'Initializes a new L{UNIXAuthorizedKeysFiles}.
@param userdb: access to the Unix user account and password database
(default is the Python module L{pwd})
@type userdb: L{pwd}-like object
@param parseKey: a callable that takes a string and returns a
L{twisted.conch.ssh.keys.Key}, mainly to be used for testing.  The
default is L{twisted.conch.ssh.keys.Key.fromString}.
@type parseKey: L{callable}'

def __init__(self, userdb=None, parseKey=keys.Key.fromString):

    self._userdb = userdb
    self._parseKey = parseKey
    if (userdb is None):
        self._userdb = pwd
