Are we conducting a review of a district court's exercise of discretion in a defense based on self-defense?

A preliminary question is whether we are conducting a review of the district court's exercise of discretion or a review de novo of an error of law. If the district court's ruling is understood as a determination that any record not known to the defendant is inadmissible as part of a defense based on self-defense, its ruling was one of law, and our review is de novo. If the district court is understood to have implicitly weighed the probative force of the evidence against its prejudicial impact on the jury by making the victim seem odious, then our review is for an abuse of discretion. As both ways of interpreting the district court's action are tenable, we shall review, first, de novo as to law, United States v. Keiser, 57 F.3d 847, 852 n. 6 (9th Cir.1995), and, second, review the postulated exercise of discretion.