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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_attr(self, **kwargs):
 """Set attributes to the Booster. Parameters **kwargs The attributes to set. Setting a value to None deletes an attribute. Returns ------- self : Booster Booster with set attributes. """

        for key, value in kwargs.items():
            if value is not None:
                if not isinstance(value, string_type):
                    raise ValueError("Only string values are accepted")
                self.__attr[key] = value
            else:
                self.__attr.pop(key, None)
        return self