Changing `self` for another instance of same object?

I want to create a class, and all objects need to have a unique identifier key, and If I attempt to create a new instance of the object with a previously existent key, the instance should be the same as the one that already existing.

Similar to a singleton class, but in this case instead of one class, there are many but different.

My first approach was this

class Master:
    existent = {}
    def __init__(self, key, value=None):
        try:
            self = Master.existent[key]
            return 
        except KeyError:
            Master.existent[key] = self
        # Rest of the __init__ method

But when I compare two objects, something like this A = Master('A', 0) and B = Master('B', 0), the B doesn't share any attributes that It should have, and if the Master class has any _method (single underscore), It also doesn't appear.

Any Idea how could I do this?

I think this is similar to the Factory Methods Pattern, but I'm still having trouble to find the parallels, or how to implemented in an elegant form.