gistfile1.txt

import bpy

bpy.ops.object.mode_set(mode = 'OBJECT')
obj = bpy.context.active_object

#deselect_all
for f in bpy.context.active_object.data.polygons:
    f.select = False    
for p in bpy.context.active_object.data.edges:
    p.select = False
    

#control variable
i=0
#select and separate
obj.data.face[i].select=True
bpy.ops.mesh.separate(type='SELECTED')