def replace_null(x, replace = None):
    """
    Replace null values

    Parameters
    ----------
    x : Expr, Series
        Column to operate on

    Examples
    --------
    >>> df = tp.Tibble(x = [0, None], y = [None, None])
    >>> df.mutate(x = tp.replace_null(col('x'), 1))
    """
    if replace == None: return x
    return x.fill_null(replace)