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
Minimum length of resulting string; strings with length less
than width be prepended with ‘0’ characters.
Fills the left side of strings with an arbitrary character.
Fills the right side of strings with an arbitrary character.
Fills the specified sides of strings with an arbitrary character.
Fills both sides of strings with an arbitrary character.
Differs from str.zfill() which has special handling
for ‘+’/’-‘ in the string.
>>> s = pd.Series(['-1', '1', '1000', 10, np.nan])
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
(str.zfill() would have moved it to the left). 1000
remains unchanged as it is longer than width.