Estonia - 1992 Constitution

Title Article 84
Chapter 5 - The President of the Republic
Article

84

Upon assuming office, the authority and duties of the President of the Republic in all elected and appointed offices shall cease, and he or she shall suspend his or her membership in political parties.

Categories - Extra-parliamentary party (Members' compatibility)
Source Gisbert H. Flanz ed. 2004. Constitutions of the Countries of the World. Dobbs Ferry, New York: Oceana Publications.