code: def df_na():
      data = {
          "Name": [
              "tom",
              "nick",
              "krish",
              np.nan,
              "peter",
              np.nan,
              "fred",
              "sam",
          ],
          "City": [
              "London",
              "Manchester",
              np.nan,
              np.nan,
              "London",
              "London",
              "Bristol",
              "Manchester",
          ],
          "Studies": [
              "Bachelor",
              "Bachelor",
              np.nan,
              np.nan,
              "Bachelor",
              "PhD",
              "None",
              "Masters",
          ],
          "Age": [20, 21, 19, np.nan, 23, 40, 41, 37],
          "Marks": [0.9, 0.8, 0.7, np.nan, 0.3, np.nan, 0.8, 0.6],
          "dob": pd.date_range("2020-02-24", periods=8, freq="T"),
      }
  
      df = pd.DataFrame(data)
  
      return df
  
file_path: feature_engine/tests/conftest.py
imports: import numpy
  import pandas
  import pytest
label_desc: Use image node_id 4 for calling a global function with example usage: df_na() and returns: df