A1.3 Exception handling answers

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

Questions 5 marks in all

[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"))

Answer:

checked 9
4
checked nine
-1

finally runs before each return completes, so checked prints first; 9 // 2 is 4, and "nine" makes int raise ValueError, giving -1.

[1 mark]When does a finally block run?

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

Answer: A. finally is for clean-up that must always happen, such as stopping motors.

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

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

Answer: A. Catch only the exceptions you can deal with, so unexpected bugs still show up.

[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)

Answer:

too far
50

80 raises ValueError, so its addition never happens; the loop carries on and the total is 20 + 30.

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

Answer: raise. raise creates the exception; a caller's try and except can then handle it.

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")

The hint students can ask for: Put the conversion and the range check inside one function, so both kinds of bad input end up as the same exception. The loop keeps asking until a call succeeds. Think about which code must run however the driving ends.

A solution

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

def read_distance(text):
    cm = int(text)
    if cm < 1 or cm > 100:
        raise ValueError("out of range")
    return cm

while True:
    text = input("Distance (1 to 100)? ")
    try:
        cm = read_distance(text)
        break
    except ValueError:
        print("rejected", text)

try:
    forward(50, distance=cm)
finally:
    stop()
    print("motors stopped")

Any program that meets the task's checks is marked correct in the simulator; this is one way, not the only way.

What this lesson is about

Questions 5 marks in all

The task: a distance that cannot crash the program

A solution