class_docstr: Select a fraction of test rows per user/item.
  
  Args:
      frac(double): the fraction of items to select for testing.
class_docstr_tok: ["Select","a","fraction","of","test","rows","per","user\/item",".","Args",":","frac","(","double",")",":","the","fraction","of","items","to","select","for","testing","."]
code: def __call__(self, udf):
      n = round(len(udf) * self.fraction)
      return udf.sort_values(self.column).iloc[-n:]
  
imports: from collections import namedtuple
  import logging
  from abc import ABC, abstractmethod
  import numpy
  import pandas
  from seedbank import numpy_rng
label_desc: Use image node_id 2 for calling the LastFrac obj's underlying member method code with example usage: obj.__call__(udf) and returns: udf