How would you ask your professor to be your supervisor? I am currently a graduate student. I've completed all the exams and I am currently looking for a supervisor for my master's dissertation. Few months ago I surveyed the professor I would like to have as my supervisor. However, he told me that probably he would have not been able to be my supervisor if I decided to graduate in July 2020 (as I intend to do), because he will probably be out of the country on the anticipated graduation date. He said that it is a moral duty for a supervisor to be there on the graduation date. Now, I really enjoyed his lectures, I consider him the best professor I have ever had and I think he is really skilled on the topic I am planning to discuss. Accordingly I would be willing to accept the risk that he will not be able to be there on the graduation date. How could I ask him to be my supervisor?
To make the answer formal, yes, you should ask him. This is best done in person, and one of the best approaches is just what you say here about his skill and your common interest in a topic. Let him know that you think he is the best person around to do this, and you won't have a problem, yourself, if he can't be at the graduation. 

What you need is his signature, I assume, and not his physical presence to approve your completion. 

He might say no, but if you don't ask, the answer is certainly no. 

Good luck.