code: def train_test_split(self, df, test_num, look_back):
      test_split_index = test_num + look_back + 1
  
      # train_df
      train_df = df[:-test_num]
      test_df = df[-test_split_index:]
      test_df = test_df.reset_index(drop=True)
      return train_df, test_df
  
file_path: analytics-zoo/pyzoo/test/zoo/chronos/detector/anomaly/test_th_detector.py
imports: import pytest
  import numpy
  import pandas
  from test.zoo.pipeline.utils.test_utils import ZooTestCase
  from zoo.chronos.forecaster.lstm_forecaster import LSTMForecaster
  from zoo.chronos.detector.anomaly import ThresholdDetector
label_desc: Use image node_id 4 for calling the TestThresholdDetector obj's underlying member method code with example usage: obj.train_test_split(df, test_num, look_back) and returns: train_df,  test_df