datas = [1, 2, 3, 4, 2, 3, 4, 1, 5]
# Either use defaultdict
from collections import defaultdict
d = defaultdict(int)
for k in datas:
d[k] += 1
# Or in situations like this just use a counter. It's self documenting and
# avoids exceptions being thrown if you index on something not in the dict
from collections import Counter
d = Counter(datas)