Two ways to create a customer account (and therefore customer ID)
This means that two accounts with the same number but a different letter could exist. It's not an issue as such as it was planned by the
generate_account_id() method but I wouldn't be happy to share my customer ID with someone else. :D Also, it means that there exists some newcomers that we can't identify based on their account number, which is sad.
There should probably be a static method (or whatever they are called in Python) to create a Customer, neither places should not be responsible for choosing the account ID.