Lists

Introduction

Lists are mutable, ordered sequences — Python's workhorse collection. Backed by a dynamic array; O(1) index, O(n) insert/remove in middle.

Understanding the topic

Core concepts to understand:

• Create: [1, 2, 3] or list().
• Append O(1), pop end O(1), insert middle O(n).
• Slicing creates a copy: xs[1:3].
• Sort in place: xs.sort(); new list: sorted(xs).

Syntax reference

Visual flow / code:

xs = [3, 1, 4, 1, 5, 9]
xs.append(2)         # [3,1,4,1,5,9,2]
xs.insert(0, 0)      # [0,3,1,4,1,5,9,2]
xs.remove(1)         # removes first 1
xs.pop()             # removes & returns last
xs.sort()            # in place
top3 = sorted(xs)[-3:]
copy = xs[:]         # full copy

Execution workflow

Real-world use

Lists are the default container for ordered data — request bodies, query results, batches. Use collections.deque if you need fast O(1) prepend.

Best practices

• Use append, not +=, in loops.
• Slice to copy — never share mutable lists by reference accidentally.

Common mistakes

• a = b = [] makes both names point to the same list.

Hands-on exercise

Interview preparation — practice these questions:

• List vs tuple?
• When use deque?
• Big-O of list ops?

Summary

In summary: Lists = mutable ordered. Append O(1), middle insert O(n).