Welcome, guest | Sign In | My Account | Store | Cart
2

This is an implementation of the singleton without using the __new__ class but by implementing the __call__ method in the metaclass.

Python, 22 lines
 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21
22
class Singleton(type):
    def __init__(self, *args):
        type.__init__(self, *args)
        self._instances = {}

    def __call__(self, *args):
        if not args in self._instances:
            self._instances[args] = type.__call__(self, *args)
        return self._instances[args]

class Test:
    __metaclass__=Singleton
    def __init__(self, *args): pass

            
ta1, ta2 = Test(), Test()
assert ta1 is ta2

tb1, tb2 = Test(5), Test(5)
assert tb1 is tb2

assert ta1 is not tb1

If you want a class instance to behave a certian way when it is called (i.e. obj() ) you implement its class __call__ method. If you want the class definition to behave in a certian way when called, you implement its metaclass __call__ method.

Usually (i.e. the type class) this method returns a new object, but you can do whatever you want here.

This specific implementation has a different instance per different init calls (good for immutable but changing classes, like numeric-type classes), but you can change that as you want.

Add a comment

Sign in to comment

Created by Daniel Brodie on Thu, 28 Apr 2005 (PSF)
Python recipes (4552)
Daniel Brodie's recipes (5)

Required Modules

  • (none specified)

Other Information and Tasks