singleton in python, with qobject

user4592357

i need a singleton class, so here's what i got:

__instances = {}


class _Singleton(type):
    def __new__(meta, name, bases, members):
        for class_ in bases:
            if class_ is not Singleton and issubclass(class_, Singleton):
                raise TypeError(("Invalid base type (as subclass of Singleton):", class_))
        return type.__new__(meta, name, bases, members)    

    def __call__(class_, *args, **kwargs):
        if class_ is Singleton:
            raise TypeError("The base Singleton type cannot be instantiated")
        if class_ not in __instances:
            __instances[class_] = super(_Singleton, class_).__call__(*args, **kwargs)
        return __instances[class_]

Singleton = type('_', (), {}) # dummy type to allow base type creation
Singleton = _Singleton('Singleton', (object, ), {})

my controller object is a subclass of QObject, and i get an error:

class Controller(QObject, '''metaclass='''Singleton):
TypeError: metaclass conflict: the metaclass of a derived class must be a (non-strict) subclass of the metaclasses of all its bases

jazzycamel

The following is my implementation (and a usage example) of a singleton I use regularly:

try: from PyQt5.QtCore import pyqtWrapperType
except ImportError:
    from sip import wrappertype as pyqtWrapperType

class Singleton(pyqtWrapperType, type):
    def __init__(cls, name, bases, dict):
        super().__init__(name, bases, dict)
        cls.instance=None

    def __call__(cls,*args,**kw):
        if cls.instance is None:
            cls.instance=super().__call__(*args, **kw)
        return cls.instance

if __name__=="__main__":
    from PyQt5.QtCore import QObject, pyqtSlot, pyqtProperty

    class MyObject(QObject, metaclass=Singleton):
        def __init__(self, parent=None, **kwargs):
            super().__init__(parent, **kwargs)

            self._x=0

        def x(self): return self._x

        @pyqtSlot(int)
        def setX(self, x): self._x=x

        x=pyqtProperty(int, x, setX)

    print(MyObject().x)
    MyObject().x=1
    print(MyObject().x)

I've actually shared this example/technique so many times now that I just quickly created a git repo (https://github.com/jazzycamel/PyQt5Singleton) and a PyPi package (https://pypi.python.org/pypi/PyQt5Singleton) so you can just install it via pip:

$ pip install PyQt5Singleton

and use it (as above):

from PyQt5.QtCore import QObject, pyqtSlot, pyqtProperty
from PyQt5Singleton import Singleton

class MyObject(QObject, metaclass=Singleton):
    def __init__(self, parent=None, **kwargs):
        super().__init__(parent, **kwargs)

        self._x=0

    def x(self): return self._x

    @pyqtSlot(int)
    def setX(self, x): self._x=x

    x=pyqtProperty(int, x, setX)

if __name__=="__main__":
    print(MyObject().x)
    MyObject().x=1
    print(MyObject().x)

Hope this helps :)