Extend polygons selection to shell

extend_polysel_to_shell.py

import maya.OpenMaya as om
import maya.cmds as cmds
def expandSelection():
   sel = om.MSelectionList()
   om.MGlobal.getActiveSelectionList(sel)
   dag = om.MDagPath()
   obj = om.MObject()
   # get the active object (dag) and its selected components (obj)
   sel.getDagPath(0, dag, obj)

   # Here we create an iterator over the selected components
   itr = om.MItMeshPolygon(dag, obj)
   # Now we add all faces of the selection to the array
   currfaces = om.MIntArray()
   while not itr.isDone():
      currfaces.append(itr.index())
      itr.next()

   # finally we reconvert the MIntArray to a simple list
   currfaces = list(currfaces)
   # and use maya.cmds to extend the polygon-selection to their shells
   cmds.polySelect(dag.fullPathName(), extendToShell = currfaces, add=True)
expandSelection()