Use argparse instead of optparse, if you are using Python >= 2.7. It does dedent
for you. You can just do:
parser.add_argument('--argument', '-a', help='''
this is my
long help text
''')
Even if you are using Python < 2.7, you can install argparse from pypi.
Note that there is a way to suppress this auto-dedent behavior. The link given by @msw is actually the section about it.