Modifying Class Object

[T]

Ok, just looking for a sanity check here, or maybe something I'm
missing.  I have a class Test, for example:

class Test:
    def __init__(self, param1, param2, param3):
        self.param1 = param1
        self.param2 = param2
        self.param3 = param3

Next, I have a dictionary mytest that contains instances of Test.  If
I want to modify one of the Test instances within my dictionary, I
have to rewrite the entire entry, correct (since Python passes by
value, not reference)?  I.e. if I wish to change just param3 of an
instance, I would have to do:

def changevalue():
    for key in mytest.keys():
        currentparam = mytest[key]
        param1 = currentparam.param1
        param2 = currentparam.param2
        param3 = currentparam.param3
        param3 = "newvalue"
        mytest[key] = Test(param1, param2, param3)

If there's an easier way to accomplish this that I'm missing, that'd
be great!