This question tests the understanding of shallow copies and mutable nested objects in Python.
Shallow Copy:
$\verb|L2 = list(L1)|$
- L2 points to a new list object $\verb|[1, 2, Y]|$.
- The elements are copied by reference:
- $\verb|L2[0]|$ and $\verb|L2[1]|$ are copies of the integers (which are immutable).
- L2[2] points to the SAME nested list object $\verb|[3, 4]|$ (labeled $X$ ) as $\verb|L1[2]|$ This is the crucial point of a shallow copy.
$\verb|L3 = L1|$
- L3 points to the SAME list object as $\verb|L1|$
\[
\begin{array}{lll}
\text{Variable} & \text{Points to} & L1[2]\ \text{points to} \\
\verb|L1| & \verb|[1, 2, X]| & \verb|[3,4]|\ (\text{Object X}) \\
\verb|L2| & \verb|[1, 2, X]| & \verb|[3,4]|\ (\text{Object X}) \\
\verb|L3| & \verb|[1, 2, X]| & \verb|[3,4]|\ (\text{Object X})
\end{array}
\]
$\verb|L1[0] = 99|$
- The list object pointed to by L1 and L3 is mutated at index 0 .
- State of L1/L3: $\verb|[99, 2, [3, 4]]|$
$\verb|L2[2][0] = 88|$
- $\verb|L2[2]|$ is the nested list object $\verb|[3,4]|$ (Object $X$ ), which is shared by L1 and L2. .
- The shared nested list is mutated at index $0$.
- State of Nested List (Object X): $\verb|[88, 4]|$
- State of L1/L3: $\verb|[99, 2, [88, 4]]|$
- State of L2: $\verb|[1, 2, [88,4]]|$
$\verb|L3.append(5)|$
- $\verb|L3|$ and $\verb|L1|$ are the same object. An element is appended to this shared object.
- State of L1/L3: $\verb|[99, 2, [88, 4], 5]|$
- State of L2: $\verb|[1,2,[88,4]]|$ (L2 is a separate outer list and is unaffected).
$L1[2][0]$ is $88.$
The length of L2 is $\mathbf{3}$.
Result: $88+3=91$.
