code: def test_zfill():
      # https://github.com/pandas-dev/pandas/issues/20868
      value = Series(["-1", "1", "1000", 10, np.nan])
      expected = Series(
          ["-01", "001", "1000", np.nan, np.nan], dtype=object
      )
      tm.assert_series_equal(value.str.zfill(3), expected)
  
      value = Series(["-2", "+5"])
      expected = Series(["-0002", "+0005"])
      tm.assert_series_equal(value.str.zfill(5), expected)
  
imports: from datetime import datetime, timedelta
  import numpy
  import pytest
  from pandas import DataFrame, Index, MultiIndex, Series
  import pandas._testing
  from pandas.core.strings.accessor import StringMethods
  from pandas.tests.strings import object_pyarrow_numpy
label_desc: Use image node_id 44 for calling a global function with example usage: test_zfill() without return types