CODEWITHSUNDEEP
pythonlistmemory
Demaunt
Demaunt

Reputation: 1243

Python memory of list in "for" loop

Using some function I create a list of million items within and check memory consumption

print ('Memory at start: {0} Mb'.format(memory_usage_psutil()))
my_list = people_list(1000000)
print ('Memory at end: {0} Mb'.format(memory_usage_psutil()))

What I get is:

Memory at start: 100.90625 Mb
Memory at end: 403.8125 Mb

So creating a list and keeping it in memory takes ~300Mb.

For example I want to count amount of items in PART of my_list WITHOUT using len()

count = 0
additional_list = my_list[0:500000]
for item in additional_list:
    count += 1
    if count == 499999:
        print ('Memory in loop: {0} Mb'.format(memory_usage_psutil()))

and results confused me.

Memory at start: 100.90625 Mb
Memory in loop: 403.6953125 Mb
Memory at end: 403.6953125 Mb

I expected that creating additional list will increase memory consumption by 150Mb but it did not happen. Can you explain why? I mean id's are different

In [30]: id(my_list)
Out[30]: 2672641621704

In [31]: id(additional_list)
Out[31]: 2672642905864

and same results come from:

for item in my_list[0:500000]:
    count += 1
    if count == 499999:
        print ('Memory in loop: {0} Mb'.format(memory_usage_psutil()))

Upvotes: 3

Views: 708

Answers (1)

Meitham
Meitham

Reputation: 9670

It's true that your two lists are different objects and have different addresses in memory. However, the individual objects in the list are the same. Using a slice basically gave you a view on some elements in your list.

You can confirm that with::

>>> map(id, my_list)[0:500000] == map(id, additional_list)

Upvotes: 2

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