How to add a new column with labels in a dataframe?
Do you have a working method to get the sentiment label for an individual sentence?

If so, I would think you could just create a method to get the sentiment given a sentence and use it's return value as the row entry.

The apply function from pandas will apply a function to every row if you specify axis=1

def getSentiment(row):
    # Do some sentiment operations here on row['Testo']
    return sentiment_label

df.apply(getSentiment, axis=1)