partialist
/ˈpɑːʃ(ə)lɪst/ /ˈpɑːʃəlɪst/noun
1A partial, prejudiced, or biased person; a person who favours one party or side unduly; a partisan, enthusiast.
2A person who holds a partial theory or view, or whose knowledge or outlook is limited; a specialist.
Origin
Late 16th century; earliest use found in Samuel Daniel (?1563–1619), poet and historian. From partial + -ist. Compare Middle French, French †partialiste.