Here is how to implement singletons in Python
Taken from: https://stackoverflow.com/questions/31875/is-there-a-simple-elegant-way-to-define-singletons
@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 |
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) |