class Base(object):

    def __init__(self, username=None, password=None, start_url=None):
        self.username  = username
        self.password  = password
        self.start_url = start_url

class Subclass(Base):

    def __init__(self, username="hoss_it87", password="whatsgoodSO", start_url="www.boss-sauce.com"):
        super(Subclass, self).__init__()

In [12]: x = Base('myuser', 'mypassword', 'www.google.com')

In [13]: x.username
Out[13]: 'myuser'

In [14]: x.start_url
Out[14]: 'www.google.com'

In [15]: y = Subclass()

In [16]: y.username

In [17]: y.start_url

In [18]: y.password

In [19]: