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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 the attribute of the Booster. Parameters **kwargs The attributes to set. Setting a value to None deletes an attribute. """

        for key, value in kwargs.items():
            if value is not None:
                if not isinstance(value, STRING_TYPES):
                    raise ValueError("Set Attr only accepts string values")
                value = c_str(str(value))
            _check_call(_LIB.XGBoosterSetAttr(
                self.handle, c_str(key), value))