`sys.getsizeof()` does not include inline values

Bug report

Bug description:

Commit tested: 5e65a1acc0b

I have a program that allocates numerous instances of this type:

@dataclass
class Node:
    cdr: Self | None

BIG_DATA = None
for i in range(50):
    BIG_DATA = Node(BIG_DATA)
    print(sys.getsizeof(BIG_DATA))

After the 28th iteration, I find that it stabilizes at printing 48. But if I run the program in GDB and trace the actual allocations, I see that it is allocating 80 bytes per iteration:

# Set a GDB-recognized breakpoint
os.kill(os.getpid(), signal.SIGTRAP)
# In GDB:
# b _PyObject_Malloc
# commands
#  silent
#  print nbytes
#  continue
# end
for i in range(50):
    BIG_DATA = Node(BIG_DATA)

I believe that the missing bytes can be accounted for in the inline values. I observe the same behavior on Python 3.13.

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-128763

報告されている sys.getsizeof() の動作から始め、dataclass Node の例を再現し、その出力を GDB の _PyObject_Malloc トレースと比較します。inline 値がどのように計上されるかを調査します。例で示された割り当てを報告サイズが反映し、既存の動作を退行させなければ完了です。