Python: make a derived class into a singleton

Question

I want to turn my derived class into a singleton in python. I would like to implement the singleton via metaclass, but I always come across the following error:

TypeError: metaclass conflict: the metaclass of a derived class must be a (non-strict) subclass of the metaclasses of all its bases

My Code:

# singleton.py
class Singleton(type):
    _instances = {}
    def __call__(cls, *args, **kwargs):
        if cls not in cls._instances:
            cls._instances[cls] = super(Singleton, cls).__call__(*args, **kwargs)
        return cls._instances[cls]

# base.py
from abc import ABC, abstractmethod

class Base(ABC):
    @abstractmethod
    def run(self, value):
        pass

# foo.py
from base import Base
from singleton import Singleton

class Foo(Base, metaclass=Singleton):
    def run(self, value):
        print(value)

# main.py
from foo import Foo

f1 = Foo()
print(f1)
f1.run(42)
f2 = Foo()
print(f2)
f2.run(24)

score 2 · Accepted Answer · answered Aug 12 '20 at 11:13

Through various other stackoverflow questions I have now come up with a solution myself.

As the error text already says, the metaclass of Foo must be a metaclass that is compatible with the base class metaclass (=ABCMeta). This means that Singleton must also inherit from ABCMeta.

New Code:

# singleton.py

from abc import ABCMeta

class Singleton(ABCMeta):
    # ...

# bar.py
from base import Base

class Bar(Base):
    def run(self, value):
        print(value)

# main.py
from foo import Foo
from bar import Bar

f1 = Foo()
print(f1)
f1.run(42)
f2 = Foo()
print(f2)
f2.run(34)

b1 = Bar()
print(b1)
b1.run(12)
b2 = Bar()
print(b2)
b2.run(21)

Output:

<foo.Foo object at 0x000001D4D512BE48>
42
<foo.Foo object at 0x000001D4D512BE48>
34
<bar.Bar object at 0x000001D4D512B0B8>
12
<bar.Bar object at 0x000001D4D512BEF0>
21

So Foo is a singleton and Bar isn't.

Bonus question:

Why doesn't Singleton(ABC) work?

ABC is just a base class.. and not metaclass.. You can derive a metaclass from another metaclass, and not from just another Class.. — Arun Kaliraja Baskaran, Aug 12 '20 at 15:59
`Base` is derived from `ABC`. This gives `Base` the metaclass `ABCMeta`. Why doesn't this also work for the (meta)class `Singleton`? — Käseknacker, Aug 14 '20 at 05:06
So, it seems that you just replaced "class Singleton(type):" to "class Singleton(ABCMeta):"? What was the 'type' used for? Was it not important? — lightbox142, Jan 21 '21 at 18:17

Python: make a derived class into a singleton

1 Answers1