from unittest import TestCase
from unittest.mock import MagicMock


class Foo:
    def __init__(self, x, y, z):
        self.x = x
        self.y = y
        self.z = z

    def __str__(self):
        return '{0}(x={1}, y={2}, z={3})'.format(self.__class__.__name__, self.x, self.y, self.z)

    def __repr__(self):
        return self.__str__()


class Test(TestCase):
    def test1(self):
        bar = MagicMock()
        a = Foo('aa', 'bb', 'cc')
        bar.x(a)
        print(bar.mock_calls)  # Prints [call.x(Foo(x=aa, y=bb, z=cc))], it's ok
        a.y = 'FooBar'
        bar.x(a)
        print(bar.mock_calls)  # Prints [call.x(Foo(x=aa, y=FooBar, z=cc)), call.x(Foo(x=aa, y=FooBar, z=cc))]. I would expect [call.x(Foo(x=aa, y=bb, z=cc)), call.x(Foo(x=aa, y=FooBar, z=cc))] here