Bank Python implementations are effectively proprietary forks of the entire Python ecosystem which are in use at many (but not all) of the biggest investment banks. Bank Python differs considerably from the common, or garden-variety Python that most people know and love (or hate).
I've said so far that a lot of data is stored in Barbara. Time to drop a bit of a bombshell: the source code is in Barbara too, not on disk. Remain composed. It's kept in a special Barbara ring called sourcecode.
it's possible to sit down, write a script and get it running in prod within the hour, which is a big deal.
Using simple Python functions, in a source controlled system, is a better middle ground than the modern-day equivalent of J2EE.
One thing I regret about software as a field is how little time is spent learning from existing systems and judging what they did well, or badly. There are only a small number of books discussing, in detail, real systems that exist.