Source code for soma.singleton

# -*- coding: utf-8 -*-

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'''
Singleton pattern.

- author: Yann Cointepas
- organization: NeuroSpin
- license: `CeCILL B <http://www.cecill.info/licences/Licence_CeCILL-B_V1-en.html>`_
'''
from __future__ import absolute_import

import atexit

__docformat__ = 'restructuredtext en'


class Singleton(object):

    '''
    Implements the singleton pattern. A class deriving from ``Singleton`` can
    have only one instance. The first instanciation will create an object and
    other instanciations return the same object. Note that the :meth:`__init__`
    method (if any) is still called at each instanciation (on the same object).
    Therefore, :class:`Singleton` derived classes should define
    :meth:`__singleton_init__`
    instead of :py:meth:`__init__` because the former is only called once.

    Example::

      from singleton import Singleton

      class MyClass( Singleton ):
        def __singleton_init__( self ):
          self.attribute = 'value'

      o1 = MyClass()
      o2 = MyClass()
      print(o1 is o2)

    '''

    @classmethod
    def get_instance(cls):
        try:
            return getattr(cls, '_singleton_instance')
        except AttributeError:
            msg = "Class %s has not been initialized" % cls.__name__
            raise ValueError(msg)

    def __new__(cls, *args, **kwargs):
        if '_singleton_instance' not in cls.__dict__:
            cls._singleton_instance = super(Singleton, cls).__new__(cls)
            singleton_init = getattr(cls._singleton_instance,
                                     '__singleton_init__', None)
            if singleton_init is not None:
                singleton_init(*args, **kwargs)
            atexit.register(cls.delete_singleton)
        return cls._singleton_instance

    def __init__(self, *args, **kwargs):
        '''
        The __init__ method of :py:class:`Singleton` derived class should do
        nothing.
        Derived classes must define :py:meth:`__singleton_init__` instead of __init__.
        '''

    def __singleton_init__(self, *args, **kwargs):
        super(Singleton, self).__init__(*args, **kwargs)

    @classmethod
    def delete_singleton(cls):
        if hasattr(cls, '_singleton_instance'):
            del cls._singleton_instance