Lists in Python

Introduction

A list is a mutable, ordered sequence that can hold elements of any type. Think of it as a dynamic array that grows and shrinks as needed.

Creating Lists

empty = []
numbers = [1, 2, 3, 4, 5]
mixed   = [1, "hello", 3.14, True, None]

# From iterables
from_string = list("hello")     # ['h', 'e', 'l', 'l', 'o']
from_range  = list(range(5))    # [0, 1, 2, 3, 4]

# Repeated elements
zeros = [0] * 5              # [0, 0, 0, 0, 0]

Indexing and Slicing

data = [10, 20, 30, 40, 50, 60, 70, 80, 90, 100]

print(data[2:5])      # [30, 40, 50]
print(data[:4])       # [10, 20, 30, 40]
print(data[6:])       # [70, 80, 90, 100]
print(data[::-1])     # Reversed
print(data[-3:])      # Last 3: [80, 90, 100]

List Methods

lst = [3, 1, 4, 1, 5, 9, 2, 6, 5]

# Adding
lst.append(7)          # Add to end — O(1)
lst.insert(0, 99)      # Insert at index — O(n)
lst.extend([10, 11])   # Add multiple

# Removing
popped = lst.pop()     # Remove last — O(1)
lst.remove(99)         # Remove by value — O(n)
lst.clear()            # Remove all

# Searching
print(lst.index(5))    # First occurrence index
print(lst.count(1))    # Count occurrences
print(5 in lst)        # Membership test

# Ordering
lst.sort()             # Sort in place — O(n log n)
lst.sort(reverse=True) # Descending
lst.reverse()          # Reverse in place

List Comprehensions

# Basic
squares = [x**2 for x in range(10)]

# With condition
evens = [x for x in range(20) if x % 2 == 0]

# Nested (flatten matrix)
matrix = [[1,2,3],[4,5,6],[7,8,9]]
flat = [val for row in matrix for val in row]

Nested Lists

matrix = [
    [1, 2, 3],
    [4, 5, 6],
    [7, 8, 9]
]
print(matrix[0][1])  # 2

Time Complexity

Operation	Complexity
`append(x)`	O(1) amortized
`pop()`	O(1)
`pop(i)`	O(n)
`insert(i, x)`	O(n)
`x in list`	O(n)
`lst[i]`	O(1)
`sort()`	O(n log n)

Why It Matters

Lists are the most commonly used data structure in Python. Understanding their internals helps you write faster code.

Common Mistakes

Using [[0]*3]*3 creates linked rows (use list comprehension instead)
Using pop(0) on large lists is O(n) — use collections.deque for queues

Interview Questions

Q1: What’s the difference between append() and extend()?

A: append() adds one element; extend() unpacks an iterable and adds each item individually.

Q2: Why does [[0]*3]*3 create linked rows?

A: * repeats the reference, not the object. All rows point to the same list. Use [[0]*3 for _ in range(3)].

Practice Exercises

Remove duplicates from a list while preserving order.
Rotate a list by k positions.
Find two numbers in a list that sum to a target.