code: def test_observation_frequency_per_bin():
      """Test empty bins are populated by a tiny amount."""
      a = pd.DataFrame({"A": [1, 2, 4]})
      b = pd.DataFrame({"A": [1, 2, 3]})
      transformer = DropHighPSIFeatures()
      a_bins, b_bins = transformer._observation_frequency_per_bin(a, b)
  
      expected_a_bins = pd.Series(
          [0.3333333, 0.333333, 0.0001, 0.333333]
      )
      expected_b_bins = pd.Series(
          [0.3333333, 0.333333, 0.333333, 0.0001]
      )
  
      pd.testing.assert_series_equal(
          a_bins.reset_index(drop=True),
          expected_a_bins,
          check_names=False,
      )
      pd.testing.assert_series_equal(
          b_bins.reset_index(drop=True),
          expected_b_bins,
          check_names=False,
      )
  
imports: from datetime import date
  import math
  import numpy
  import pandas
  import pytest
  from sklearn.datasets import make_classification
  from feature_engine.selection import DropHighPSIFeatures
label_desc: Use image node_id 25 for calling a global function with example usage: test_observation_frequency_per_bin() without return types