In python, how to do unit test on a function without return value?

I have some code like:

class HelloTest:

    def foo(self, msg):
        self.bar(msg.upper())

    def bar(self, msg):
        print(msg)

Using the unittest standard library, we can use methods like assertEqual, assertTrue etc. to verify what is returned from a function.

Since foo does not return a value explicitly, how can I test that it does the right thing?