def RotateExtend(plyP,sLength):
 l=plyP.length
 ptX=plyP.positionAlongLine (l/2).firstPoint
 ptX0=plyP.firstPoint
 ptX1=plyP.lastPoint
 dX=float(ptX1.X)-float(ptX0.X)
 dY=float(ptX1.Y)-float(ptX0.Y)
 lenV=math.sqrt(dX*dX+dY*dY)
 sX=-dY*sLength/lenV;sY=dX*sLength/lenV
 leftP=arcpy.Point(ptX.X+sX,ptX.Y+sY)
 rightP=arcpy.Point(ptX.X-sX, ptX.Y-sY)
 array = arcpy.Array([leftP,ptX])
 section=arcpy.Polyline(array)
 return section
#-----------------
RotateExtend( !Shape!, ! !SHIFT!)