Python: Find Instance of a class by value

Question

i created much instances of a class. Then I want to find one instance by its name. But I get the error message TypeError: get() missing 1 required positional argument: 'value'.

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

    def get(self, value):
        if self.value == value:
            return self
        else:
            return None

test_obj = Test('foobar')

print(test_obj.value)

instance = Test.get('foobar')

if instance:
    print(instance.value)

Answer 1

Re-reading your question again, I think all of us have missed the point so far. You wanted to check all instances of the class Test to see if an instance has the value 'foobar' (in this case, test_obj. Referencing this answer, you can modify your code like so:

class Test(object):
    # class attribute to keep track of class instances
    instances = []
    def __init__(self, value):
        self.value = value
        Test.instances.append(self)

    # class method to access the get method without any instance
    @classmethod
    def get(cls, value):
        return [inst for inst in cls.instances if inst.value == value]

You can then create multiple tests:

test1 = Test(1)
test2 = Test(2)
test3 = Test(3)

instance = Test.get(3)
# [<__main__.Test object at 0x03F29CD0>]

instance[0].value
# 3

It makes sense for me to return a list of instances instead of one single instance. If you however is only interested in the first match, you can modify the return statement accordingly.

---

Original answer:

instance = Test.get('foobar') is the problem. You're referencing Test by its class, not its instance. So naturally the instance method .get(self, value) is looking for the first argument self for the instance.

Usually if you already have an instance (e.g. Test().get('foobar')) then the instance is passed into the instance method as self by default.

You could still call the instance method, but you just need to explicitly pass the instance in:

Test.get(test, 'foobar')