mirror in 2.5

1 - Bpy ops

i tested this and get this from console report

bpy.ops.mesh.primitive_arrow1_add(l1=10, w1=0.4, w2=0.4, l2=2, w3=0.5, ang11=36)
bpy.ops.object.modifier_add(type=‘MIRROR’)

but if i change the Y or Z axis nothing happen in report console

so how do you wr rite this in script if you need an other axis then x?

2- bpy struct

now there must also be a way to select an object and do a mirror about an axis but how ?

Thanks