What is the internal load factor of a sets in Python

Question

I am trying to find out what the internal load factor is for the Python sets. For dictionary which uses a hash table with a load factor of 0.66 (2/3) is. The number of buckets start at 8 and when the 6th key is inserted the number of buckets increases to 16 The table below shows the shift in buckets.

bucket	shift
8	5
16	10
32	21
64	42
128	85

This can be seen with de following Python code where the size of a dictionary and sets is shows with the getsizeof method:

import sys
d = {}
s = set()

for x in range(25):
    d[x] = 1
    s.add(x)
    print(len(d), sys.getsizeof(d), sys.getsizeof(s))

# of elements	memory used for dict	memory used for sets
1	232	216
2	232	216
3	232	216
4	232	216
5	232	728
6	360	728
7	360	728
8	360	728
9	360	728
10	360	728
11	640	728
12	640	728
13	640	728
14	640	728
15	640	728
16	640	728
17	640	728
18	640	728
19	640	2264
20	640	2264
21	640	2264
22	1176	2264
23	1176	2264
24	1176	2264
25	1176	2264

The above table shows that the shift in the buckets correct is for the dictionary, but not for the sets. The memory in sets is different.

I am trying to find out what the load factor is for a set. Is that also 2/3? Or am I doing something wrong with the code?

Answer 1

Currently, it's about 3/5. See the source:

if ((size_t)so->fill*5 < mask*3)
    return 0;
return set_table_resize(so, so->used>50000 ? so->used*2 : so->used*4);

fill is the number of occupied table cells (including "deleted entry" markers), and mask is 1 less than the total table capacity.