The IBAN Only Dictionary

The “Integer only list” is an example of the employment of a subclassed built-in list to check the types of elements being added to the list. How about checking the values of the keys being used when new elements are added to the dictionary?

For those of you who have taken the course “Programming Essentials in Python”, the IBAN Validator should be well know. But, if you haven’t taken the course, now is a good moment familiarize yourself with IBAN and ways to validate it.

We'll use the IBAN Validator to ensure that our banking app dictionary contains only validated IBANs (keys) and info about the associated balance (value).

Well, what is this IBAN then?

IBAN is an algorithm used by European banks to specify account numbers. The standard name IBAN (International Bank Account Number) provides a simple and fairly reliable method of validating the account numbers against simple typos that can occur during rewriting of the number, e.g., from paper documents, like invoices or bills, into computers.

You can find more details here: https://en.wikipedia.org/wiki/International_Bank_Account_Number.

An IBAN-compliant account number consists of:

• a two-letter country code taken from the ISO 3166-1 standard (e.g., FR for France, GB for the United Kingdom, DE for Germany, and so on)
• two check digits used to perform the validity checks – fast and simple, but not fully reliable, tests, showing whether a number is invalid (distorted by a typo) or seems to be good;
• the actual account number (up to 30 alphanumeric characters – the length of that part depends on the country)

The standard says that validation requires the following steps (according to Wikipedia):

• (step 1) Check that the total IBAN length is correct as per the country (this program won't do that, but you can modify the code to meet this requirement if you wish; note: you have to teach the code all the lengths used in Europe)
• (step 2) Move the four initial characters to the end of the string (i.e., the country code and the check digits)
• (step 3) Replace each letter in the string with two digits, thereby expanding the string, where A = 10, B = 11 ... Z = 35;
• (step 4) Interpret the string as a decimal integer and compute the remainder of that number on division by 97; If the remainder is 1, the check digit test is passed and the IBAN might be valid.

Look at the genuine code in the editor. In a moment we'll modify it, but first let's analyze it:

• line 03: ask the user to enter the IBAN (the number can contain spaces, as they significantly improve number readability...
• line 04: ...but remove them immediately;
• line 05: the entered IBAN must consist of digits and letters only – if it doesn't...
• line 06: ...output the message;
• line 07: the IBAN mustn't be shorter than 15 characters (this is the shortest variant, used in Norway)
• line 08: if it is shorter, the user is informed;
• line 09: moreover, the IBAN cannot be longer than 31 characters (this is the longest variant, used in Malta)
• line 10: if it is longer, make an announcement;
• line 11: start the actual processing;
• line 12: move the four initial characters to the number's end, and convert all letters to upper case (step 02 of the algorithm)
• line 13: this is the variable used to complete the number, created by replacing the letters with digits (according to the algorithm's step 03)
• line 14: iterate through the IBAN;
• line 15: if the character is a digit...
• line 16: just copy it;
• line 17: otherwise...
• line 18: ...convert it into two digits (note the way it's done here)
• line 19: the converted form of the IBAN is ready – make an integer out of it;
• line 20: is the remainder of the division of iban2 by 97 equal to 1?
• line 21: if yes, then success;
• line 22: otherwise...
• line 23: ...the number is invalid.

Let's add some test data (all these numbers are valid – you can invalidate them by changing any character).

• British: GB72 HBZU 7006 7212 1253 00
• French: FR76 30003 03620 00020216907 50
• German: DE02100100100152517108