16 Pa. Stat. § 1404
If any vacancy shall occur in the office of district attorney in a county of the second class A through eighth class, the judges of the court of common pleas shall, upon a showing that the first assistant district attorney satisfies the requirements of section 1401, appoint the first assistant district attorney to fill the office of district attorney and to discharge the duties of the district attorney until the first Monday in January following the next municipal election occurring not less than ninety days after the occurrence of the vacancy. If the first assistant district attorney is unwilling or unable to serve or does not satisfy the requirements of section 1401, the judges of the court of common pleas shall fill the vacancy by the appointment of a competent person who satisfies the requirements of section 1401 to fill the office until the first Monday in January following the next municipal election occurring not less than ninety days after the occurrence of the vacancy.
16 P.S. § 1404