`scprep.select.columns` should return a series if only one column is selected

[dburkhardt]

Is your feature request related to a problem? Please describe. Currently, selecting a single column from a dataframe using scprep.select.columns (e.g. using exact_word=...) returns a dataframe with shape (n, 1). This makes it difficult to use the column as a boolean mask or pass to plt.hist.

Describe the solution you'd like If a single columns is selected, return a series instead of a columns. It wouldn't be a bad idea to generate warning text if multiple columns are selected using exact_word because that's usually used for when I have the exact gene symbol but am using data loaded with gene_id='both'. In this use case I expect a single column match my gene symbol.

Describe alternatives you've considered Using scanpy ;)

[scottgigante]

I can solve the first problem but the point about exact_word returning more than one hit I'm not so convinced about. It doesn't seem preposterous to me that you could expect to have multiple hits with any one word and not be upset about it (obviously not the case with 10X data but that is not the only data type we support). I lean more towards just leaving it as a dataframe if there are multiple hits -- you'll discover soon enough what's going on anyway.

[scottgigante]

@dburkhardt should the same behaviour apply to select_rows?

[dburkhardt]

Yes because if you pass in a list of series to pd.DataFrame then you get out a DF with 2 rows.