Lernziele dieses Kapitels
- You build if-elif-else structures for decision trees
- You understand nested conditions and when they make sense
- You use Truthy and Falsy in Python for elegant checks
- You expand PyBuddy with a mood check
if, elif, else
Conditions control the program flow. In Python indentation (4 spaces) is mandatory — it replaces the curly braces from JavaScript.
The flow is always: if checks the first condition, elif (else if) checks alternatives, else catches all remaining cases.
Warning
Don't forget the colon : at the end of every condition! Otherwise Python throws a SyntaxError.
# Grade calculator with if-elif-else
prozent = float(input("Percentage: "))
if prozent >= 89:
note = 1
elif prozent >= 76:
note = 2
elif prozent >= 63:
note = 3
elif prozent >= 50:
note = 4
else:
note = 5
print(f"Your grade: {note}")Du kennst bereits JavaScript aus dem JS-Quest. Hier ist der direkte Vergleich:
if prozent >= 89:
note = 1
elif prozent >= 76:
note = 2
else:
note = 3if (prozent >= 89) {
note = 1;
} else if (prozent >= 76) {
note = 2;
} else {
note = 3;
}: instead of curly braces. elif in Python is else if in JavaScript. The logic is identical!Nested Conditions
Conditions inside conditions enable complex logic. Imagine you first check whether a player is online, and then whether they have enough mana for a spell.
But be careful: Beyond 3 nesting levels code quickly becomes unreadable. Then functions (Chapter 8) are the better choice.
# Nested conditions: Login system
name = input("Username: ")
passwort = input("Password: ")
if name == "admin":
if passwort == "geheim123":
print(" Login successful!")
else:
print(" Wrong password.")
else:
print(" User not found.")
# More compact with and:
if name == "admin" and passwort == "geheim123":
print(" Login successful!")
else:
print(" Access denied.")Truthy & Falsy
In Python certain values are automatically considered False — this is called Falsy. All other values are Truthy (i.e. True).
- Falsy:
0,0.0,""(empty string),[](empty list),None,{} - Truthy: Everything else, e.g.
"Hello",42,[1, 2]
This enables very elegant checks.
# Truthy & Falsy in Python
name = input("Your name: ")
# Elegant instead of name != "":
if name:
print(f"Hello {name}!")
else:
print("You entered nothing.")
# Also with lists
aufgaben = []
if not aufgaben:
print("No tasks available.")
aufgaben = ["Python lernen"]
if aufgaben:
print(f"You have {len(aufgaben)} task(s).")Du kennst bereits JavaScript aus dem JS-Quest. Hier ist der direkte Vergleich:
if name:
print("Hello!")
if not aufgaben:
print("Empty")if (name) {
console.log("Hello!");
}
if (!aufgaben.length) {
console.log("Empty");
}if name: or if not aufgaben:. In JavaScript you need .length for arrays and empty strings are also falsy — but empty arrays are truthy!match-case for Pattern Matching
From Python 3.10 onwards there is match-case — an elegant replacement for long if-elif chains when you check a value against multiple patterns. It is comparable to switch-case in other languages, but much more powerful.
# match-case instead of if-elif
befehl = input("Command: ").lower()
match befehl:
case "hello":
print(" Hello!")
case "help":
print(" Available commands: hello, status, exit")
case "status":
print(" PyBuddy is online.")
case "exit":
print(" Bye!")
case _:
print(" Unknown command.")Conditions in Action: Quest Status
Imagine an RPG where the player must complete a quest. You need to check: Does he have the item? Is his level high enough? Does he have enough time?
# Quest check in an RPG
level = 12
item = "Dragon scale"
zeit = 45 # minutes
if level >= 10 and item == "Dragon scale" and zeit <= 60:
print(" Quest completed!")
print(" Reward: 500 XP + Legendary Sword")
elif level < 10:
print("⛔ Your level is too low. Required: Level 10")
elif item != "Dragon scale":
print("⛔ You are missing the required item.")
else:
print(" Time expired!")Warm-Up: Grade Calculator
Write a program that reads a percentage and outputs the school grade (1-5). Use if-elif-else.
Hinweis: p = float(input("Percent: "))
if p >= 89: n = 1
elif p >= 76: n = 2
elif p >= 63: n = 3
elif p >= 50: n = 4
else: n = 5
print(f"Grade: {n}")
p = float(input("Percent: "))
if p >= 89: n = 1
elif p >= 76: n = 2
elif p >= 63: n = 3
elif p >= 50: n = 4
else: n = 5
print(f"Grade: {n}")Challenge: Password Strength Checker
Check a password for strength: at least 8 characters, contains a number, contains an uppercase letter. Use nested conditions and len().
Hinweis: pw = input("Password: ")
has_digit = any(c.isdigit() for c in pw)
has_upper = any(c.isupper() for c in pw)
if len(pw) < 8:
print(" Too short")
elif not has_digit:
print(" No number")
elif not has_upper:
print(" No uppercase letter")
else:
print(" Strong!")
pw = input("Password: ")
has_digit = any(c.isdigit() for c in pw)
has_upper = any(c.isupper() for c in pw)
if len(pw) < 8:
print(" Too short")
elif not has_digit:
print(" No number")
elif not has_upper:
print(" No uppercase letter")
else:
print(" Strong!")PyBuddy Checkpoint: Mood Check
PyBuddy asks for the mood (1-5) and outputs appropriate responses. Use if-elif-else and f-Strings.
Hinweis: # pybuddy/main.py
stimmung = int(input(" How is your mood (1-5)? "))
if stimmung >= 4:
print(" Great! Let's be productive!")
elif stimmung == 3:
print(" Good! A good day for learning.")
elif stimmung >= 1:
print(" No problem. Every day is different.")
else:
print(" Please enter a number between 1 and 5.")
# pybuddy/main.py
stimmung = int(input(" How is your mood (1-5)? "))
if stimmung >= 4:
print(" Great! Let's be productive!")
elif stimmung == 3:
print(" Good! A good day for learning.")
elif stimmung >= 1:
print(" No problem. Every day is different.")
else:
print(" Please enter a number between 1 and 5.")In Undertale the complete game behavior changes based on if-else decisions: Do you kill enemies or spare them? The game saves your decision in a variable and checks it with conditions — exactly what you can do now!
Zusammenfassung
- if → elif → else for decision trees
- Nesting possible, but max. 2-3 levels
- Truthy/Falsy: empty, 0, None = automatically False
- match-case (Python 3.10+) for pattern matching
- Conditions control the entire program flow