事实证明这是一条红鲱鱼。增加字典大小的规则在 cPython 2.7 - 3.2 和 cPython 3.3 之间发生了变化,在 cPython 3.4 中又发生了变化(尽管此更改仅在发生删除时适用)。我们可以使用以下代码来确定字典何时扩展:
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
请记住,字典在达到 2/3 满时会调整大小,我们可以看到 cPython 2.7 字典实现在扩展时大小会增加四倍,而 cPython 3.5/3.6 字典实现大小仅增加一倍。
这在评论中进行了解释字典源代码 https://github.com/python/cpython/blob/master/Objects/dictobject.c#L401:
/* 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
*/