code: def test_fit_raises_error_if_df_contains_na(df_enc_na):
      # test case 4: when dataset contains na, fit method
      with pytest.raises(ValueError):
          encoder = MeanEncoder()
          encoder.fit(
              df_enc_na[["var_A", "var_B"]], df_enc_na["target"]
          )
  
file_path: feature_engine/tests/test_encoding/test_mean_encoder.py
imports: import numpy
  import pandas
  import pytest
  from feature_engine.encoding import MeanEncoder
label_desc: Use image node_id 4 for calling a global function with example usage: test_fit_raises_error_if_df_contains_na(df_enc_na) without return types