<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def expand_default(self, scale, discrete=(0, 0.6, 0, 0.6), continuous=(0.05, 0, 0.05, 0)):
 """ Expand a single scale """

        if is_waive(scale.expand):
            if isinstance(scale, scale_discrete):
                return discrete
            elif isinstance(scale, scale_continuous):
                return continuous
            else:
                name = scale.__class__.__name__
                msg = "Failed to expand scale '{}'".format(name)
                raise PlotnineError(msg)
        else:
            return scale.expand