Why is there a difference in sys.getsizeof for two differently created but equal lists?

The difference comes from "over-allocation". From source code:

 /* This over-allocates proportional to the list size, making room * for additional growth. The over-allocation is mild, but is * enough to give linear-time amortized behavior over a long * sequence of appends() in the presence of a poorly-performing * system realloc(). * Add padding to make the allocated size multiple of 4. * The growth pattern is: 0, 4, 8, 16, 24, 32, 40, 52, 64, 76, ... * Note: new_allocated won't overflow because the largest possible value * is PY_SSIZE_T_MAX * (9 / 8) + 6 which always fits in a size_t. 

But the exact behavior is implementation detail!

In Python 3.10:

import sys lst1 = list(range(1, 10)) lst2 = [item for item in range(1, 10)] lst3 = [1, 2, 3, 4, 5, 6, 7, 8, 9] lst4 = [] for i in range(1, 10): lst4.append(i) print(sys.getsizeof(lst1)) # 136 print(sys.getsizeof(lst2)) # 184 print(sys.getsizeof(lst3)) # 136 print(sys.getsizeof(lst4)) # 184 

In Python 3.5.1:

import sys lst1 = list(range(1, 10)) lst2 = [item for item in range(1, 10)] lst3 = [1, 2, 3, 4, 5, 6, 7, 8, 9] lst4 = [] for i in range(1, 10): lst4.append(i) print(sys.getsizeof(lst1)) # 192 print(sys.getsizeof(lst2)) # 192 print(sys.getsizeof(lst3)) # 136 print(sys.getsizeof(lst4)) # 192 

I know surely that this happens when .append() is called. (same thing with list-comprehension). That's why you see in both versions, lst2 and lst4 have the largest size.

Seems like in Python 3.10, for lst1, interpreter says OK from range object, I know (__len__ and __length_hint__) it requires 10 elements so I build a list of 10 elements. No need for over-allocation.