Python Tutorials

Choose a lesson to begin

Hello World and Print Statements
Learn the basics of printing output in Python
Beginner
Variables and Data Types
Store and work with different types of data
Beginner
Math and Arithmetic
Perform calculations and mathematical operations
Beginner
F-Strings and Formatting
Format text output with embedded variables
Beginner
String Methods
Transform and manipulate text with built-in methods
Beginner
Lists and For Loops
Store multiple items and iterate through them
Beginner
List Comprehensions and Generator Expressions
Master concise, Pythonic ways to create and transform sequences
Intermediate
Dictionaries and Advanced Data Structures
Master key-value mappings, nested structures, and efficient data organization
Intermediate
Functions, Decorators, and Functional Programming
Write reusable code with functions, decorators, lambda expressions, and functional patterns
Intermediate
Object-Oriented Programming (OOP)
Master classes, objects, inheritance, and OOP design patterns
Advanced
Error Handling and Exceptions
Handle errors gracefully with try-except, custom exceptions, and error recovery
Intermediate
File I/O and Data Persistence
Read and write files, work with JSON/CSV, and persist application data
Advanced

Lists and For Loops

Lists let you store multiple items in a single variable. For loops let you process each item automatically. Together, they're one of Python's most powerful features!

What Is a List?

A list is a collection of items in square brackets [], separated by commas:

fruits = ["apple", "banana", "orange"]
numbers = [10, 20, 30, 40]
mixed = ["Vidar", 22, True, 3.14]

Creating Lists

# empty list
empty = []
# list of strings
colors = ["red", "green", "blue"]
# list of numbers
scores = [95, 87, 92, 100]
# list of mixed types (works, but avoid it)
stuff = ["text", 42, True]

Printing Lists

fruits = ["apple", "banana", "orange"]
print(fruits)
# output: ['apple', 'banana', 'orange']
# in an f-string
print(f"Fruits: {fruits}")
# output: Fruits: ['apple', 'banana', 'orange']

For Loops - The Basics

A for loop runs code for each item in a list:

fruits = ["apple", "banana", "orange"]
for fruit in fruits:
    print(fruit)

Output: 

apple
banana
orange

How it works:

  • Takes the first item ("apple") and stores it in fruit
  • Runs the indented code
  • Takes the next item ("banana") and repeats
  • Continues until all items are processed

    • Loop Syntax

    for item in list_name:
    # this code runs for each item
    print(item)

    Key parts:

    Variable Naming in Loops

    Use descriptive names:

    # good - clear what we're looping through
for color in colors:
    print(color)
for score in scores:
    print(score)
# bad - unclear
for x in colors:
    print(x)

    Using Loop Variables

    The loop variable (fruit, score, etc.) holds the current item:

    fruits = ["apple", "banana", "orange"]
for fruit in fruits:
    print(f"I like {fruit}")

    Output: 

    I like apple
I like banana
I like orange

    Loops with Numbers

    numbers = [10, 20, 30, 40, 50]
for num in numbers:
    doubled = num * 2
    print(f"{num} doubled is {doubled}")

    Output: 

    10 doubled is 20
20 doubled is 40
30 doubled is 60
40 doubled is 80
50 doubled is 100

    Accumulating Values

    Use a variable to build up a total:

    scores = [95, 87, 92, 88]
total = 0
for score in scores:
    total = total + score
print(f"Total: {total}")
# output: Total: 362

    How it works:

  • Start with total = 0
  • First loop: total = 0 + 95 → total = 95
  • Second loop: total = 95 + 87 → total = 182
  • Third loop: total = 182 + 92 → total = 274
  • Fourth loop: total = 274 + 88 → total = 362

    • Counting Items

    fruits = ["apple", "banana", "orange", "grape"]
count = 0
for fruit in fruits:
    count = count + 1
print(f"We have {count} fruits")
# output: We have 4 fruits

    Real-World Examples

    Process Shopping List

    shopping_list = ["milk", "bread", "eggs", "butter"]
print("Shopping List:")
for item in shopping_list:
    print(f"- {item.upper()}")

    Calculate Average Score

    scores = [85, 92, 78, 95, 88]
total = 0
for score in scores:
    total = total + score
average = total / 5
print(f"Average score: {average}")

    Format Names

    names = ["Vidar", "bob", "charli"]
for name in names:
    formatted = name.upper()
    print(f"Hello, {formatted}!")

    Inventory Check

    products = ["laptop", "mouse", "keyboard", "monitor"]
print("Inventory Check:")
for product in products:
    print(f"✓ {product} - in stock")

    Indentation Is Critical!

    Correct: 

    for num in [1, 2, 3]:
    print(num)      # indented - inside loop
    print("---")    # indented - inside loop
print("Done!")      # not indented - runs once after loop

    Wrong: 

    for num in [1, 2, 3]:
print(num)  # eRROR! Must be indented

    Common Patterns

    Print each item: 

    for item in items:
    print(item)

    Sum all numbers: 

    total = 0
for num in numbers:
    total = total + num

    Count items: 

    count = 0
for item in items:
    count = count + 1

    Transform each item: 

    for name in names:
    upper_name = name.upper()
    print(upper_name)

    Try It Out!

  • Create a list of 5 numbers and print each one
  • Make a list of names and print "Hello, [name]!" for each
  • Create a list of prices and calculate the total
  • Make a list of words and count how many there are
  • Create a to-do list and print each item with a checkbox: "☐ Task"

    • Common Mistakes

    Forgetting the colon: 

    # wrong
for item in items
    print(item)
# right
for item in items:
    print(item)

    Forgetting to indent: 

    # wrong
for item in items:
print(item)
# right
for item in items:
    print(item)

    Using the list name instead of the loop variable: 

    # wrong
for fruit in fruits:
    print(fruits)  # Prints the whole list each time!
# right
for fruit in fruits:
    print(fruit)   # Prints each fruit

    Next Steps

    You now know the fundamentals of Python! Practice these concepts, and you'll be ready to build real programs. Consider exploring:

    Keep coding! 🐍

    🐍 Python Runner