Get index of recently appended item

Question

Is there a straightforward way to get the index of an item I just appended to a list? I need to keep track of the last added item.

I came up with two possible solutions:

# Workaround 1
# The last added is the one at index len(li) - 1
>> li = ['a', 'b', 'c',]
>> li.append('d')
>> last_index = len(li) - 1
>> last_item = li[len(li) - 1]

# Workaround 2
# Use of insert at index 0 so I know index of last added
>> li = ['a', 'b', 'c',]
>> li.insert(0, 'd')
>> last_item = li[0]

Is there a trick to get the index of an appended item?

If there's not, which of the above would you use and why? Any different workaround you suggest?

Answer 1

li[-1] is the last item in the list, and hence the one that was most recently appended to its end:

>>> li = [1, 2, 3]
>>> li.append(4)
>>> li[-1]
4

If you need the index, not the item, then len(li) - 1 is just fine, and very efficient (since len(li) is computed in constant time - see below)

---

In the source of CPython, len for lists is mapped to function list_length in Objects/listobject.c:

static Py_ssize_t
list_length(PyListObject *a)
{
    return Py_SIZE(a);
}

Py_SIZE is just a macro for accessing the size attribute of all Python objects, defined in Include/object.h:

#define Py_SIZE(ob)     (((PyVarObject*)(ob))->ob_size)

Hence, len(lst) is essentially a single pointer dereference.