class_docstr: Randomly select a fraction of test rows per user/item.
  
  Args:
      frac(float): the fraction items to select for testing.
class_docstr_tok: ["Randomly","select","a","fraction","of","test","rows","per","user\/item",".","Args",":","frac","(","float",")",":","the","fraction","items","to","select","for","testing","."]
code: def __init__(self, frac, rng_spec=None):
      self.fraction = frac
      self.rng = numpy_rng(rng_spec)
  
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 1 to create a new SampleFrac object from inherited base classes: PartitionMethod with example: obj = SampleFrac(frac, rng_spec)