Answers

F6.1 Defensive design and validation

Robust programs · GCSE · OCR J277 2.3.1, AQA 8525 3.2.11, Edexcel 1CP2 3.2.3 · about 15 min

What this lesson is about

Anticipating misuse; presence, type, range, length, format and look-up checks.

• What goes wrong without it
• The validation checks
• Checking in order
• Asking until it is valid
• Length, format and look-up
• Anticipating misuse

Questions 6 marks in all

1. [1 mark]Which validation check makes sure something was entered at all?

   1. APresence check
   2. BRange check
   3. CLength check
   4. DFormat check
   Answer: A. A presence check rejects an empty answer.

2. [1 mark]A distance must be from 1 to 50. Which check is that?

   1. ARange check
   2. BPresence check
   3. CLook-up check
   4. DType check
   Answer: A. A range check tests a value is between limits.

3. [1 mark]What does this program print?

   for answer in ["30", "", "lots", "-5"]:
       print(answer.isdigit())

   Answer:

   True
   False
   False
   False

   isdigit is True only when every character is a digit, and False for an empty string.

4. [1 mark]Why check the type of an answer before its range?

   1. AConverting text that is not a number to int would crash
   2. BRange checks are slower
   3. CType checks are required first by Python
   4. DIt makes no difference
   Answer: A. int("lots") raises a ValueError, so check it is a number first.

5. [1 mark]A user types 30 when they meant 20, and the range is 1 to 50. What does validation do?

   1. AAccepts it: validation checks data is sensible, not correct
   2. BRejects it
   3. CChanges it to 20
   4. DAsks them to type it twice
   Answer: A. Validation cannot know what the user meant; it only rejects impossible values.

6. [1 mark]Checking a colour is one of red, green or blue is which check?

   1. ALook-up check
   2. BLength check
   3. CRange check
   4. DPresence check
   Answer: A. A look-up check compares the input with a list of allowed values.

The task: a validated drive

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

far = int(input("How far, 1 to 50 cm? "))
forward(60, distance=far)

from bugbot import *
connect()
def ask_distance():
    while True:
        answer = input("How far, 1 to 50 cm? ")
        if answer == "":
            print("you must type something")
        elif not answer.isdigit():
            print("that is not a whole number")
        elif not 1 <= int(answer) <= 50:
            print("it must be between 1 and 50")
        else:
            return int(answer)

far = ask_distance()
forward(60, distance=far)