code: def stats(self, columns):
          
          assert (not isinstance(columns, basestring)), "columns should be a " \
                                                        "list of strs,  " \
                                                        "not a str!"
          assert isinstance(columns, list), "columns should be a list!"
  
          from pyspark.sql import functions as F
          functions = [F.min, F.max, F.avg, F.count]
          aggs = list(
              self._flatmap(lambda column: map(lambda f: f(column), functions),
                            columns))
          return PStats(self.from_schema_rdd(self._schema_rdd.agg(*aggs)))
docs: Compute the stats for each column provided in columns.
          Parameters
          ----------
          columns : list of str, contains all columns to compute stats on.