convert vertex color to vertex alpha

from pymel.core import*
import maya.cmds as mc
import time
start = time.clock()
sl = selected()
color = polyColorPerVertex(sl[0].vtx, q = 1, rgb=1, nun=1)# Get RGB
lencolor = len(color)/3
colorSum = []
x = 0
z = 3
pRange = len(sl[0].vtx)
window = mc.window(t="Progress Bar")
mc.columnLayout()
progressControl = mc.progressBar(maxValue=pRange, width=300)
mc.showWindow( window )
for i in range(0,lencolor): # to convert RGB into A
    sums = sum(color[x:z])/3
    colorSum.append(sums)
    x += 3
    z += 3
i = 0
for v in sl[0].vtx: # assign RGB to A
    progressInc = mc.progressBar(progressControl, edit=True, pr=i+1)
    MeshVertex.setColor(v,colorSum[i])
    i += 1
elapsed = (time.clock() - start)
print ("done, took:"), elapsed