[semlar]

This will call your function whenever the player target changes..

frame:SetScript('OnEvent', function()
    if UnitIsPlayer("target") then
        frametext:SetText(UnitName("target"))
        classtext:SetText(UnitClass("target") .. " " .. UnitRace("target"))
        healthtext:SetText(UnitHealthMax("target") .. " Max HP")
        partybutton()
    end
end)
frame:RegisterEvent('PLAYER_TARGET_CHANGED')