Why is a Python2.7 dict using more space than a Python3 dict?

Question

I read about Raymond Hettinger 's new method of compact dicts . This explains why dicts in Python 3.6 use less memory than dicts in Python 2.7-3.5. However, there seems to be a difference between the memory used in Python 2.7 and 3.3-3.5 dicts. Test code:

import sys

d = {i: i for i in range(n)}
print(sys.getsizeof(d))

• Python 2.7: 12568
• Python 3.5: 6240
• Python 3.6: 4704

As already mentioned, I understand the savings from 3.5 to 3.6, but I'm curious about the reason for the savings between 2.7 and 3.5.

Answer 1

, . dicts cPython 2.7 - 3.2 cPython 3.3 cPython 3.4 ( ). , , , dict:

import sys

size_old = 0
for n in range(512):
    d = {i: i for i in range(n)}
    size = sys.getsizeof(d)
    if size != size_old:
        print(n, size_old, size)
    size_old = size

Python 2.7:

(0, 0, 280)
(6, 280, 1048)
(22, 1048, 3352)
(86, 3352, 12568)

Python 3.5

0 0 288
6 288 480
12 480 864
22 864 1632
44 1632 3168
86 3168 6240

Python 3.6:

0 0 240
6 240 368
11 368 648
22 648 1184
43 1184 2280
86 2280 4704

, dicts 2/3, , cPython 2.7 dict , , cPython 3.5/3.6 dict .

dict:

/* GROWTH_RATE. Growth rate upon hitting maximum load.
 * Currently set to used*2 + capacity/2.
 * This means that dicts double in size when growing without deletions,
 * but have more head room when the number of deletions is on a par with the
 * number of insertions.
 * Raising this to used*4 doubles memory consumption depending on the size of
 * the dictionary, but results in half the number of resizes, less effort to
 * resize.
 * GROWTH_RATE was set to used*4 up to version 3.2.
 * GROWTH_RATE was set to used*2 in version 3.3.0
 */