pythonlist

sys.getsizeof(list) returns less than the sum of its elements

I'm curious - why does the sys.getsizeof call return a smaller number for a list than the sum of its elements?

import sys
lst = ["abcde", "fghij", "klmno", "pqrst", "uvwxy"]
print("Element sizes:", [sys.getsizeof(el) for el in lst])
print("Sum of sizes: ", sum([sys.getsizeof(el) for el in lst]))
print("Size of list: ", sys.getsizeof(lst))

The above prints

Element sizes: [42, 42, 42, 42, 42]
Sum of sizes:  210
Size of list:  112

How come?

Solution

  • You are getting the size of the actual list object. As the list object stores pointers to objects its memory size is bound to be different (and lower) than the sum of its elements.

    By analogy, it’s like getting the size of an array of pointers in C.