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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 GetDefault(self, fd=None, default=None):
 """Returns a default attribute if it is not set."""

    if callable(self.default):
      return self.default(fd)

    if self.default is not None:
      # We can't return mutable objects here or the default might change for all
      # objects of this class.
      if isinstance(self.default, rdfvalue.RDFValue):
        default = self.default.Copy()
        default.attribute_instance = self
        return self(default)
      else:
        return self(self.default)

    if isinstance(default, rdfvalue.RDFValue):
      default = default.Copy()
      default.attribute_instance = self

    return default