Python : Argument based Singleton

问题

I'm following this link and trying to make a singleton class. But, taking arguments (passed while initiating a class) into account so that the same object is returned if the arguments are same.

So, instead of storing class name/class reference as a dict key, I want to store passed arguments as keys in dict. But, there could be unhashable arguments also (like dict, set itself).

What is the best way to store class arguments and class objects mapping? So that I can return an object corresponding to the arguments.

Thanks anyways.

EDIT-1 : A little more explanation. Let's say there is class as follows

class A:
    __metaclass__ == Singleton
    def __init__(arg1, arg2):
        pass

Now, A(1,2) should always return the same object. But, it should be different from A(3,4)

I think, the arguments very much define the functioning of a class. Let's say if the class is to make redis connections. I might want to create 2 singletons objects with diff redis hosts as parameters, but the underlying class/code could be common.

回答1:

As theheadofabroom and me already mentioned in the comments, there are some odds when relying on non-hashable values for instance caching or memoization. Therefore, if you still want to do exactly that, the following example does not hide the memoization in the __new__ or __init__ method. (A self-memoizing class would be hazardous because the memoization criterion can be fooled by code that you don't control).

Instead, I provide the function memoize which returns a memoizing factory function for a class. Since there is no generic way to tell from non-hashable arguments, if they will result in an instance that is equivalent to an already existing isntance, the memoization semantics have to be provided explicitly. This is achieved by passing the keyfunc function to memoize. keyfunc takes the same arguments as the class' __init__ method and returns a hashable key, whose equality relation (__eq__) determines memoization.

The proper use of the memoization is in the responsibility of the using code (providing a sensible keyfunc and using the factory), since the class to be memoized is not modified and can still be instantiated normally.

def memoize(cls, keyfunc):
    memoized_instances = {}

    def factory(*args, **kwargs):
        key = keyfunc(*args, **kwargs)
        if key in memoized_instances:
            return memoized_instances[key]

        instance = cls(*args, **kwargs)
        memoized_instances[key] = instance
        return instance

    return factory


class MemoTest1(object):
    def __init__(self, value):
        self.value = value

factory1 = memoize(MemoTest1, lambda value : value)

class MemoTest2(MemoTest1):
    def __init__(self, value, foo):
        MemoTest1.__init__(self, value)
        self.foo = foo

factory2 = memoize(MemoTest2, lambda value, foo : (value, frozenset(foo)))

m11 = factory1('test')
m12 = factory1('test')
assert m11 is m12

m21 = factory2('test', [1, 2])

lst = [1, 2]
m22 = factory2('test', lst)

lst.append(3)
m23 = factory2('test', lst)

assert m21 is m22
assert m21 is not m23

I only included MemoTest2 as a sublclass of MemoTest1 to show that there is no magic involved in using regular class inheritance.

来源：https://stackoverflow.com/questions/39033946/python-argument-based-singleton

标签

python

dictionary

arguments

singleton

hashable