Your class definitions are incorrect: you do not need an area or circumference class, they should b e methods of your main class. Also you never create any objects of those classes so nothing gets done. I suggest you go to the
9. Classes — Python 3.9.10 documentation[
^] section of the Python tutorials and look at the explanations and samples.
Also, you do not need to convert numbers to strings and keep them internally, that will be done automatically when you print them, like this:
pi = 3.14
radius = 5
circumference = radius * 2 * pi
print("radius = {}, circumference = {}".format(radius, circumference))