for col in cols_with_missing:
X_train_plus[col + '_was_missing'] = X_train_plus[col].isnull()
X_valid_plus[col + '_was_missing'] = X_valid_plus[col].isnull()
报错：unsupported operand type(s) for +: 'int' and 'str'
修改：for col in cols_missing:
X[str(col) + '_was_missing'] = X[col].isnull()