--- title: "When things go wrong" subtitle: "Deeper Python in the console · Errors and the adventure · Lesson 4" --- # When things go wrong **Name:** {{field-24}} **Date:** {{field-15}} **Class:** {{field-15}} > **Key question:** Apps you trust never crash when you mistype: they catch the mistake, say what they wanted, and ask again. :::teacher-note **In one sentence:** Students catch errors with try and except so typed mistakes get a reply instead of a crash, compare it with checking first (isdigit), and catch two kinds of error in one program. **Students should be able to say:** I can use try and except to catch an error and carry on. I can name the kind of error to catch (ValueError, KeyError, ZeroDivisionError). I can explain why a caught error is not the same as a sensible answer. **Width:** the unmarked parts are the floor every student does (predict, run, investigate and make). Typical adds the modify practice; wider adds the real-world contexts. They add contexts, not difficulty. Students work in the online tutorial alongside this sheet. ::: ## Predict `int("twelve")` cannot work. Inside `try`, what happens to the next line, and does the program stop? ```python text = "twelve" try: age = int(text) print("Next year: " + str(age + 1)) except ValueError: print("That is not a number.") print("Done") ``` :::keep-together **What does it print?** ☐ Only Done ☐ Next year: 13 then Done ☐ It stops with a ValueError ☐ That is not a number. then Done My prediction, and why: {{lines-3}} ::: :::teacher-note **What happens:** That is not a number. then Done. int("twelve") causes a ValueError, so Python skips the rest of try and runs the except lines. Then the program carries on after them. ::: ## Run Run the program in the tutorial. Was your prediction right? What was different? {{lines-2}} ## Investigate ```python while True: text = input("How many tickets? ") try: tickets = int(text) break except ValueError: print("Please type a whole number, such as 3.") print("That will be $" + str(tickets * 12) + ".") ``` :::keep-together ### Which answers pass? **Which answer does int() accept, so the loop stops?** ☐ 3.5 ☐ 3 tickets ☐ -2 ☐ three ::: :::teacher-note **Answer:** -2 int("-2") is the number -2, so nothing goes wrong and the loop stops, charging $-24. A caught error is not the same as a sensible answer: the program still has to check the number. ::: :::keep-together ### Try or check first? **Compared with isdigit() in the console bridge, what does try let you do?** ☐ Skip the input ☐ Never get errors at all ☐ Turn letters into numbers ☐ Just try it, and handle the error if it happens ::: :::teacher-note **Answer:** Just try it, and handle the error if it happens isdigit() checks first; try does it and catches the error. Python programmers often say it is easier to ask forgiveness than permission. ::: :::keep-together ### Spot the bug **This should say There is no such room, but it stops with KeyError: attic. Click the line that is wrong.** ``` 1 rooms = {"hall": "A long hall."} 2 try: 3 print(rooms["attic"]) 4 except ValueError: 5 print("There is no such room.") ``` Line: {{field-15}} Why is it wrong? {{lines-2}} ::: :::teacher-note **Answer:** line 4 except ValueError only catches a ValueError. A missing key is a KeyError, so it should be except KeyError:. ::: :::typical ## Modify A canteen with two kinds of mistake: an item that is not on the menu (`KeyError`) and a number that is not a number (`ValueError`). Try both. **One to ten.** Only accept **1 to 10** of an item. **Quit.** Let the customer type `quit` instead of an item. Work in the tutorial. Plan first: what will you change, and how will you test it? {{box-4}} :::teacher-note **One possible solution.** Other correct programs are fine; check that the program runs and does what the task asks. **One to ten:** ```python prices = {"pie": 5, "drink": 3, "fruit": 2} while True: item = input("What would you like? ").strip().lower() try: price = prices[item] break except KeyError: print("We have: " + ", ".join(prices)) while True: text = input("How many? ") try: count = int(text) if count >= 1 and count <= 10: break print("Choose 1 to 10.") except ValueError: print("Please type a whole number.") print("That will be $" + str(price * count) + ".") ``` **Quit:** ```python prices = {"pie": 5, "drink": 3, "fruit": 2} price = 0 while True: item = input("What would you like? ").strip().lower() if item == "quit": break try: price = prices[item] break except KeyError: print("We have: " + ", ".join(prices)) if item != "quit": while True: text = input("How many? ") try: count = int(text) break except ValueError: print("Please type a whole number.") print("That will be $" + str(price * count) + ".") else: print("Goodbye!") ``` ::: ::: ## Make a safe calculator Ask for two whole numbers and print the first divided by the second. Catch **two** kinds of mistake, each with its own `except` and its own message: a typing mistake (`ValueError`) and dividing by zero (`ZeroDivisionError`). After either, ask again. Work in the tutorial. Plan first: what will you change, and how will you test it? {{box-4}} :::teacher-note **One possible solution.** Other correct programs are fine; check that the program runs and does what the task asks. ```python while True: try: a = int(input("First number: ")) b = int(input("Divide it by: ")) print("Answer: " + str(a / b)) break except ValueError: print("Please type whole numbers.") except ZeroDivisionError: print("You can't divide by zero. Try again.") ``` ::: :::wider ## Where this leads Next: the text adventure, built from the parser, the bag, the map and safe input. ::: ## Exit ticket :::keep-together **try: n = int("3.5") except ValueError: n = 0. What is n?** {{lines-3}} ::: :::teacher-note **Answer:** 0: int("3.5") causes a ValueError, so the except line runs. :::