def COUNT_DISTINCT(src_column):
    """
    Builtin unique counter for groupby. Counts the number of unique values

    Example: Get the number of unique ratings produced by each user.

    >>> sf.groupby("user",
    ...    {'rating_distinct_count':tc.aggregate.COUNT_DISTINCT('rating')})
    """
    return '__builtin__count__distinct__', [src_column]
