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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def password_get(username=None):
 """ Retrieves a password from the keychain based on the environment and configuration parameter pair. If this fails, None is returned. """

    password = keyring.get_password('supernova', username)
    if password is None:
        split_username = tuple(username.split(':'))
        msg = ("Couldn't find a credential for {0}:{1}. You need to set one "
               "with: supernova-keyring -s {0} {1}").format(*split_username)
        raise LookupError(msg)
    else:
        return password.encode('ascii')