Invert BlendShape weights

Maya_InvertShapeWeights.py

from maya import cmds
import pymel.core as pm

sel = pm.ls(selection=True)
print sel[0]
blends = sel[0].getShape().inputs(type='blendShape')
if len(blends) < 1:
    print "No blend shapes on this node!"
else:
    
    vertexCount = pm.polyEvaluate(sel[0], vertex=True)
    weights = blends[0].inputTarget[0].baseWeights.get()
    newWeights = [1.0 - x for x in weights]
    
    for i, wt in enumerate(weights):
        blends[0].inputTarget[0].baseWeights[i].set(newWeights[i])