How to fix issues with using classes in variables

The format which I'm using to execute some code based on which class the variable 'tester' is appears not to be working. How can I write the code such that the variable 'var' in the class 'testClass()' is changed

I've tried changing the format from "if tester == testClass():" to "if testClass() in [tester]:", but neither work nor return errors. Changing 'var = second' to 'var = "second"' doesn't appear to help either, and neither does removing any brackets.

The code is as follows:

class fillClass():
    fill = True
class testClass():
    var = "first"
    filler = True
tester = testClass()
oldVar = tester.var
print(oldVar,"is older variable")
second = "second"
if tester == testClass():
    class testClass():
        var = second
        filler = True
tester = testClass()
newVar = tester.var
print(newVar,"is newer variable")

I would expect the output

first is older variable
second is newer variable
>>> 

but the actual output is

first is older variable
first is newer variable
>>> 

How can I fix this issue?

Many thanks!