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