code: def zfill(self, width):
          
          result = str_pad(self._parent, width, side='left', fillchar='0')
          return self._wrap_result(result)
docs: Pad strings in the Series/Index by prepending '0' characters.
  
          Strings in the Series/Index are padded with '0' characters on the
          left of the string to reach a total string length  `width`. Strings
          in the Series/Index with length greater or equal to `width` are
          unchanged.
  
          Parameters
          ----------
          width : int
              Minimum length of resulting string; strings with length less
              than `width` be prepended with '0' characters.
  
          Returns
          -------
          Series/Index of objects
  
          See Also
          --------
          Series.str.rjust : Fills the left side of strings with an arbitrary
              character.
          Series.str.ljust : Fills the right side of strings with an arbitrary
              character.
          Series.str.pad : Fills the specified sides of strings with an arbitrary
              character.
          Series.str.center : Fills boths sides of strings with an arbitrary
              character.
  
          Notes
          -----
          Differs from :meth:`str.zfill` which has special handling
          for '+'/'-' in the string.
  
          Examples
          --------
          >>> s = pd.Series(['-1', '1', '1000', 10, np.nan])
          >>> s
          0      -1
          1       1
          2    1000
          3      10
          4     NaN
          dtype: object
  
          Note that ``10`` and ``NaN`` are not strings, therefore they are
          converted to ``NaN``. The minus sign in ``'-1'`` is treated as a
          regular character and the zero is added to the left of it
          (:meth:`str.zfill` would have moved it to the left). ``1000``
          remains unchanged as it is longer than `width`.
  
          >>> s.str.zfill(3)
          0     0-1
          1     001
          2    1000
          3     NaN
          4     NaN
          dtype: object