在 Python 中动态访问类实例“名称”

Question

In plain english: I am creating class instances dynamically in a for loop, the class then defines a few attributes for the instance. I need to later be able to look up those values in another for loop.

Sample code:

class A:
    def __init__(self, name, attr):
        self.name=name
        self.attr=attr

names=("a1", "a2", "a3")
x=10
for name in names:
    name=A(name, x)
    x += 1
...
...
...
for name in names:
    print name.attr

How can I create an identifier for these instances so they can be accessed later on by "name"?

I've figured a way to get this by associating "name" with the memory location:

class A:
    instances=[]
    names=[]
    def __init__(self, name, attr):
        self.name=name
        self.attr=attr
        A.instances.append(self)
        A.names.append(name)

names=("a1", "a2", "a3")
x=10
for name in names:
    name=A(name, x)
    x += 1
...
...
...
for name in names:
    index=A.names.index(name)
    print "name:  " + name
    print "att:  " + str(A.instances[index].att)

This has had me scouring the web for 2 days now, and I have not been able to find an answer. Maybe I don't know how to ask the question properly, or maybe it can't be done (as many other posts seemed to be suggesting).

Now this 2nd example works, and for now I will use it. I'm just thinking there has to be an easier way than creating your own makeshift dictionary of index numbers and I'm hoping I didn't waste 2 days looking for an answer that doesn't exist. Anyone have anything?

Thanks in advance, Andy

Update: A coworker just showed me what he thinks is the simplest way and that is to make an actual dictionary of class instances using the instance "name" as the key.

Answer 1

Sometimes keeping it simple is best. Having a dict that stores your instances with their names as the keys would be both straightforward and fairly simple to implement.

class A:
    instances={}
    def __init__(self, name, attr):
        self.name=name
        self.attr=attr
        A.instances[name] = self

and then to get the proper instance, just...

instance = A.instances[name]

Answer 2

No need to put the instance dict inside the class. Just create a dict, inst in the local scope:

class A:
    def __init__(self, name, attr):
        self.name=name
        self.attr=attr

inst={}
names=("a1", "a2", "a3")
x=10
for name in names:
    inst[name]=A(name, x)
    x += 1

Then, whenever you want to access a certain instance by name, just use inst[name] :

for name in names:
    print inst[name].attr

Answer 3

Yes, the dictionary approach should work well, and can be dovetailed into the class itself.

class A:
    _instances = {}

    @classmethod
    def get(cls, name):
        return A._instances[name]

    def __init__(self, name, attr):
        self.name=name
        self.attr=attr
        A._instances[name] = self

a = A('foo', 10)
aa = A.get('foo')

If you want to play around with __new__ , you can even make this transparent:

a = A('foo', 10)
aa = A('foo') # 'a' and 'aa' refer to the same instance.

This is a bit complicated, so I'll leave it to you to research (and of course ask another question on SO if you get stuck).