def zfill(string, width):
    """zfill(x, width) -> string
    Pad a numeric string x with zeros on the left, to fill a field
    of the specified width.  The string x is never truncated.
    """
    size = len(string)
    if size >= width: return string
    sign = ''
    if string[0] in ('-', '+'):
        sign, string = string[0], string[1:]
    return sign + '0' * (width - size) + string