Chapter 5 · Lesson 1

Lists Deep Dive

Lists are Python's most versatile built-in data structure. They are ordered, mutable, and can hold items of any type — including other lists.

Creating Lists

python

empty = []
numbers = [1, 2, 3, 4, 5]
mixed  = [42, "hello", 3.14, True]
nested = [[1, 2], [3, 4], [5, 6]]

Essential List Methods

Method	Purpose
`append(x)`	Add x to the end
`extend(iterable)`	Add all items from iterable
`insert(i, x)`	Insert x before index i
`remove(x)`	Remove first occurrence of x
`pop([i])`	Remove & return item at index i (default: last)
`sort()`	Sort in place
`reverse()`	Reverse in place
`index(x)`	Return first index of x
`count(x)`	Count occurrences of x
`copy()`	Return a shallow copy

Slicing

Slice syntax is list[start:stop:step]. All parts are optional:

python

nums = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
nums[2:5]    # [2, 3, 4]
nums[:3]     # [0, 1, 2]
nums[7:]     # [7, 8, 9]
nums[::2]    # [0, 2, 4, 6, 8]  — every other item
nums[::-1]   # [9, 8, ..., 0]   — reversed

Slices never raise an IndexError, making them safe for boundary cases.

List Multiplication & Concatenation

python

[0] * 5          # [0, 0, 0, 0, 0]
[1, 2] + [3, 4]  # [1, 2, 3, 4]

Warning: [[]] * 3 creates three references to the same inner list. Use a list comprehension when you need independent sublists: [[] for _ in range(3)].

Sorting with a Key Function

sort() and the built-in sorted() both accept a key parameter — a callable applied to each element before comparison:

python

words = ["banana", "fig", "apple", "kiwi"]
words.sort(key=len)          # sort by string length
# ["fig", "kiwi", "apple", "banana"]

people = [("Alice", 30), ("Bob", 25), ("Carol", 27)]
people.sort(key=lambda p: p[1])  # sort by age

Use reverse=True in either function for descending order.

Checking Membership & Length

python

3 in [1, 2, 3, 4]   # True
len([10, 20, 30])   # 3

Lists vs. Other Sequences

Use a list when you need a mutable ordered collection.
Use a tuple when the collection should not change.
Use a deque (from collections) for efficient front/back appends.

Code Examples

Core List Methodspython

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

# append — add to end
fruits.append("date")
print(fruits)

# insert — add at index
fruits.insert(1, "avocado")
print(fruits)

# remove — delete by value
fruits.remove("banana")
print(fruits)

# pop — delete by index (returns the item)
removed = fruits.pop(0)
print("Removed:", removed)
print(fruits)

# extend — add multiple items
fruits.extend(["elderberry", "fig"])
print(fruits)

append() adds a single item, extend() merges another iterable, insert() places an item at a specific index, remove() deletes by value, and pop() removes by index while returning the deleted element.

Slicing & Multiplicationpython

nums = list(range(10))  # [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
print("Original:", nums)

print("nums[2:6]  =", nums[2:6])
print("nums[:4]   =", nums[:4])
print("nums[6:]   =", nums[6:])
print("nums[::2]  =", nums[::2])
print("nums[::-1] =", nums[::-1])

# Multiplication
zeros = [0] * 5
print("zeros:", zeros)

# Safe independent sublists
matrix = [[] for _ in range(3)]
matrix[0].append(1)
print("matrix:", matrix)  # other rows unaffected

Slices use start:stop:step notation and always return a new list. The step -1 efficiently reverses a list. List comprehension is preferred over multiplication when you need independent mutable sublists.

Sorting with Key Functionspython

# Sort strings by length
words = ["banana", "fig", "apple", "kiwi", "date"]
words.sort(key=len)
print("By length:", words)

# sorted() returns a new list; sort() modifies in place
original = [3, 1, 4, 1, 5, 9, 2, 6]
ascending  = sorted(original)
descending = sorted(original, reverse=True)
print("Ascending: ", ascending)
print("Descending:", descending)
print("Original unchanged:", original)

# Sort list of tuples by second element
people = [("Alice", 30), ("Bob", 25), ("Carol", 27)]
people.sort(key=lambda person: person[1])
print("By age:", people)

The key parameter accepts any callable. sort() modifies the list in-place and returns None, while sorted() returns a brand-new sorted list without touching the original.

Quick Quiz

1. What is the difference between list.sort() and sorted(list)?

2. What does nums[::2] return for nums = [0, 1, 2, 3, 4]?

3. Which method removes an item by VALUE (not by index)?

4. Why is [[] for _ in range(3)] preferred over [[]] * 3 for a 2-D list?

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Chapter 5 · Lesson 1

Lists Deep Dive

Lists are Python's most versatile built-in data structure. They are ordered, mutable, and can hold items of any type — including other lists.

Creating Lists

python

empty = []
numbers = [1, 2, 3, 4, 5]
mixed  = [42, "hello", 3.14, True]
nested = [[1, 2], [3, 4], [5, 6]]

Essential List Methods

Method	Purpose
`append(x)`	Add x to the end
`extend(iterable)`	Add all items from iterable
`insert(i, x)`	Insert x before index i
`remove(x)`	Remove first occurrence of x
`pop([i])`	Remove & return item at index i (default: last)
`sort()`	Sort in place
`reverse()`	Reverse in place
`index(x)`	Return first index of x
`count(x)`	Count occurrences of x
`copy()`	Return a shallow copy

Slicing

Slice syntax is list[start:stop:step]. All parts are optional:

python

nums = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
nums[2:5]    # [2, 3, 4]
nums[:3]     # [0, 1, 2]
nums[7:]     # [7, 8, 9]
nums[::2]    # [0, 2, 4, 6, 8]  — every other item
nums[::-1]   # [9, 8, ..., 0]   — reversed

Slices never raise an IndexError, making them safe for boundary cases.

List Multiplication & Concatenation

python

[0] * 5          # [0, 0, 0, 0, 0]
[1, 2] + [3, 4]  # [1, 2, 3, 4]

Warning: [[]] * 3 creates three references to the same inner list. Use a list comprehension when you need independent sublists: [[] for _ in range(3)].

Sorting with a Key Function

sort() and the built-in sorted() both accept a key parameter — a callable applied to each element before comparison:

python

words = ["banana", "fig", "apple", "kiwi"]
words.sort(key=len)          # sort by string length
# ["fig", "kiwi", "apple", "banana"]

people = [("Alice", 30), ("Bob", 25), ("Carol", 27)]
people.sort(key=lambda p: p[1])  # sort by age

Use reverse=True in either function for descending order.

Checking Membership & Length

python

3 in [1, 2, 3, 4]   # True
len([10, 20, 30])   # 3

Lists vs. Other Sequences

Use a list when you need a mutable ordered collection.
Use a tuple when the collection should not change.
Use a deque (from collections) for efficient front/back appends.

Code Examples

Core List Methodspython

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

# append — add to end
fruits.append("date")
print(fruits)

# insert — add at index
fruits.insert(1, "avocado")
print(fruits)

# remove — delete by value
fruits.remove("banana")
print(fruits)

# pop — delete by index (returns the item)
removed = fruits.pop(0)
print("Removed:", removed)
print(fruits)

# extend — add multiple items
fruits.extend(["elderberry", "fig"])
print(fruits)

append() adds a single item, extend() merges another iterable, insert() places an item at a specific index, remove() deletes by value, and pop() removes by index while returning the deleted element.

Slicing & Multiplicationpython

nums = list(range(10))  # [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
print("Original:", nums)

print("nums[2:6]  =", nums[2:6])
print("nums[:4]   =", nums[:4])
print("nums[6:]   =", nums[6:])
print("nums[::2]  =", nums[::2])
print("nums[::-1] =", nums[::-1])

# Multiplication
zeros = [0] * 5
print("zeros:", zeros)

# Safe independent sublists
matrix = [[] for _ in range(3)]
matrix[0].append(1)
print("matrix:", matrix)  # other rows unaffected

Sorting with Key Functionspython

# Sort strings by length
words = ["banana", "fig", "apple", "kiwi", "date"]
words.sort(key=len)
print("By length:", words)

# sorted() returns a new list; sort() modifies in place
original = [3, 1, 4, 1, 5, 9, 2, 6]
ascending  = sorted(original)
descending = sorted(original, reverse=True)
print("Ascending: ", ascending)
print("Descending:", descending)
print("Original unchanged:", original)

# Sort list of tuples by second element
people = [("Alice", 30), ("Bob", 25), ("Carol", 27)]
people.sort(key=lambda person: person[1])
print("By age:", people)

The key parameter accepts any callable. sort() modifies the list in-place and returns None, while sorted() returns a brand-new sorted list without touching the original.

Quick Quiz

1. What is the difference between list.sort() and sorted(list)?

2. What does nums[::2] return for nums = [0, 1, 2, 3, 4]?

3. Which method removes an item by VALUE (not by index)?

4. Why is [[] for _ in range(3)] preferred over [[]] * 3 for a 2-D list?

Was this lesson helpful?