, cls
).__new__(cls, *args, **kwargs)
return cls._logger
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# mymodule.py
__all__ = ["mysingleton"]
class __MySingletonLike(object):
def __init__(self):
self._variable = 42
@property
def variable(self):
return self._variable
@variable.setter
def variable(self, value):
check_value(value) # imaginary validation
self._variable = value
mysingleton = __MySingleton()