Week 2 Code Examples: Python Fundamentals

This page walks through every lecture topic from Week 2 with working code examples. These are the building blocks of every Python program you'll ever write — getting these solid now pays off all semester.

How to use this page: Copy any example into a .py file and run it. Change the values and see what happens — that's the fastest way to build intuition.

1. Variables and Data Types

A variable is a named container that stores a value. Python figures out the data type automatically based on what you assign.

Key concept: Variables are how programs remember things. Every piece of information your program works with — a name, a score, a yes/no answer — lives in a variable.

The four basic types

          name     = "Alice"   # str   — text, always in quotes
age      = 17        # int   — whole number
height   = 5.6       # float — decimal number
is_ready = True      # bool  — True or False (capital T/F, no quotes)

print(name)      # Alice
print(age)       # 17
print(height)    # 5.6
print(is_ready)  # True

# Check what type a variable is
print(type(name))      # 
print(type(age))       # 
print(type(is_ready))  # 
        

Variables can change

          score = 0
print(score)   # 0

score = score + 10   # add 10 to whatever score currently is
print(score)   # 10

score += 5     # shorthand for score = score + 5
print(score)   # 15

score -= 3
print(score)   # 12
        

Common mistake: score = score + 1 is NOT the same as math's "score equals score plus one." In Python, the right side is evaluated first, then stored in the variable on the left. So it means "take the current value of score, add 1, and store the result back in score."

Type conversion

input() always returns a string — even if the user types a number. Use int() or float() to convert.

          raw = "42"         # this is the string "42", NOT the number 42
number = int(raw)  # now it's the integer 42

print(raw + raw)     # "4242" — string concatenation!
print(number + number)  # 84  — numeric addition

# The typical pattern for numeric input:
age_str = input("How old are you? ")  # returns a string
age     = int(age_str)                 # convert to int so you can do math
print(f"Next year you'll be {age + 1}")
        

2. String Formatting

f-strings (formatted string literals) let you embed variables directly inside a string. Put an f before the opening quote and use {curly braces} around any variable or expression.

          name  = "Bob"
score = 87
grade = "B+"

# Old way — error-prone and hard to read
print("Hello " + name + ", you scored " + str(score) + " (" + grade + ")")

# f-string — clean and readable
print(f"Hello {name}, you scored {score} ({grade})")

# Expressions work too — not just variables
print(f"Double your score: {score * 2}")
print(f"Your name has {len(name)} letters")
        

Formatting numbers

          price   = 4.9999
balance = 1234567.5

print(f"Price: ${price:.2f}")          # Price: $5.00  (2 decimal places)
print(f"Balance: ${balance:,.2f}")     # Balance: $1,234,567.50  (commas + 2dp)
print(f"Percent: {0.876:.1%}")         # Percent: 87.6%
        

3. Boolean Logic and Comparisons

Comparisons produce True or False. These are the building blocks of every decision your program makes.

          x = 10

print(x == 10)   # True  — equal to
print(x != 5)    # True  — not equal to
print(x > 8)     # True  — greater than
print(x < 8)     # False — less than
print(x >= 10)   # True  — greater than or equal to
print(x <= 10)   # True  — less than or equal to
        

Watch out: = assigns a value. == compares two values. Using = when you mean == is one of the most common beginner bugs.

Combining conditions with and / or / not

          age    = 20
has_id = True

# and — both must be True
print(age >= 18 and has_id)   # True

# or — at least one must be True
print(age < 16 or has_id)     # True

# not — flips True to False and vice versa
print(not has_id)              # False

# Real-world example
temperature = 72
is_raining  = False

if temperature > 65 and not is_raining:
    print("Good weather for a walk!")
else:
    print("Maybe stay inside.")
        

4. If / Elif / Else — Making Decisions

Conditionals let your program take different paths based on a condition. Python uses indentation (4 spaces) to define what's inside each branch.

Key concept: Only one branch runs. Python checks conditions top to bottom and executes the first one that is True, then skips the rest.

Basic if / elif / else

          score = 82

if score >= 90:
    print("Grade: A")
elif score >= 80:
    print("Grade: B")   # ← this runs because 82 >= 80
elif score >= 70:
    print("Grade: C")
else:
    print("Grade: F")

# Only ONE of these prints — the first true condition wins
        

Nested conditionals

          age     = 20
has_ticket = True

if age >= 18:
    if has_ticket:
        print("Welcome — enjoy the show!")
    else:
        print("You're old enough, but you need a ticket.")
else:
    print("Sorry, this event is 18+.")
        

Checking membership with in

          valid_colors = ["red", "green", "blue"]
choice = "green"

if choice in valid_colors:
    print(f"{choice} is a valid color")
else:
    print(f"{choice} is not valid")
        

5. Getting User Input

input() pauses the program, shows a prompt, and waits for the user to type something and press Enter. It always returns a string.

          name = input("What is your name? ")
print(f"Hello, {name}!")

# Validate input — keep asking until they give something valid
color = input("Pick red, green, or blue: ").lower()
while color not in ["red", "green", "blue"]:
    print("That's not a valid choice.")
    color = input("Pick red, green, or blue: ").lower()

print(f"You picked {color}.")
        

Input validation in action

This pattern — ask, validate, loop back if invalid — is something you'll use constantly. Here it is with a numeric input:

          while True:
    raw = input("Enter a number between 1 and 10: ")
    try:
        number = int(raw)
        if 1 <= number <= 10:
            break           # valid — exit the loop
        else:
            print("Out of range. Try again.")
    except ValueError:
        print("That's not a number. Try again.")

print(f"You entered: {number}")
        

6. While Loops — Repeating Until a Condition is Met

A while loop keeps running its body as long as the condition is True. Use it when you don't know in advance how many times you'll need to loop.

Key concept: The loop checks its condition before every iteration. As soon as the condition becomes False, the loop stops and Python moves on.

Basic while loop

          countdown = 5

while countdown > 0:
    print(f"T-minus {countdown}...")
    countdown -= 1   # IMPORTANT: always move toward the exit condition

print("Liftoff!")

# Output:
# T-minus 5...
# T-minus 4...
# T-minus 3...
# T-minus 2...
# T-minus 1...
# Liftoff!
        

Infinite loops: If the condition never becomes False, the loop runs forever and freezes your program. Always make sure something inside the loop moves you toward the exit condition. Press Ctrl+C to stop a runaway loop.

while True with break

A common pattern: loop forever on purpose, and use break to exit when you're ready.

          total = 0
count = 0

while True:
    raw = input("Enter a number (or 'done' to stop): ")

    if raw.lower() == "done":
        break     # exit the loop immediately

    total += int(raw)
    count += 1

if count > 0:
    print(f"Sum: {total}, Average: {total / count:.1f}")
else:
    print("No numbers entered.")
        

7. The random Module

Python's random module generates unpredictable values — essential for games, simulations, and testing.

          import random

# random.randint(a, b) — random integer between a and b, inclusive
dice = random.randint(1, 6)
print(f"You rolled a {dice}")

# random.choice(list) — pick one item from a list at random
options = ["heads", "tails"]
flip = random.choice(options)
print(f"Coin flip: {flip}")

# random.random() — random float between 0.0 and 1.0
chance = random.random()
print(f"Random float: {chance:.4f}")

# random.shuffle(list) — shuffle a list in place
deck = ["Ace", "King", "Queen", "Jack", "10"]
random.shuffle(deck)
print(f"Shuffled deck: {deck}")
        

8. Putting It All Together — A Complete Example

Here's a number guessing game that uses variables, input, conditionals, a while loop, and random together. Notice how each concept plays its own role.

          import random

secret = random.randint(1, 20)   # computer picks a number
guesses = 0
max_guesses = 5

print("I'm thinking of a number between 1 and 20.")
print(f"You have {max_guesses} guesses.")

while guesses < max_guesses:
    raw = input("\nYour guess: ")

    try:
        guess = int(raw)
    except ValueError:
        print("Please enter a whole number.")
        continue    # skip the rest of this loop iteration, go back to top

    guesses += 1
    remaining = max_guesses - guesses

    if guess == secret:
        print(f"Correct! You got it in {guesses} guess(es).")
        break
    elif guess < secret:
        print(f"Too low. {remaining} guess(es) left.")
    else:
        print(f"Too high. {remaining} guess(es) left.")
else:
    # The 'else' on a while loop runs only if the loop finished naturally (no break)
    print(f"\nOut of guesses! The number was {secret}.")

# Output (example run):
# I'm thinking of a number between 1 and 20.
# You have 5 guesses.
#
# Your guess: 10
# Too high. 4 guess(es) left.
#
# Your guess: 5
# Too low. 3 guess(es) left.
#
# Your guess: 7
# Correct! You got it in 3 guess(es).
        

Notice: The try/except block handles bad input gracefully. The continue keyword skips the rest of the loop body when input is invalid. The break keyword exits immediately when the player wins. These small tools make programs much more robust.