Python: how to refer to an instance name?

Question

I'm collecting instances using the following code:

class Hand():
    instances = []
    def __init__(self):
        Hand.instances.append(self)
        self.value = 5

    def do_something(self, a):
        self.value = self.value * a

class Foo():
    def __init__(self):
        pass

    def insty(self):
        self.hand1 = Hand()
        self.hand2 = Hand()


foo = Foo()
foo.insty()
print Hand.instances

for hand in Hand.instances:
    print "how do I print the instance name?"

The last line is just a way to learn how to to access the instance name so i can call the 'do_something' method on each instance in order.

How do I access the instance name for each instance of Hand?

Answer 1

If you mean how to get hand1 from the instance you assigned to self.hand1, the answer is that you can't. When you do self.hand1 = Hand(), you tell the Foo object it has a Hand, but the Hand object has no knowledge that it has been assigned to a Foo. You could do this:

h = Hand()
self.bob = h
self.larry = h

Now what is the "name" of that Hand supposed to be? You assigned the same hand to both "bob" and "larry", so there's no way it can have a single unique name.

If you want to have a name for each hand, you need to tell the hand what name you want to give it. You would have to modify your Hand code to allow you to pass a name to the constructor, then create the Hand with Hand("some name").

You can of course give the hands "names" by assigning attributes on them:

self.hand1 = Hand()
self.hand1.name = "hand 1"

. . . but these names are not special or "automatic" in any way.

The bottom line is that if you want something to have a name, you need to decide how to handle that name. You need write your own code that gives it its name, and your own code that retrieves its name.

Answer 2

I don't know if this would solve your problem or not. I needed to get instance names in order to do clear error reporting. Everywhere I looked, folks said "variables don't have names! The name is just a pointer to the thing!"

But it turns out that getting instance names in python is pretty straightforward. Here's how I did it:

import gc
def instance_names(self):
    referrers = gc.get_referrers(self)
    result = []
    dict_of_things = {}
    for item in referrers:
        if isinstance(item, dict):
            dict_of_things = item
    for k, v in dict_of_things.items():
        if v == self:
            result.append(k)
    if not result:
        result = ['unnamed instance']
    return result