Add a Bezier circle curve and an empty, make the empty animate by following the circle curve

empty_following_a_bezier_circle_curve.py

import bpy

last_frame = 90
# set the lenght of the animation
bpy.context.scene.frame_end = last_frame

# add a Bezier circle curve into the scene
bpy.ops.curve.primitive_bezier_circle_add()
bezier_circle_obj = bpy.context.active_object

# set the number of frames it would take to traverse the curve
bezier_circle_obj.data.path_duration = last_frame

# add an empty into the scene
bpy.ops.object.empty_add()
empty_obj = bpy.context.active_object

# add the "follow path" constraint
bpy.ops.object.constraint_add(type="FOLLOW_PATH")

empty_obj.constraints["Follow Path"].target = bezier_circle_obj
bpy.ops.constraint.followpath_path_animate(constraint="Follow Path", owner="OBJECT")
empty_obj.constraints["Follow Path"].use_curve_follow = True