def left_fill(s, n, x="0"):
    """Cross platform string left fill method, defaults to zero filling.

    Parameters
    ----------
    s : str, string to left pad
    n : int, number of total places to pad to
    x : str, optional, character to fill, defaults to "0"
    """
    sl = len(s)
    zn = n - sl
    if zn > 0:
        return zn*"0" + s
    else:
        return s