Validation and Verification

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Textbook Chapter 9.1

Verification and Validation

Data entered by users can not be trusted. There are two basic methods of trying to catch mistakes in user entered data:

Verification: Getting a user (or another system) to verify that the data entered is correct. E.g.

When setting up an account you need to enter your password twice to check for typos (aka double entry)
When submitting an important form, e.g., a bank transfer, you are presented a summary page and asked to double check before pressing submit (aka visual check)
Credit cards are checked against their credit card company before payment is accepted

Validation: Automatic, but simple checks that can be done by the current program alone. E.g.,

Checking a number is in the correct range, e.g., age of user is between 12 and 150
Checking an email is if the correct structure user_name@domain.tld
Checking a credit card number is potentially correct using Luhn's Algorithm. Or some other check-digit style verification such as that used in an ISBN.

Verification and Validation

The validation sequence

Implementation of Verification

Implementation of Validation

Exercises

Verification

Verification - New password

Validation - Library Card Number

Validation - Computer Science Books

Validation - Date

The validation sequence

Presence/Existence - has data been entered for that field/variable
Type - is the data entered of the correct type (integer, number, string, etc...)
Range/Length/Format/etc - does that data satisfy some further constraints such as
- Range: Numerical data types or elements in a list (e.g., post codes)
- Length: Of allowed strings
- Format: Date, Email, or Postal Address
- Character Check: Make sure all characters entered are allowed
- Check digits (credit cards, ISBN, etc...)

Implementation of Verification

E.g., New password input

while True:

    password = input("Enter your new password: ")

    check = input("Reenter your password to check: ")

    if password == check:

        break

    print("Passwords do not match, please try again!")

# update password in database!

print("Great! Password is now updated")

See textbook for more examples (page 227)

The basic logic of such a verification check is easy, but it gets messier once you want to add user feedback.

REPEAT

  INPUT password

  INPUT check

UNTIL password = check

OUTPUT "Done!"

An exercises below is to rewrite using a WHILE loop.

Implementation of Validation

E.g., Check user supplied age - see textbook page 229 for a different implementation

age = -1 # prime the loop

while age <= 0:

  age_str = input("Enter your age in years: ")

  if age_str.isdigit():

    age = int(age_str)

  if age > 0:

    break

  print("Age must be a positive integer")

print(f"{age} is a good age to be!")

A more modern / Pythonic approach would be to use Try-Catch to use the exception system to handle bad input. See e.,g., https://study.com/academy/lesson/data-validation-exception-handling-in-python.html

As pseudocode variables are not explicitly typed, it is hard to write pseudocode presence and type checks. However, range & length &c checks are still ok.

Here is a pseudocode equivalent

REPEAT

  INPUT age

  IF age <= 0

    OUTPUT "Age should be a

          positive whole number"

  END IF

UNTIL age > 0

OUTPUT age

OUTPUT "is a good age to be!"

Exercises

Verification

A user is signing up for a website. Write a program that will ask for a username, date of birth and email for the new account. Echo (repeat) them back to the user and ask them to confirm / verify the data to continue or restart. Loop until verified.

# Example SessionPlease enter your preferred username: aissharkPlease enter your date of birth: 03/02/1993Please enter your email address: ais@ais.edu.auIs the following date correct: username = aisshark, DOB = 03/02/1993, email = ais@ais.edu.au (Y/N)? NPlease enter your preferred username: aissharkPlease enter your date of birth: 03/02/1993Please enter your email address: ais@ais.edu.sgIs the following date correct: username = aisshark, DOB = 03/02/1993, email = ais@ais.edu.sg (Y/N)? YGreat! Your account is being set up :)

Verification - New password

Rewrite the new password example from above to use a WHILE-DO-ENDWHILE loop (instead of a REPEAT-UNTIL loop). Here is the pseudocode outline, don't forget about the user prompts!

INPUT password, check

WHILE password <> check DO

  INPUT password, check

ENDWHILE

Validation - Library Card Number

A library card for a school library has to be 5 digits long and the last digit is a check digit that makes the sum of digits 0 modulo 10.

E.g.:

12340, 56126, 45678 are valid
12345, 56789, 11223 are not valid
123, 1X456 are not valid

Write a program that will repeatedly ask for a library number until a valid one is given. You should check in order: Presence, Type, Length, Check Digit.

Validation - Computer Science Books

Write a program that given a number/string will check that it is a valid ISBN-10 or ISBN-13. Test it by getting the ISBNs of some IGCSE computer science books [Cambridge, Kinokuniya]. (Structured Version of Exercise)

Validation - Date

Ask a user for their birthdate in the form dd/mm/yyyy. Use string slices and methods to confirm that is is of the correct format.

Bonus: Check that it is also a birthdate that is in the past!

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