def __dict__(self):
        """Return only those attributes that have non-null values.  This
           enables usage like 'environment(variables=var(toolchain))'"""
        return {key: getattr(self, key) for key in self.__attrs__
                if getattr(self, key)}
Return only those attributes that have non-null values.  This
           enables usage like 'environment(variables=var(toolchain))'
hpccm/toolchain.py