Python returns MagicMock object instead of return_value

I have a python file a.py which contains two classes A and B.

class A(object):
    def method_a(self):
        return "Class A method a"

class B(object):
    def method_b(self):
        a = A()
        print a.method_a()

I would like to unittest method_b in class B by mocking A. Here is the content of the file testa.py for this purpose:

import unittest
import mock
import a


class TestB(unittest.TestCase):

    @mock.patch("a.A")
    def test_method_b(self, mock_a):
        mock_a.method_a.return_value = "Mocked A"
        b = a.B()
        b.method_b()


if __name__ == "__main__":
    unittest.main()

I expect to get Mocked A in the output. But what I get is:

<MagicMock name="A().method_a()" id="4326621392">

Where am I doing wrong?