Umbrella or Not

Asking a question before acting

Everything you have written so far ran straight down the page, every line, every time. That is fine for a banner. It is useless for advice, because advice depends on the situation.

if is how a program asks a question before it acts. You give it something to check, and underneath it a block of lines that run only when the answer is yes. Follow that with else and a second block, and you have covered the no as well. Exactly one of the two blocks runs. Never both, never neither.

Two small things do all the work of holding that together:

• The if line ends in a colon. The colon means the lines below belong to this question.
• Those lines are indented, pushed in by four spaces. The indentation is what marks them as the block. Python reads it the way you read a box on a paper form: what is inside the box is inside.

One equals sign or two

The question itself goes between if and the colon, and it is usually a comparison — two values held up against each other:

• == asks whether two values are the same
• >= asks whether the left one is at least as big as the right one

That first one catches almost everybody once, because one equals sign and two equals signs are completely different instructions:

• = stores. rain_chance = 70 puts 70 into rain_chance. It is a command, and it changes something.
• == compares. rain_chance == 70 asks whether what is in there happens to be 70. It is a question, and it changes nothing.

Python will not accept = inside an if, because storing a value is not a question it can answer. It stops and reports a syntax error before anything at all runs. If you ever see that, count the equals signs first.

A worked example

A cinema till, not a weather report, so the answer to this exercise stays yours to write:

age = 14
adult_from = 18

print("Age ......", age)
if age >= adult_from:
    print("Ticket type: adult")
else:
    print("Ticket type: child")
print("Enjoy the film.")

Running that prints three lines:

Age ...... 14
Ticket type: child
Enjoy the film.

14 is not at least 18, so the first block is skipped over entirely and the second one runs. Only one ticket line ever appears. Notice the last line too: it is not indented, so it sits outside the question and prints either way. Change age to 30 and the middle line changes while the other two stay put.

Your turn

The editor holds a rain report. Two numbers sit at the top: rain_chance is today's forecast, and umbrella_mark is the point at which carrying an umbrella is worth the bother.

Run it before changing anything. The card prints, and then tells you to leave the umbrella at home on a day with a seventy percent chance of rain. That is because the test currently reads:

if rain_chance == 0:

which asks whether the chance is exactly zero — a question with nothing to do with umbrellas, and one whose answer today is no.

Replace that test with one asking whether rain_chance has reached umbrella_mark: at the mark or above it counts as take it. Everything else stays exactly as it is.

Then change rain_chance to 20 and run it again. The same program should now tell you to leave the umbrella at home. That second run is the real proof — it shows the program is reading the numbers rather than printing a fixed answer.

If something goes wrong

The likeliest slip is the equals sign. Written as if rain_chance = umbrella_mark:, Python stops on the spot with a syntax error and nothing runs, not even the card above. That is Python saying it expected a question and got an instruction. Add the second equals sign, or the comparison you actually meant.

Next likeliest is the colon going missing from the end of that line. Same kind of message, different cause: Python ran out of line while still waiting for the colon that opens the block.

If the message mentions indentation, the spaces in front of the print lines have shifted. They are not decoration — they are the only thing telling Python which lines belong to which answer. Put them back, run it again. Nothing here can break.