"unexpected indent" error when using notepad++ for creating django function

I am following this book to learn django using notepad++, something interesting happens, when I type the function using notepad++ for the following script:

def current_datetime(request):
    now = datetime.datetime.now()
    html = "<html><body>It is now %s.</body></html>" % now
    return HttpResponse(html)

It gives me an error like this:

IndentationError at /time/

('unexpected indent', ('M:\\DjangoStack\\projects\\beta_01\\..\\beta_01\\hello_world\\views.py', 12, 1, '\thtml = "<html>"\n'))

But when I paste it directly from the book, it is OK. I wonder why, should I do some settings in notepad++? Thanks.

update 01

I use 4 space bars to create the indent in notepad++, I tried 1 tab and seems the problem is fixed