Python — Lists

Creating lists

Lists are Python's most versatile collection — ordered, mutable, and允许 duplicates:

fruits = ["apple", "banana", "cherry"]
numbers = [1, 2, 3, 4, 5]
mixed = [1, "hello", 3.14, True]
empty = []

Accessing elements

Lists are zero-indexed:

fruits = ["apple", "banana", "cherry"]

print(fruits[0])   # apple
print(fruits[2])   # cherry
print(fruits[-1])  # cherry (last element)
print(fruits[-2])  # banana (second to last)

Slicing

Extract sublists with [start:stop:step]:

numbers = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

print(numbers[2:5])    # [2, 3, 4]
print(numbers[:4])     # [0, 1, 2, 3]
print(numbers[6:])     # [6, 7, 8, 9]
print(numbers[::2])    # [0, 2, 4, 6, 8]
print(numbers[::-1])   # [9, 8, 7, 6, 5, 4, 3, 2, 1, 0]

Slicing creates a new list — the original is unchanged.

Modifying lists

Lists are mutable — you can change, add, and remove elements:

fruits = ["apple", "banana", "cherry"]

# Change an element
fruits[1] = "blueberry"
print(fruits)  # ["apple", "blueberry", "cherry"]

# Change a slice
fruits[0:2] = ["grape", "kiwi"]
print(fruits)  # ["grape", "kiwi", "cherry"]

Adding elements

fruits = ["apple", "banana"]

# append — add to end
fruits.append("cherry")
print(fruits)  # ["apple", "banana", "cherry"]

# insert — add at specific position
fruits.insert(1, "blueberry")
print(fruits)  # ["apple", "blueberry", "banana", "cherry"]

# extend — add multiple items
fruits.extend(["grape", "kiwi"])
print(fruits)  # ["apple", "blueberry", "banana", "cherry", "grape", "kiwi"]

# concatenation — creates a new list
new = fruits + ["mango"]

Removing elements

fruits = ["apple", "banana", "cherry", "banana"]

# remove — first occurrence of value
fruits.remove("banana")
print(fruits)  # ["apple", "cherry", "banana"]

# pop — remove and return element at index
last = fruits.pop()
print(last)    # banana
print(fruits)  # ["apple", "cherry"]

# pop at index
first = fruits.pop(0)
print(first)   # apple

# del — remove by index or slice
numbers = [0, 1, 2, 3, 4]
del numbers[2]
print(numbers)  # [0, 1, 3, 4]

del numbers[1:3]
print(numbers)  # [0, 4]

# clear — empty the list
fruits.clear()
print(fruits)  # []

List operations

a = [1, 2, 3]
b = [4, 5, 6]

# Concatenation
print(a + b)       # [1, 2, 3, 4, 5, 6]

# Repetition
print(a * 3)       # [1, 2, 3, 1, 2, 3, 1, 2, 3]

# Length
print(len(a))      # 3

# Membership
print(2 in a)      # True
print(5 not in a)  # True

# Min, max, sum
numbers = [3, 1, 4, 1, 5, 9]
print(min(numbers))  # 1
print(max(numbers))  # 9
print(sum(numbers))  # 23

List methods

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

# Sort — modifies in place
numbers.sort()
print(numbers)  # [1, 1, 2, 3, 4, 5, 6, 9]

# Sort in reverse
numbers.sort(reverse=True)
print(numbers)  # [9, 6, 5, 4, 3, 2, 1, 1]

# sorted() — returns a new sorted list
original = [3, 1, 4, 1, 5]
new_sorted = sorted(original)
print(original)    # [3, 1, 4, 1, 5] — unchanged
print(new_sorted)  # [1, 1, 3, 4, 5]

# reverse — reverses in place
numbers.reverse()
print(numbers)  # [1, 1, 2, 3, 4, 5, 6, 9]

# count occurrences
print(numbers.count(1))  # 2

# find index
print(numbers.index(5))  # 4

# copy — shallow copy
copy = numbers.copy()

Iterating over lists

fruits = ["apple", "banana", "cherry"]

# Basic iteration
for fruit in fruits:
    print(fruit)

# With index
for index, fruit in enumerate(fruits):
    print(f"{index}: {fruit}")

# With index using range
for i in range(len(fruits)):
    print(f"{i}: {fruits[i]}")

List comprehensions

A concise way to create lists:

# Traditional way
squares = []
for x in range(10):
    squares.append(x ** 2)

# List comprehension
squares = [x ** 2 for x in range(10)]
print(squares)  # [0, 1, 4, 9, 16, 25, 36, 49, 64, 81]

# With condition
evens = [x for x in range(20) if x % 2 == 0]
print(evens)  # [0, 2, 4, 6, 8, 10, 12, 14, 16, 18]

# With transformation
words = ["hello", "world"]
upper_words = [word.upper() for word in words]
print(upper_words)  # ["HELLO", "WORLD"]

Nested lists

Lists can contain other lists:

matrix = [
    [1, 2, 3],
    [4, 5, 6],
    [7, 8, 9]
]

print(matrix[0][0])  # 1
print(matrix[1][2])  # 6

# Flatten a nested list
flat = [item for row in matrix for item in row]
print(flat)  # [1, 2, 3, 4, 5, 6, 7, 8, 9]

Unpacking

numbers = [1, 2, 3, 4, 5]

# Unpack all
a, b, c, d, e = numbers
print(a, b, c)  # 1 2 3

# Unpack with star
first, *middle, last = numbers
print(first)   # 1
print(middle)  # [2, 3, 4]
print(last)    # 5

Common patterns

# Check if list is empty
if not fruits:
    print("No fruits")

# Find duplicates
from collections import Counter
counts = Counter([1, 2, 2, 3, 3, 3])
duplicates = [item for item, count in counts.items() if count > 1]

# Remove duplicates (preserves order)
unique = list(dict.fromkeys([1, 2, 2, 3, 3, 3]))

Mini Practice

1. Create a list of your five favorite movies and print each one
2. Slice a list to get every other element
3. Use a list comprehension to create a list of squares from 1 to 20
4. Sort a list of names alphabetically and in reverse
5. Flatten a nested list [[1,2], [3,4], [5,6]] into [1,2,3,4,5,6]

Next: tuples — immutable sequences →