Iterate through all the shells of mesh

enum_shells.py

from pymel.all import *

def getShellFaces( poly, str = 0 ): 
	shells = set() 
	faces = set() 
	total = polyEvaluate( poly, s=1) 

	for f in xrange( polyEvaluate( poly, f=1 ) ): 

		if len( shells ) >= total: 
			break 
		if f in faces: 
			continue 

		shell = polySelect( poly, ets=f ) 
		faces.update( shell ) 

		if str: 
			val = "%s.f[%d:%d]" % ( poly, min( shell ), max( shell ) ) 
		else: 
			val = min( shell ) 

		shells.add( val ) 

	return list( shells ) 

poly = ls( sl=1 )

# get the first face of each shell to be selected 
# later with polySelect 
faceIds = getShellFaces( poly ) 
for f in faceIds: 
	polySelect( poly, ets=f )