What is the test for determining whether a legislatively prescribed punishment is constitutionally excessive?

"We add that the determination of whether a legislatively prescribed punishment is constitutionally excessive is not a duty which the courts eagerly assume or lightly discharge. Here, as in other contexts, ' "mere doubt does not afford sufficient reason for a judicial declaration of invalidity. Statutes must be upheld unless their unconstitutionality clearly, positively and unmistakably appears." ' [Citation omitted.] When such a showing is made, however, we must forthrightly meet our responsibility 'to ensure that the promise of the Declaration of Rights is a reality to the individual.' [Citation omitted.]" (Id., at pp. 414-415, 105 Cal.Rptr. 217, 503 P.2d 921; see also People v. Wingo (1975) 14 Cal.3d 169, 174, 121 Cal.Rptr. 97, 534 P.2d 1001.)