Rain or sun?
If it is raining, I play a board game inside. Else, I play soccer outside.
04CONDITIONALS
Ask. Then choose.
A conditional lets code choose a path after a yes-or-no question.
IN REAL LIFE
You decide like this every day.
A decision is a question with paths. Python does the same thing you do.
Hungry?
If I am hungry, I eat an apple. If not, I just keep playing.
Homework first
If my homework is done, I can play video games. Else, homework first.
LOOK FIRST
You already make choices this way.
If asks the question
“Is it raining?” Python checks whether the answer is true.
The indented step is one path
If it is raining, the next step can be: take an umbrella.
Else is the other path
If the answer is no, Python can follow a different step.
if raining:
print("umbrella")take umbrellaelse:
print("cap")wear capSTEP BY STEP
How Python decides.
Follow the code one line at a time, like PyBot does.
battery = 20A box keeps the number 20.if battery < 30:Python asks: is 20 less than 30? Yes. The answer is True. print("charge")True, so this indented line runs.else:Skipped. The answer was already True. print("play")Skipped too. Only one path runs.print("bye")Not indented, so it always runs at the end.sunny = FalseA yes-or-no box that keeps False.if sunny:Python asks: is sunny True? No. print("sunglasses")Skipped. There is no else, so this path just does nothing.print("let's go")Not indented, so it always runs.Three rules to remember.
Indentation shows the path
Lines pushed right with 4 spaces belong to the path. Lines on the left run no matter what.
The colon opens the path
if, elif, and else always end with : right before their path starts.
Only one path runs
Python never runs two paths of the same if. It picks one, then keeps going.
MORE THAN TWO PATHS
elif asks one more question.
Python asks from top to bottom and stops at the first True.
temperature = 15A box keeps 15 degrees.if temperature > 25:Is 15 greater than 25? No. Go to the next question. print("shorts")Skipped.elif temperature > 10:Is 15 greater than 10? Yes! print("jacket")This path runs.else:Skipped. A path already ran. print("coat")else runs only when every question was False.RUN IT
Help PyBot check its battery.
Guess first. Then run real Python and check.
Guess the result
battery = 20
if battery < 30:
print("charge")
else:
print("play")What will Python show?
OUTPUT
PyBot's hint
Ready. Press Run Python.
Python runs here thanks to Pyodide ♥
Pyodide 314.0.3 runs CPython in your browser with WebAssembly.
Open-source under the Mozilla Public License 2.0. License
Now change one thing: Change 20 to a bigger number, like 80, so the other path runs. Run again.
YOUR TURN
Follow the true path.
Try each tiny question. A wrong answer becomes something to review.
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Park or movie?
Family rule: if it is sunny, we go to the park. Else, we watch a movie. Today it is raining.
What does the family do?
It is raining
raining = True
if raining:
print("umbrella")Which path runs?
Battery is low
battery_low = True
if battery_low:
print("charger")Which path runs?
It is not raining
raining = False
if raining:
print("umbrella")
else:
print("cap")Which path does else choose?
No else here
sunny = False
if sunny:
print("sunglasses")What does Python show?
The line on the left
raining = True
if raining:
print("umbrella")
print("go out")Which words show?
Three paths
temperature = 30
if temperature > 25:
print("shorts")
elif temperature > 10:
print("jacket")
else:
print("coat")Which path runs?
How many paths?
A program has an if and an else. The answer to the question is True.
How many paths run?
Fix PyBot's code
Something is missing at the end of the if line.
Goal output
Time to charge!OUTPUT
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BIG IDEA
A conditional is a question with paths.
Python checks first. Then it follows the matching path.