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Lists

List Methods

append, extend, insert, remove, pop, sort

Interview: Essential methods — tests understanding of in-place vs returning-new-list operations

Last Updated: June 12, 2026 • 10 min read

Python lists provide a rich set of methods for adding, removing, searching, and rearranging elements. A critical distinction is whether methods modify the list in place (returning None) or return a new list.

Adding Elements

  • append(x): Add single element to end — O(1) amortized
  • extend(iterable): Add all elements from iterable to end — O(k) where k is length of iterable
  • insert(i, x): Insert element at index i — O(n) because elements must shift

Removing Elements

  • remove(x): Remove first occurrence of value x — O(n). Raises ValueError if not found
  • pop([i]): Remove and return element at index i (default: last) — O(1) for last, O(n) for arbitrary
  • clear(): Remove all elements — O(n)
  • del lst[i]: Delete by index (not a method, but common)

Searching and Counting

  • index(x, [start], [end]): Find index of first occurrence — O(n). Raises ValueError if not found
  • count(x): Count occurrences of x — O(n)

Sorting and Reversing

  • sort(key=None, reverse=False): In-place sort using Timsort — O(n log n). Returns None!
  • reverse(): In-place reversal — O(n). Returns None!
  • sorted() vs sort(): sorted() returns a new list; .sort() modifies in place
  • Custom sorting: lst.sort(key=lambda x: x.age) — sort by attribute

Common Pitfall

lst.sort() and lst.reverse() return None. Writing result = lst.sort() gives you None, not a sorted list. This is a deliberate design choice: it signals that the operation modifies in place.

Use Cases

Implementing stacks (append/pop) and queues (deque)

Data cleaning: removing duplicates, sorting, filtering

Priority queues with sorted insertion

Building result collections with append in loops

Common Mistakes

Assigning result of sort()/reverse() to a variable — they return None

Using remove() without checking if item exists (raises ValueError)

Using pop(0) for queues — O(n); use collections.deque instead

Confusing append([1,2]) with extend([1,2]) — append adds the list as one element