from collections import defaultdict
 data = [(2010, 2), (2009, 4), (1989, 8), (2009, 7)]
 d = defaultdict(list)
print (d) # output --> defaultdict(, {})
 for year, month in data:
     d[year].append(month) 
print (d) # output --> defaultdict(, {2009: [4, 7], 2010: [2], 1989: [8]})