class TestClass(object):

    def __init__(self, param_str=''):
        self.the_str = ''
        for c in param_str:
            if c.isalpha():
                self.the_str += c

    def __add__(self, param):
        if type(param) == TestClass:
            the_str = self.the_str + param.the_str
            return TestClass(the_str)
        else:
            return self

    def __str__(self):
        return 'Value: {}'.format(self.the_str)


inst1 = TestClass('abc')
inst2 = TestClass('123ijk')
sumInst1 = inst1 + inst2
sumInst2 = inst1 + 'xyz'
print(inst1)  # Line 1
print(sumInst1)  # Line 2
print(sumInst2)  # Line 3
print(isinstance(sumInst2, TestClass))  # Line 4
