test.py

class Either(object):
    pass

class Left(Either):
    def __init__(self, v): self.v = v
    def is_left(self): return True
    def is_right(self): return False
    def value(self): return self.v

class Right(Either):
    def __init__(self, v): self.v = v
    def is_left(self): return False
    def is_right(self): return True
    def value(self): return self.v

errval = Left("This value is unoccupied!")
goodval = Right(1)

def doSomething(val):
    if val.is_left():
        print(val.value())
        exit()

    print(val.value() + 1)

doSomething(goodval)
doSomething(errval)