From http://tech-artists.org/forum/showthread.php?5055-How-does-Maya-determine-the-primary-UV-set-and-the-UV-set-order

new_gist_file_0

'''
mayaScene.uv

namespace for uv manipulation and queries
'''

import maya.cmds as cmds


def get_names(obj):
    '''
    returns a name:index dictionary of the uvsets on the supplied object
    '''
    uvindices = cmds.polyUVSet(obj, q=True, uvn=True)
    uvnames = cmds.polyUVSet(obj, q=True, auv=True)
    return dict(zip(uvnames, uvindices))


def get_indices(obj):
    '''
    returns am index:name dictionary of the uvsets on the supplied object
    '''
    uvindices = cmds.polyUVSet(obj, q=True, uvn=True) or []
    uvnames = cmds.polyUVSet(obj, q=True, auv=True) or []
    return dict(zip(uvindices, uvnames))


def collapse(obj, map=None):
    '''
    Deletes all UV channes except the first. If the optional map parameter is supplied, that map will be copied into 
    the first map channe;

    Returns a string list of the deleted channel names
    '''

    indices = get_indices(obj)
    if map:
        cmds.polyUVSet(obj, cp=True, uvs=map, nuv=indices[0])
    else:
        cur_map = cmds.polyUVSet(obj, q=True, cuv=True)[0]
        if cur_map != indices[0]:
            cmds.polyUVSet(obj, cp=True, uvs=cur_map, nuv=indices[0])
    index_list = [k for k in indices]
    index_list.sort()
    index_list.reverse()
    index_list.pop()
    delenda = []
    for item in index_list:
        cmds.polyUVSet(obj, delete=True, uvs=indices[item])
        delenda.append(indices[item])
    return delenda


def delete_empty_uvs(obj):
    '''
    Deletes all empty UV channels on the supplied object

    @note: the first UV channel is not touched, even it if is empty, since it cannot be deleted.
    '''
    indices = get_indices(obj)
    index_list = [k for k in indices]
    index_list.sort()
    index_list.reverse()
    index_list.pop()
    delenda = []
    for item in index_list:
        ct = cmds.polyEvaluate(obj, uv=True, uvs=indices[item])
        if ct == 0:
            cmds.polyUVSet(obj, delete=True, uvs=indices[item])
            delenda.append(indices[item])
    return delenda


def find_multiple_uv_sets():
    '''
    Returns a list of all the meshes in the scene with multiple UV sets
    '''
    lvi = lambda uv_obj: len(cmds.polyUVSet(uv_obj, q=True, auv=True)) > 1
    meshes = cmds.ls(type='mesh', ni=True)
    return filter(lvi, meshes)


def force_map_1(obj, map=None):
    '''
    collapse UV sets and make sure that the surviving UV channel is named 'map1' for finicky programs that care about 
    UV set names
    '''
    delete_empty_uvs(obj)
    collapse(obj, map)
    cmds.polyUVSet(obj, rename=True, newUVSet="map_rename_temp")
    cmds.polyUVSet(obj, rename=True, newUVSet="map1")