Question

Query in Python interpreter:

Python 2.7.3 (default, Apr 10 2012, 23:31:26) [MSC v.1500 32 bit (Intel)] on win
32
Type "help", "copyright", "credits" or "license" for more information.
>>> k = [i for i in xrange(9999999)]
>>> import sys
>>> sys.getsizeof(k)/1024/1024
38
>>>

And here – see how much it takes from RAM:

Memory usage after statement del k:

And after gc.collect():

Why list of integers with expected size of 38Mb takes 160Mb?

UPD: This part of question was answered (almost immediately and multiple times :))

Okay – here is another riddle:

Python 2.7.3 (default, Apr 10 2012, 23:31:26) [MSC v.1500 32 bit (Intel)] on win
32
Type "help", "copyright", "credits" or "license" for more information.
>>> import sys

>>> str = 'abcdefg'
>>> sys.getsizeof(str)
28
>>> k = []
>>> for i in xrange(9999999):
...     k.append(str)
...
>>> sys.getsizeof(str)*9999999/1024/1024
267

How much do you think it will consume now?

_{(source: i.imm.io)}

Size of str is 28, vs 12 in past example. So, expected memory usage is 267Mb – even more then with integers. But it takes only ~40Mb!

Answer 1

sys.getsizeof() is not very useful because it accounts often for only a part of what you expect. In this case, it accounts for the list, but not all integer objects that are in the list. The list takes roughly 4 bytes per item. The integer objects take another 12 bytes each. For example, if you try this:

k = [42] * 9999999
print sys.getsizeof(k)

you’ll see that the list still takes 4 bytes per item, i.e. around 40MB, but because all items are pointers to the same integer object 42, the total memory usage is not much more than 40MB.