code: def unique(df, columns=None):
      
      if columns is None:
          columns = df.columns
      for col in columns:
          if not df[col].is_unique:
              raise AssertionError("Column {!r} contains non-unique values".format(col))
      return df
docs: Asserts that columns in the DataFrame only have unique values.
  
      Parameters
      ----------
      df : DataFrame
      columns : list
        list of columns to restrict the check to. If None, check all columns.
  
      Returns
      -------
      df : DataFrame
        same as the original