Python : AttributeError: 'NoneType' object has no attribute 'append'

Question

My program looks like

# global
item_to_bucket_list_map = {}

def fill_item_bucket_map(items, buckets):
    global item_to_bucket_list_map

    for i in range(1, items + 1):
        j = 1
        while i * j <= buckets:
            if j == 1:
                item_to_bucket_list_map[i] = [j]
            else:
                item_to_bucket_list_map[i] = (item_to_bucket_list_map.get(i)).append(j)
            j += 1
        print "Item=%s, bucket=%s" % (i, item_to_bucket_list_map.get(i))


if __name__ == "__main__":
    buckets = 100
    items = 100
    fill_item_bucket_map(items, buckets)

When I run this, it throws me

AttributeError: 'NoneType' object has no attribute 'append'

Not sure why this would happen? When I am already creating a list at start of each j

Answer 1

[...]
for i in range(1, items + 1):
    j = 1
    while i * j <= buckets:
        if j == 1:
            mylist = []
        else:
            mylist = item_to_bucket_list_map.get(i)
        mylist.append(j)
        item_to_bucket_list_map[i] = mylist
        j += 1
    print "Item=%s, bucket=%s" % (i, item_to_bucket_list_map.get(i))

The while loop, however, can be simplified to

    for j in range(1, buckets / i + 1): # + 1 due to the <=
        if j == 1:
            mylist = []
        else:
            mylist = item_to_bucket_list_map.get(i)
        mylist.append(j)
        item_to_bucket_list_map[i] = mylist