List Methods & Advanced Operations

Why read this? Beyond basic CRUD, lists have powerful built-in methods for sorting, searching, and transforming. List comprehensions — a concise way to create lists — are one of Python's most elegant features and appear heavily in exams.

🎯 Learning Objectives

1. Use all major list methods confidently
2. Create new lists using list comprehensions
3. Sort lists with custom keys
4. Understand shallow vs deep copying
5. Use lists as stacks and queues

📋 Prerequisites

• Lists — Basics & Operations
• For Loops & range()

📖 Core Content

15.1 List Methods — Complete Reference

15.2 List Comprehensions — The Pythonic Way

python
Run Inline
# runnable
# Traditional way
squares = []
for i in range(10):
    squares.append(i ** 2)
print("Traditional:", squares)
# List comprehension
squares2 = [i ** 2 for i in range(10)]
print("Comprehension:", squares2)
# With condition
evens = [i for i in range(20) if i % 2 == 0]
print("Evens:", evens)
# Transform and filter
names = ["alice", "bob", "charlie", "dave"]
upper_long = [n.upper() for n in names if len(n) > 3]
print("Upper long names:", upper_long)

pseudo
Traditional: [0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
Comprehension: [0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
Evens: [0, 2, 4, 6, 8, 10, 12, 14, 16, 18]
Upper long names: ['ALICE', 'CHARLIE']

text
[expression for variable in iterable if condition]

15.3 Sorting with Custom Keys

python
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# runnable
words = ["python", "java", "c", "javascript", "go", "rust"]
# Sort by length
words.sort(key=len)
print("By length:", words)
# Sort by last character
words.sort(key=lambda w: w[-1])
print("By last char:", words)
# Sort descending
words.sort(key=len, reverse=True)
print("By length desc:", words)
# sorted() preserves original
original = [3, 1, 4, 1, 5]
sorted_copy = sorted(original, reverse=True)
print("Original:", original)
print("Sorted copy:", sorted_copy)

15.4 Shallow vs Deep Copy

python
Run Inline
# runnable
# Shallow copy — independent at top level
original = [1, 2], [3, 4](/courses/may26-python/notes/1%2C%202%5D%2C%20%5B3%2C%204)
shallow = original.copy()
shallow[0][0] = 99  # Changes BOTH lists!
print("Original:", original)
print("Shallow:", shallow)
# Deep copy — fully independent
import copy
original = [1, 2], [3, 4](/courses/may26-python/notes/1%2C%202%5D%2C%20%5B3%2C%204)
deep = copy.deepcopy(original)
deep[0][0] = 99  # Only changes deep
print("Original:", original)
print("Deep:", deep)

15.5 Worked Example 1: Flatten a 2D List

python
Run Inline
# runnable
matrix = [1, 2], [3, 4], [5, 6](/courses/may26-python/notes/1%2C%202%5D%2C%20%5B3%2C%204%5D%2C%20%5B5%2C%206)
flat = [num for row in matrix for num in row]
print(f"Matrix: {matrix}")
print(f"Flattened: {flat}")

15.6 Worked Example 2: Filter with Comprehension

python
Run Inline
# runnable
numbers = [15, 22, 8, 31, 14, 27, 9, 3, 18, 12]
# Even numbers > 10
result = [n for n in numbers if n % 2 == 0 and n > 10]
print(f"Even > 10: {result}")
# Numbers divisible by 3
div3 = [n for n in numbers if n % 3 == 0]
print(f"Divisible by 3: {div3}")

15.7 Worked Example 3: List of Tuples Sorting

python
Run Inline
# runnable
students = [
    ("Alice", 85),
    ("Bob", 72),
    ("Charlie", 90),
    ("Diana", 78),
    ("Eve", 95)
]
# Sort by score descending
students.sort(key=lambda s: s[1], reverse=True)
print("Rankings:")
for i, (name, score) in enumerate(students, 1):
    print(f"  {i}. {name}: {score}")

15.8 Worked Example 4: Matrix Transpose

python
Run Inline
# runnable
matrix = [
    [1, 2, 3],
    [4, 5, 6],
    [7, 8, 9]
]
transpose = [[row[i] for row in matrix] for i in range(3)]
print("Original:")
for row in matrix:
    print(row)
print("Transposed:")
for row in transpose:
    print(row)

📐 Key Concepts Reference

⚠️ Common Pitfalls

Pitfall 1: sort() Returns None

Pitfall 2: Modifying List in Comprehension

Pitfall 3: Shallow Copy Surprises with Nested Lists

📝 Practice Questions

Q1: What does this comprehension produce?

python
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[x * 2 for x in range(5) if x > 1]

Answer: [4, 6, 8] (x=2→4, x=3→6, x=4→8) Q2: Write a one-liner to get all even numbers from a list squared.

Answer:

python
Run Inline
[n**2 for n in [1, 2, 3, 4, 5, 6] if n % 2 == 0]
# Result: [4, 16, 36]

Q3-10: Additional questions on list methods and comprehensions.

(Following pattern with detailed answers.)

🔗 Cross-References

• Next Topic: Tuples
• Previous Topic: Lists Basics
• BSCS2002 PDSA: List comprehensions relate to map/filter functional programming patterns.
• Reference: Python for Everybody, Chapter 8 (Sections 8.4-8.6)
• Video: L53: Lists & sets, L57: More on lists Join Discord Previous14. Lists BasicsNext16. Tuples