Worksheet

A1.3 Exception handling

Programming techniques and object-oriented programming · A level · AQA 7517 4.1.1.9 · about 20 min

What this lesson is about

try, except, else and finally, raising your own exceptions, and keeping the motors safe when something goes wrong.

• An unhandled exception
• try and except
• else, finally and as
• Raising your own
• Exceptions or checks?

Questions 5 marks in all

1. [1 mark]What does this program print?

   def half(text):
       try:
           n = int(text)
       except ValueError:
           return -1
       else:
           return n // 2
       finally:
           print("checked", text)
   
   print(half("9"))
   print(half("nine"))

2. [1 mark]When does a finally block run?

   1. AEvery time the try statement finishes, whether or not an exception was raised
   2. BOnly when an exception is raised
   3. COnly when no exception is raised
   4. DOnly when the exception is not caught

3. [1 mark]Why is a bare except: that catches every exception usually a bad idea?

   1. AIt also hides errors you did not expect, such as a misspelt variable name
   2. BIt makes the program run more slowly
   3. CPython does not allow it
   4. DIt stops finally blocks from running

4. [1 mark]What does this program print?

   def check(cm):
       if cm > 50:
           raise ValueError("too far")
       return cm
   
   total = 0
   for cm in [20, 80, 30]:
       try:
           total = total + check(cm)
       except ValueError as error:
           print(error)
   print(total)

5. [1 mark]Which Python keyword is used to signal an exception yourself, for example when a value is out of range?

The task: a distance that cannot crash the program

Write a function read_distance(text) that takes a string text and returns it as an integer. It must raise a ValueError in two cases: when text is not a whole number (let int raise it) and when the number is below 1 or above 100 (raise it yourself with raise ValueError). In the main program, keep asking Distance (1 to 100)? with input until read_distance succeeds. Each time it raises ValueError, print rejected <text>, where <text> is exactly what was typed. The task types twenty, 250, -5 and then 25. When you have a valid distance, drive forward that many cm inside a try block whose finally block calls stop() and prints motors stopped.

# the two lines every program starts with: the commands, then the robot
from bugbot import *
connect()

cm = int(input("Distance (1 to 100)? "))
forward(50, distance=cm)
print("motors stopped")

Plan your program here, then type it in and press Run.

Do it on the robot
www.bugbotlab.com/learn/a1-3-exception-handling/
The simulator checks it and tells you when it passes. Nothing to install, no account.

Challenges

1. Add a second except for TypeError. Can read_distance ever raise one, when input always returns a string?
2. Give up after three rejected answers by raising an exception of your own, and catch it in the main program.
3. Put print(1 / 0) inside the try block that drives. Does motors stopped still print? Does the program carry on afterwards?