def get_impress_positions(df: pd.DataFrame) -> pd.Series:
    """Returns a column of impression positions.

       Note: This method assumes that impressions are sorted in ascending order of their position.
    """
    positions = df.groupby('request_id').cumcount() + 1
    positions = positions.reset_index(drop=True)

    return positions


# Unit test: Column level
def test_get_impress_position(logs):
    impress_events = logs.explode('impressions')
    impress_positions = get_impress_positions(impress_events)

    # Since each impress log has 4 items, expect impression positions to be five sets of 1-4
    pd.testing.assert_series_equal(impress_positions, pd.Series([1, 2, 3, 4] * 5))