CODEWITHSUNDEEP
pythonlisttypestuplessizeof
Digvijay Patil
Digvijay Patil

Reputation: 37

Size comparison between Tuple and List

Why the size of the 2nd list decreased with the increase of the element 'XYZ'?, In the case of the tuple, it increased as expected.

import sys
l1 = [1,2,3,4, "Quant Tading", "Python"]
t1 = (1,2,3,4, "Quant Tading", "Python")
print(sys.getsizeof(l1)) #output 152
print(sys.getsizeof(t1)) #output 88

import sys
l1 = [1,2,3,4, "Quant Tading", "Python", "xyz"]
t1 = (1,2,3,4, "Quant Tading", "Python", "xyz")
print(sys.getsizeof(l1)) # why size decreased to 120?
print(sys.getsizeof(t1)) # output 96

Upvotes: 1

Views: 236

Answers (1)

aaron
aaron

Reputation: 43103

This is a regression1 in Python 3.9 that has been fixed2 in Python 3.10 (3.10.5).

In Python v3.9+ there was a regression in the amount of used memory for list-literals, due to switching to using list_extend() to allocate memory for the new list to accomodate the literal elements. ...

Prior to v3.9, the byte-code for making a list from a literal had the "BUILD_LIST" opcode with an explicit length argument, allowing allocation of the exact amount of memory needed for the literal. As of v3.9, the LIST_EXTEND opcode is used, instead. I believe the simplest way of restoring the old behavior is to change list_extend() to not overallocate when the list being extended currently has 0 elements.

Python 3.8 and Python 3.10:

import sys
l1 = [1,2,3,4, "Quant Tading", "Python"]
t1 = (1,2,3,4, "Quant Tading", "Python")
print(sys.getsizeof(l1)) # output 104
print(sys.getsizeof(t1)) # output 88

import sys
l2 = [1,2,3,4, "Quant Tading", "Python", "xyz"]
t2 = (1,2,3,4, "Quant Tading", "Python", "xyz")
print(sys.getsizeof(l2)) # output 112
print(sys.getsizeof(t2)) # output 96

You can verify on https://www.python.org/shell/ (Python 3.10.5 when this answer was posted).

[1] Regression in overallocation for literal list initialization in v3.9+ (issues/87740)
[2] bpo-39829: Optimize __len__() being called twice in the list() constructor (pull/31816)

Upvotes: 2

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