Question

What is happening here?

What do the brackets do here test2 = A()?

How can I make object A() callable.

class A(object):
    @property
    def a(self):
        "an important attribute"
        return ["a value","dsfsdfsd"]

test1 = A

test2 = A()


print test1().a                                

print test2().a      

Answer 1

(I’ve edited my previous anwser, which was working but as pointed out, not really a good practice)

There is slight confusion between object representation in Python here :

test1 = A 
print test1 #return : <class '__main__.A'>

In python you can copy a class, because a class in also an object in Python. You can see as an alias, but it is more useful than that.

test2 = A() 
print test2 #return : <__main__.A object at 0x0126DF90>

Here you have created an A object at a certain adress.

print test1().a

What you are doing is in reality : A().a . In other word you create a temporary object and then call the a property. It works in your example but it is not more useful than a static method since the object is temporary (usually class methods take are used to view/modify an object’s internal state).

print test2().a # == [A()]().a
This does not work because you want to call an object as a method. To do so you have to add a __call__ special method in the class description. If your a property is important, that’s what I would do :

class State(object):
    " Dummy Finite state machine "

    def __init__(self)
         self.count = 0

    @property
    def state(self):
        return self.count % 4

    def __call__(self):
        return self.state

    def next(self):
        self.count +=1

s = State() # State object
print s() # return 0
s.next() # count = 1
s.next() # count = 2
s.next() # count = 3
s.next() # count = 4
s.next() # count = 5
print s() # return 5 % 4 =1