If all you need to do is launch Excel, the best way to do it is to use LaunchServices to do it.
If you have PyObjC (which you do if you're using the Python that Apple pre-installs on 10.6 and later; otherwise, you may have to install it):
import Foundation
ws = Foundation.NSWorkspace.sharedWorkspace()
ws.launchApplication_('Microsoft Excel')
If not, you can always use the open tool:
import subprocess
subprocess.check_call(['open', '-a', 'Microsoft Excel'])
Either way, you're effectively launching Excel the same way as if the user double-clicked the app icon in Finder.
If you want to make Excel do something simple like open a specific document, that's not much harder. Look at the NSWorkspace or open documentation to see how to do whatever you want.
If you actually want to control Excel—e.g., open a document, make some changes, and save it—you'll want to use its AppleScript interface.
Apple's recommended way of doing that is via ScriptingBridge, or using a dual-language approach (write AppleScripts and execute them via NSAppleScript—which, in Python, you do through PyObjC). However, I'd probably use appscript (get the code from here). Despite the fact that it's been abandoned by its original creator, and is only being sparsely maintained, and will probably eventually stop working with some future OS X version, it's still much better than the official solutions.
Here's a sample (untested, because I don't have Excel here):
import appscript
excel = appscript.app('Microsoft Excel')
excel.workbooks[1].column[2].row[2].formula.set('=A2+1')
os.startfile("something.xls")on mac?