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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 set(self, new_fields, subset=None):
 """Assign the registered fields based on a dictionary Argument: | ``new_fields`` -- the dictionary with the data to be assigned to the attributes Optional argument: | ``subset`` -- a list of names to restrict the fields that are effectively overwritten """

        for name in new_fields:
            if name not in self._fields and (subset is None or name in subset):
                raise ValueError("new_fields contains an unknown field '%s'." % name)
        if subset is not None:
            for name in subset:
                if name not in self._fields:
                    raise ValueError("name '%s' in subset is not a known field in self._fields." % name)
                if name not in new_fields:
                    raise ValueError("name '%s' in subset is not a known field in new_fields." % name)
        if subset is None:
            if len(new_fields) != len(self._fields):
                raise ValueError("new_fields contains too many fields.")
        for name, attr in self._fields.items():
            if name in subset:
                attr.set(new_fields[name])