class_docstr: Partition methods select test rows for a user or item.  Partition methods
  are callable; when called with a data frame, they return the test rows.
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docstr: Subset a data frame.
  
  Args:
      udf(pandas.DataFrame):
          The input data frame of rows for a user or item.
  
  Returns:
      pandas.DataFrame:
          The data frame of test rows, a subset of ``udf``.
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code: def __call__(self, udf):
      """
      Subset a data frame.
  
      Args:
          udf(pandas.DataFrame):
              The input data frame of rows for a user or item.
  
      Returns:
          pandas.DataFrame:
              The data frame of test rows, a subset of ``udf``.
      """
      pass
  
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 for calling the PartitionMethod obj's underlying member method code with example usage: obj.__call__(udf) without return types