From http://tech-artists.org/forum/showthread.php?4922-Looking-to-improve-the-speed-of-one-of-my-PyMEL-functions #Util

new_gist_file_0.py

def getUVShells(shape, uvSet=None):
    
    uvSets = cmds.polyUVSet(shape, q=True, allUVSets=True)
    if not uvSet or uvSet not in uvSets:
        uvSet = cmds.polyUVSet(shape, q=True, cuv=True)[0]

    selectionList = om.MSelectionList()
    selectionList.add(shape)

    mDag = om.MDagPath()
    selectionList.getDagPath(0, mDag)
    
    meshFn = om.MFnMesh(mDag)
    
    uvShellArray = om.MIntArray()
    shells = om.MScriptUtil()
    shellsPtr = shells.asUintPtr()
    
    meshFn.getUvShellsIds(uvShellArray, shellsPtr, uvSet)
    
    uvShells = defaultdict(list)
    for i, shellId in enumerate(uvShellArray):
        uvShells[shellId].append('{0}.map[{1}]'.format(shape, i))
    
    return uvShells.values()