Access a single value for a row/column label pair.
If the index is not unique, all matching pairs are returned as an array.
Similar to loc, in that both provide label-based lookups. Use at if you only need to
get a single value in a DataFrame or Series.
Unlike pandas, pandas-on-Spark only allows using at to get values but not to
Warning: If row_index matches a lot of rows, large amounts of data will be
fetched, potentially causing your machine to run out of memory.
When label does not exist in DataFrame
>>> psdf = ps.DataFrame([[0, 2, 3], [0, 4, 1], [10, 20, 30]],
... index=[4, 5, 5], columns=['A', 'B', 'C'])
A B C
4 0 2 3
5 0 4 1
5 10 20 30
Get value at specified row/column pair
>>> psdf.at[4, 'B']
Get array if an index occurs multiple times
>>> psdf.at[5, 'B']
array([ 4, 20])