def is_non_neg_int(s):
    """
    Returns True iff string s is a non-negative number.

    Parameters
    ----------
    s : str

    Returns
    -------
    bool

    """
    # s.isdigit() with s equal to
    # '0', '1', '001' gives True,
    # '-1', '1.' gives False
    if not s.isdigit():
        return False
    if s[0] == '0' and len(s) > 1:
        return False
    return True