What do I do if I want to make an if statement that is dependent on an output? (Im running python)

Ok so I'm making a tip calculator and I want to make it so that if it prints a message because the input was not a float, then it reads the fact that it prints that message and goes back to the original input and makes you try again so can i do something like: if print("No stupid use a number."): return input("Bill before tax.")