what gets returned when you return self?

Question

what gets returned when you return 'self' inside a python class? where do we exactly use return 'self'? In the below example what does self exactly returns

class Fib:
'''iterator that yields numbers in the Fibonacci sequence'''

    def __init__(self, max):
        self.max = max

    def __iter__(self):
        self.a = 0
        self.b = 1
        return self

    def __next__(self):
        fib = self.a
        if fib > self.max:
            raise StopIteration
        self.a, self.b = self.b, self.a + self.b
        print(self.a,self.b,self.c)
        return fib

Answer 1

Python treats method calls like object.method() approximately like method(object). The docs say that "call x.f() is exactly equivalent to MyClass.f(x)". This means that a method will receive the object as the first argument. By convention in the definition of methods, this first argument is called self.

So self is the conventional name of the object owning the method.

Now, why would we want to return self? In your particular example, it is because the object implements the iterator protocol, which basically means it has __iter__ and __next__ methods. The __iter__ method must (according to the docs) "Return the iterator object itself", which is exactly what is happening here.

As an aside, another common reason for returning self is to support method chaining, where you would want to do object.method1().method2().method3() where all those methods are defined in the same class. This pattern is quite common in libraries like pandas.

Answer 2

The keyword self is used to refer to the instance that you are calling the method from.

This is particularly useful for chaining. In your example, let's say we want to call __next__() on an initialized Fib instance. Since __iter__() returns self, the following are equivalent :

obj = Fib(5)
obj.__iter__() # Initialize obj 
obj.__next__()

And

obj = Fib(5).__iter__() # Create AND initialize obj
obj.__next__()

In your particular example, the self keyword returns the instance of the Fib class from which you are calling __iter__() (called obj in my small snippet).

Hope it'll be helpful.