def METHOD_NAME(series, dropna=True, combine_size=None):
    """
    Return number of unique elements in the object.
    Excludes NA values by default.
    Parameters
    ----------
    dropna : bool, default True
        Don't include NaN in the count.
    combine_size : int, optional
        The number of chunks to combine.
    Returns
    -------
    int
    See Also
    --------
    DataFrame.nunique: Method nunique for DataFrame.
    Series.count: Count non-NA/null observations in the Series.
    Examples
    --------
    >>> import mars.dataframe as md
    >>> s = md.Series([1, 3, 5, 7, 7])
    >>> s.execute()
    0    1
    1    3
    2    5
    3    7
    4    7
    dtype: int64
    >>> s.nunique().execute()
    4
    """
    op = DataFrameNunique(
        dropna=dropna,
        combine_size=combine_size,
        output_types=[OutputType.scalar],
        use_arrow_dtype=options.dataframe.use_arrow_dtype,
    )
    return op(series)