From Scratch Programming Class

Session 4: Conditionals — combining decisions

Phase 3 — Python basics · Session 4 of 16

What we’re learning today

You already know if, elif, and else from Phase 2. Today we add three small but powerful words — and, or, and not — that let you combine multiple conditions into one decision. By the end of class, you’ll have built a “rollercoaster eligibility checker” that asks the user several things and tells them if they can ride.

You’ll need to remember from last time

• f-strings — f"Hello, {name}!".
• Comparison operators — >, <, ==, !=, >=, <=.
• if/elif/else from Phase 2 Session 6 — “if condition, do this; elif condition, do that; else, do the other.”
• int(input(...)) for numeric input.

Part A: and, or, not

Open Thonny and start a new file. Save it as decisions.py.

In real life, decisions usually involve multiple things at once. “Can I ride the rollercoaster?” depends on age AND height. “Can I get my driver’s license?” depends on age OR a special permit. “Should I take an umbrella?” depends on whether it’s NOT sunny.

Python has three words for combining conditions:

and — both conditions

age = 15
height = 60

if age >= 13 and height >= 50:
    print("You can ride!")
else:
    print("Sorry, not yet.")

The if runs only if both parts are true. Age must be at least 13, and height must be at least 50. If either fails, the combined condition is false.

Try changing age = 12. The condition fails (because 12 isn’t

= 13). Try changing height = 40. Same — fails because height isn’t >= 50.

or — at least one

name = input("What's your name? ")

if name == "Sam" or name == "Alex" or name == "Jordan":
    print("Welcome, friend!")
else:
    print(f"Hi, {name}.")

The if runs if the name matches any of the three. The or operator means “at least one of these has to be true.”

not — the opposite

is_raining = True

if not is_raining:
    print("Leave the umbrella at home.")
else:
    print("Bring your umbrella!")

not flips a true to false (or vice versa). not True is False. not False is True.

not is most useful when the variable already has a yes/no meaning. not is_raining reads naturally: “if it’s not raining.”

Combining all three

You can mix them. Here’s a full example:

age = 14
has_permission = True

if (age >= 13 or has_permission) and not age < 8:
    print("You can play this game.")
else:
    print("Not allowed.")

This checks: “if (you’re 13 or older OR you have permission), AND (you’re NOT under 8), then you can play.”

The parentheses group the conditions like math. and is checked after or by default — using parentheses makes the order explicit and makes the code easier to read.

Tip: when conditions get complicated, put each part on its own line. Python lets you split a long condition with backslashes or by surrounding it with parentheses. Most programmers use parentheses. We’ll see that in the rollercoaster project.

Checkpoint: You’ve used and, or, and not in at least one program, and you’ve seen how Python evaluates a combined condition. This is the natural stop point if class is cut short.

Part B: The rollercoaster eligibility checker

Time to build something that uses all this.

What you’re building

A program that asks the user their age and height, then tells them whether they can ride the rollercoaster. The rules:

• You must be at least 13 years old.
• You must be at least 48 inches tall.

Both conditions must be true to ride.

Base goal

print("Welcome to the Rollercoaster Eligibility Checker!")

age = int(input("How old are you? "))
height = int(input("How tall are you, in inches? "))

if age >= 13 and height >= 48:
    print("You can ride! Have fun!")
else:
    print("Sorry, you can't ride this one.")

Save. Run. Try a few combinations:

• age = 15, height = 60 — should pass.
• age = 10, height = 70 — should fail (too young).
• age = 14, height = 40 — should fail (too short).

That’s the base goal. Working program, simple combined condition.

Stretch — tell them WHY

A vague “sorry, you can’t ride” isn’t very helpful. Use multiple branches to tell the user exactly what’s wrong:

if age >= 13 and height >= 48:
    print("You can ride! Have fun!")
elif age >= 13 and height < 48:
    print("You're old enough, but you're a little too short.")
elif age < 13 and height >= 48:
    print("You're tall enough, but you're a little too young.")
else:
    print("You're both too young and too short. Come back in a few years!")

Now the program gives specific feedback for each failure case. Four possible outcomes; one branch per outcome.

Extension — refactor with intermediate variables

The repeated age >= 13 and height >= 48 checks are awkward. Save the results in named variables for cleaner code:

print("Welcome to the Rollercoaster Eligibility Checker!")

age = int(input("How old are you? "))
height = int(input("How tall are you, in inches? "))

old_enough = age >= 13
tall_enough = height >= 48

if old_enough and tall_enough:
    print("You can ride! Have fun!")
elif old_enough:
    print("You're old enough, but you're a little too short.")
elif tall_enough:
    print("You're tall enough, but you're a little too young.")
else:
    print("You're both too young and too short. Come back in a few years!")

Notice what’s different:

• old_enough = age >= 13 saves the result of the comparison (a True or False value) in a variable. This is called a boolean — a value that’s either True or False.
• The if statements now read more like English.
• The repetition is gone.

This is a common trick in real code: when conditions are repeated or complex, give them names.

You could push this even further:

if old_enough and tall_enough:
    print("You can ride! Have fun!")
else:
    if not old_enough:
        print("- You're too young.")
    if not tall_enough:
        print("- You're too short.")
    print("Try again later!")

Now you list all the failures, not just the first one. That uses not to invert each check.

Wrap-up

Before we leave, share with the room:

• For the kids who built the base — did your eligibility check work for the test cases?
• For the stretch — was anyone surprised by the “old enough but too short” message?
• For the extension — did the named variables version feel cleaner?

You learned today the three small words that make Python’s conditionals genuinely powerful. and, or, not turn simple yes/no checks into real decisions about complicated situations. Every program that handles “what should we do here?” ends up using these.

You also met booleans — values that are True or False. Booleans are the third major type after numbers and strings, and you’ll see them constantly from here on.

If you missed this session

Open Thonny and start a new file. Save as decisions.py. Then:

1. Try basic and, or, not examples:

   age = 15
   if age >= 13 and age < 20:
       print("Teenager.")
   

2. Build the rollercoaster eligibility checker (Part B base):

   • Ask for age and height
   • Check age >= 13 and height >= 48
   • Print “you can ride” or “sorry”

3. Try the stretch — separate branches for each failure case.

About 30 minutes. If you get stuck, ask your buddy at the start of next class.

Stretch and extension ideas

• The order of or and and matters when they’re mixed. True or True and False is True because and is evaluated first. Use parentheses when in doubt: (True or True) and False is False.
• True and False (with capital T and F) are Python’s built-in booleans. You can assign them directly: is_raining = True.
• The bool() function converts other values to True/False. bool(1) is True; bool(0) is False; bool("") is False; bool("hello") is True. Try them.
• Compound conditions with three or more parts: if x > 0 and y > 0 and z > 0: — works for any number of parts.

What’s next

Next week we deepen loops and meet the Thonny debugger — a tool that lets you watch your program run line by line. The debugger is the single most useful thing for figuring out why your program isn’t doing what you thought it would. By the end of next week, you’ll have a new debugging superpower.