TST: Fixing Series.str.zfill() to behave same as str.zfill() from standard library (pandas-dev#20868)

Author: atulagrwl

doc/source/whatsnew/v0.24.0.txt

@@ -84,6 +84,7 @@ Other Enhancements
 - :meth:`Series.nlargest`, :meth:`Series.nsmallest`, :meth:`DataFrame.nlargest`, and :meth:`DataFrame.nsmallest` now accept the value ``"all"`` for the ``keep`` argument. This keeps all ties for the nth largest/smallest value (:issue:`16818`)
 - :class:`IntervalIndex` has gained the :meth:`~IntervalIndex.set_closed` method to change the existing ``closed`` value (:issue:`21670`)
 - :func:`~DataFrame.to_csv` and :func:`~DataFrame.to_json` now support ``compression='infer'`` to infer compression based on filename (:issue:`15008`)
+- :func:`Series.str.zfill()` now matches with standard string library zfill (:issue:`20868`)
 -
 
 .. _whatsnew_0240.api_breaking:

pandas/tests/test_strings.py

@@ -2242,6 +2242,9 @@ def test_zfill(self):
         expected = Series(['00001', np.nan, '00aaa', np.nan, '45678', np.nan])
         tm.assert_series_equal(result, expected)
 
+        with tm.assert_raises_regex(TypeError, "width must be of integer"):
+            values.str.zfill('5')
+
     def test_split(self):
         values = Series(['a_b_c', 'c_d_e', NA, 'f_g_h'])