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

def zfill(self, width):
        """
        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
        """
        result = str_pad(self._parent, width, side='left', fillchar='0')
        return self._wrap_result(result)