Working with Python Lists

What is a List in Python?

A list in Python is one of the most commonly used data structures. Think of it as a collection of items that can be stored in a specific order.

Key Characteristics

Ordered — Items have a specific order, so each item has an index starting from 0. The last element always have index -1.

Mutable —Unlike strings, Lists are mutable. You can change, add, or remove items after the list is created

Heterogeneous — A list can contain different data types: numbers, strings, booleans, other lists, etc.

Dynamic — You can grow or shrink the list as needed.

Creating Lists

# Empty list
empty_list = []

# List of numbers
numbers = [10, 20, 30, 40]

# List of strings
fruits = ["apple", "banana", "cherry"]

# Mixed data types
mixed = [1, "two", 3.5, True]

# List of lists
nested = [[1,2], [3,4], [5,6]]

Accessing Elements

Elements can be accessed via Index. As mentioned first element always have index 0 and the last element have index -1.

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

fruits[0]   # "apple" → first element
fruits[-1]  # "cherry" → last element
fruits[0:2] # ["apple", "banana"] → slicing
fruits[4]   # Error -> cannot access element 

Why Lists Are Useful

Lists can Store collections of related data

Iterate easily using loops

Perform mathematical or logical operations

Can be nested (list of lists) for tables or matrices.

# A shopping list
shopping = ["milk", "bread", "eggs"]

# Add an item
shopping.append("butter")

# Remove an item
shopping.remove("bread")

# Loop through items
for item in shopping:
    print(item)

# Output:
# milk
# eggs
# butter
"milk" in shopping   #True

# Find all the methods in List
dir(list) 

#Clear the list 
shopping.clear() 

#extend
shopping.extend(['jam', 'oats',])

#Count
numbers=[1,2,3,3,4,4,5,6,6,7,8,8]
print(numbers.count(4))
#2

# copy
new_numbers=numbers.copy()
print(new_numbers)

Adding Items

fruits = ["apple", "banana", "cherry"]
fruits.append("orange")        # Add at end
fruits.insert(1, "kiwi")       # Add at index 1
fruits.extend(["mango","grape"])  # Add multiple

Removing Items

fruits.remove("banana")        # Remove first occurrence
fruits.pop()                   # Remove last
fruits.pop(1)                  # Remove index 1
fruits.clear()                 # Remove all items

Searching & Counting

fruits.index("cherry")         # Get index
fruits.count("apple")          # Count occurrences

Copying

copy_fruits = fruits.copy()    # Shallow copy
same_list = fruits             # References same list

Common Operations / Functions

len(fruits)       # Number of items
sum(numbers)      # Sum of numbers
min(numbers)      # Minimum
max(numbers)      # Maximum

List Comprehensions (Very Useful)

squares = [x**2 for x in range(5)]           # [0,1,4,9,16]
even = [x for x in range(10) if x % 2 == 0]  # [0,2,4,6,8]

Iterating Over Lists

for fruit in fruits:
    print(fruit)

for i, fruit in enumerate(fruits):
    print(i, fruit)

Combining & Repeating

a = [1,2]
b = [3,4]
c = a + b           # [1,2,3,4]
d = a * 3           # [1,2,1,2,1,2]

Slicing Shortcuts

numbers[::-1]       # Reverse list
numbers[::2]        # Every second item
numbers[-3:]        # Last 3 items
numbers[:-3]        # All except last 3

Quick Reference Table for List Methods

 Method               | Description                                        |
--------------------  | -------------------------------------------------- |
 `append(x)`          | Add `x` at the end                                 |
 `insert(i,x)`        | Insert `x` at index `i`                            |
 `extend(iterable)`   | Add all items from another iterable                |
 `remove(x)`          | Remove first occurrence of `x`                     |
 `pop([i])`           | Remove and return item at index `i` (default last) |
 `clear()`            | Remove all items                                   |
 `index(x)`           | Return index of first occurrence                   |
 `count(x)`           | Count occurrences of `x`                           |
 `sort()`             | Sort ascending                                     |
 `sort(reverse=True)` | Sort descending                                    |
 `reverse()`          | Reverse list in place                              |
 `copy()`             | Return a shallow copy                              |