Why do two identical lists have a different memory footprint?

Question

I created two lists l1 and l2, but each one with a different creation method:

import sys

l1 = [None] * 10
l2 = [None for _ in rang         


        

Answer 1

As noted in this question the list-comprehension uses list.append under the hood, so it will call the list-resize method, which overallocates.

To demonstrate this to yourself, you can actually use the dis dissasembler:

>>> code = compile('[x for x in iterable]', '', 'eval')
>>> import dis
>>> dis.dis(code)
  1           0 LOAD_CONST               0 ( at 0x10560b810, file "", line 1>)
              2 LOAD_CONST               1 ('')
              4 MAKE_FUNCTION            0
              6 LOAD_NAME                0 (iterable)
              8 GET_ITER
             10 CALL_FUNCTION            1
             12 RETURN_VALUE

Disassembly of  at 0x10560b810, file "", line 1>:
  1           0 BUILD_LIST               0
              2 LOAD_FAST                0 (.0)
        >>    4 FOR_ITER                 8 (to 14)
              6 STORE_FAST               1 (x)
              8 LOAD_FAST                1 (x)
             10 LIST_APPEND              2
             12 JUMP_ABSOLUTE            4
        >>   14 RETURN_VALUE
>>>

Notice the LIST_APPEND opcode in the disassembly of the code object. From the docs:

LIST_APPEND(i)

Calls list.append(TOS[-i], TOS). Used to implement list comprehensions.

Now, for the list-repetition operation, we have a hint about what is going on if we consider:

>>> import sys
>>> sys.getsizeof([])
64
>>> 8*10
80
>>> 64 + 80
144
>>> sys.getsizeof([None]*10)
144

So, it seems to be able to exactly allocate the size. Looking at the source code, we see this is exactly what happens:

static PyObject *
list_repeat(PyListObject *a, Py_ssize_t n)
{
    Py_ssize_t i, j;
    Py_ssize_t size;
    PyListObject *np;
    PyObject **p, **items;
    PyObject *elem;
    if (n < 0)
        n = 0;
    if (n > 0 && Py_SIZE(a) > PY_SSIZE_T_MAX / n)
        return PyErr_NoMemory();
    size = Py_SIZE(a) * n;
    if (size == 0)
        return PyList_New(0);
    np = (PyListObject *) PyList_New(size);

Namely, here: size = Py_SIZE(a) * n;. The rest of the functions simply fills the array.