Proper way to mock classes and assert on calls to methods

read is a method on instances of Foo. You want to check for the mock return_value to access the instance. After all, you create the instance by calling foo.Foo():

foo_instance = self.foo_mock.return_value
foo_instance.read.assert_called_once_with('foo')

Note that you are patching foo.Foo; using bar.foo.Foo is the same object, but a round-about way of specifying it.