The Python interpreter will recognize spaces or tabs characters. The only gottcha is you must never mix spaces and tabs, pick one or the other. I'd guess that you have mixed them and are unable to tell in the script editor-- The mixing is causing the IndentationError.
The specification (PEP-8) recommends spaces, most developers use spaces, so unless you have a really good reason not to (the file is already in all tabs), I'd say go with spaces. I always edit in an external editor (vim) that inserts only spaces and will highlight tabs characters to warn me.
