Hi Mike,
Lambda functions in python are anonymous functions. If I defined func = lambda x : x + 3 , func(4) would return 7.
In this case func is a lambda function which returns x[1].Current.Value for an input x. The way that func is defined implies that x is a list with an indicator at index 1.
We can actually rewrite
func = lambda x: x[1].Current.Value
top3 = {x[0]: x[1].Current.Value for x in sorted(self.data.items(), key=func, reverse=False)[:3]}
in one line, which is how it is usually written.
top3 = {x[0]: x[1].Current.Value for x in sorted(self.data.items(), key=lambda x: x[1].Current.Value, reverse=False)[:3]}
Learn more about sorting in python with lambdas here.