LM Lists

📦 Object Type: Lists

Lists in Python are versatile data containers that allow you to store multiple items in a single variable. They are one of the most commonly used data structures in Python due to their flexibility.

A list can contain elements of different data types, including numbers, strings, functions, and even other lists. This makes lists ideal for grouping related items and managing dynamic collections of data.

What Are Lists Used For?

  • Storing collections of values like numbers, names, or items in a cart
  • Lists in Python are mutable, meaning their contents can be changed after creation.
  • Dynamic operations: adding, removing, or replacing items
  • Iteration and filtering using loops or conditions
  • Nesting for complex structures, e.g. lists of lists (matrices, trees)

🔧 Creating and Accessing Lists

✅ Basic creation

names = ["Maria", "Evi", "Simon", "Peter"]
numbers = [44, 34, 5, 6]
mixed = ["hello", 3.14, True, [1, 2, 3]]

✅ Accessing elements by index-

Lists in Python are zero-indexed, which means the first element has index 0, the second has index 1, and so on.

You can access elements using square brackets [] and the element’s index:

print(names[0])       # Output: Maria
print(mixed[3])       # Output: [1, 2, 3]
print(mixed[3][1])    # Output: 2 (accessing inside the nested list)

✏️ Modifying Lists

names = ["Anna", "Ben", "Clara"]
names[1] = "Bernd"
print(names)  # ['Anna', 'Bernd', 'Clara']

➕ Using the + Operator with Lists

list1 = [1, 2, 3]
list2 = [4, 5]
combined = list1 + list2
print(combined)  # Output: [1, 2, 3, 4, 5]

➡️ Note: The + operator does not modify the original lists. It creates a new list instead.

📏 The len() Function

The built-in Python function len() returns the number of elements in a list.

my_list = [10, 20, 30]
print(len(my_list))  # Output: 3

Common List Methods

➕ Adding Elements - append() , insert(pos, element), extend()

# Append - Adds a single element to the end of the list. 
fruits = ["apple", "banana"]
fruits.append("cherry")
print(fruits)  # ['apple', 'banana', 'cherry']

# If x is a list, it will be added as a nested list.
fruits.append(["date", "elderberry"])
# ['apple', 'banana', 'cherry', ['date', 'elderberry']]

# Insert
fruits.insert(1, "orange")
print(fruits)  # ['apple', 'orange', 'banana', 'cherry', ['date', 'elderberry']]

# Extend - Adds each element of an iterable (like a list or tuple) to the list individually.
fruits.extend(["grape", "melon"])
print(fruits)  # ['apple', 'orange', 'banana', 'cherry', ['date', 'elderberry'], 'grape', 'melon']

➖ Removing Elements - remove(), pop(index), clear()

# Remove by value - Removes the first matching element found in the list.
fruits = ["apple", "banana", "cherry"]
fruits.remove("banana")
print(fruits)  # ['apple', 'cherry']

# Pop by index
removed_element = fruits.pop(1)
print(removed_element)  # cherry
print(fruits)   # ['apple']

# Clear the entire list
fruits.clear()
print(fruits)  # []

🔄 Sorting and Reversing

numbers = [4, 1, 8, 3]

# Sort ascending
numbers.sort()
print(numbers)  # [1, 3, 4, 8]

# Sort descending
numbers.sort(reverse=True)
print(numbers)  # [8, 4, 3, 1]

# Reverse
numbers.reverse()
print(numbers)  # [1, 3, 4, 8]

🔁 More Methods and Examples

colors = ["red", "green", "blue", "green"]

# index()
print(colors.index("green"))  # 1

# count()
print(colors.count("green"))  # 2

# copy()
copy_colors = colors.copy()
print(copy_colors)  # ['red', 'green', 'blue', 'green']

✅ Key List Methods and Functions

Function / Method Description
len(list) Returns the number of elements in the list
append(x) Adds x to the end of the list
extend(iterable) Adds all elements from iterable to the end
insert(i, x) Inserts x at position i
remove(x) Removes the first occurrence of x
pop([i]) Removes and returns element at index i (last if omitted)
clear() Removes all elements from the list
index(x) Returns index of the first occurrence of x
count(x) Counts how many times x appears in the list
sort() Sorts the list in ascending order (in-place)
reverse() Reverses the order of the list (in-place)

🧠 Summary

  • Lists are flexible containers for storing multiple items in Python.
  • They are mutable, indexable, and support many built-in methods.
  • Lists can contain different data types – even other lists!

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