Before pronouncing sentence, the defendant shall be asked whether he has anything to say why the sentence should not be pronounced against him. The only reasons which can be shown, on account of which sentence cannot be pronounced, are:
1. That the defendant has received a pardon from the proper authority, on the presentation of which, legally authenticated, he shall be discharged.
2. That the defendant is incompetent to stand trial; and if evidence be shown to support a finding of incompetency to stand trial, no sentence shall be pronounced, and the court shall proceed under Chapter 46B; and
3. When a person who has been convicted escapes after conviction and before sentence and an individual supposed to be the same has been arrested he may before sentence is pronounced, deny that he is the person convicted, and an issue be accordingly tried before a jury, or before the court if a jury is waived, as to his identity.