code: def _assert_all_na(join_chunk, source_columns, join_col):
      for c in source_columns:
          if c in join_col:
              continue
          assert join_chunk[c].isna().all()
  
imports: import re
  import numpy
  import pytest
  import pandas.util._test_decorators
  import pandas
  from pandas import Categorical, DataFrame, Index, MultiIndex, Series, Timestamp, bdate_range, concat, merge, option_context
  import pandas._testing
label_desc: Use image node_id 5 for calling a global function with example usage: _assert_all_na(join_chunk, source_columns, join_col) without return types