asterisk = s.find('*')
if asterisk == -1:
    return s
else:
    return s[:asterisk] + '*' + '-' * len(s[asterisk + 1:])

print '{:-<{}}'.format(s[:s.find('*') + 1] or s, len(s))

>>> s = 'Hello world!*la-la'
>>> print '{:-<{}}'.format(s[:s.find('*') + 1] or s, len(s))
Hello world!*-----

>>> def f(s):
...     l, c, r = s.partition('*')
...     return l + c + '-' * len(r)
... 
>>> f('abc*def*ghi')
'abc*-------'
>>> f('abcdefghi')
'abcdefghi'
>>>