python Singleton

gistfile1.py

# -*- coding: utf-8
__author__ = 'Charlie Qiu <qbowater@gmail.com>'
"""
A implementation of singleton from stackoverflow.

http://stackoverflow.com/questions/42558/python-and-the-singleton-pattern

Example:

    @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. 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.

    Limitations: The decorated class cannot be inherited from.

    """
    def __init__(self, decorated):
        self._decorated = decorated

    def instance(self, *args, **kwargs):
        """
        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(*args, **kwargs)
            return self._instance

    def __call__(self):
        raise TypeError('Singletons must be accessed through `instance()`.')

    def __instancecheck__(self, inst):
        return isinstance(inst, self._decorated)