Question

Here I’m subclassing a wxPython class and defining a class method called singleton.

class AddressCellAttr(wx.grid.GridCellAttr):
    _instance = None

    def __init__(self):
        wx.grid.GridCellAttr.__init__(self)

        self.SetTextColour('#0000FF')

    @classmethod
    def singleton(cls):
        if cls._instance == None:
            cls._instance = cls()

        return cls._instance

class ValidAddressCellAttr(AddressCellAttr):
    def __init__(self):
        AddressCellAttr.__init__(self)

        self.SetTextColour('#00FF00')

class CorrectedAddressCellAttr(AddressCellAttr):
    def __init__(self):
        AddressCellAttr.__init__(self)

        self.SetTextColour('#FFFF00')

class InvalidAddressCellAttr(AddressCellAttr):
    def __init__(self):
        AddressCellAttr.__init__(self)

        self.SetTextColour('#FF0000')

class UnparsableAddressCellAttr(AddressCellAttr):
    def __init__(self):
        AddressCellAttr.__init__(self)

        self.SetTextColour('#555555')

The rest of the classes are subclasses of the first subclass. I figured that the singleton class method would work for all the subclasses as well since it operates on the class, and the subclass is indeed a separate class.

What happens is that after I call singleton once on AddressCellAttr, the singleton method returns that same object on all the subclasses too. Why does this happen?

Answer 1

Not sure why you think you need a singleton pattern here, but in any case, you should really be doing this in __new__.

class Singleton(object):

    def __new__(cls):

        try:
            return cls._instance
        except AttributeError:
            cls._instance = object.__new__(cls)
            return cls._instance

Just make sure all subclasses call parent __new__(), and remember that the signature for __new__() and __init__() must match.