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

import pymel.all as pm

def get_selected_uv_shells():
    sel = pm.selected(o = True)[0]
    selected_uvs = set(uv for uv in pm.selected() if isinstance(uv, pm.MeshUV))
    # getUvShellsIds returns a list of shell_ids, and the number of shells
    uv_shell_ids, num_shells = sel.getUvShellsIds()
    # You still need to actually get the MeshUVs yourself    
    # Using sets instead of sublists because checking if an item is in a set is much faster
    all_shells = [set() for i in xrange(num_shells)]
    for uv_index, uv_shell_id in enumerate(uv_shell_ids):
        all_shells[uv_shell_id].add(sel.uvs[uv_index])
    # If you've not selected any UVs, I'm assuming you want them all.
    # Converting your selection to uvs before getting selected_uvs would probably be more accurate
    if not selected_uvs:
        return all_shells
    selected_shells = []
    # This goes through and finds shells that contain one of the selected uvs
    while selected_uvs:
        test_uv = selected_uvs.pop()
        for shell in all_shells:
            if test_uv in shell:
                break
        selected_shells.append(shell)
        # No need to check against that shell again
        all_shells.remove(shell)
        # Also we can get rid of all the uvs from that shell
        # This way we don't end up iterating through all the UVs
        selected_uvs.difference_update(shell)
    return selected_shells
get_selected_uv_shells()