The approach taken is a bit different I think, since they rely heavily on `mypy` plugins to reach type safety and functional constructs otherwise impossible to get, without runtime inspections.
0: https://github.com/dry-python 1: https://github.com/dry-python/returns
1
===================
user: Optional[User]
discount_program: Optional['DiscountProgram'] = None
if user is not None:
balance = user.get_balance()
if balance is not None:
credit = balance.credit_amount()
if credit is not None and credit > 0:
discount_program = choose_discount(credit)
===================
user: Optional[User]
discount_program: Maybe['DiscountProgram'] = Maybe.from_optional(
user,
lambda real_user: real_user.get_balance(),
lambda balance: balance.credit_amount(),
lambda credit: choose_discount(credit) if credit > 0 else None,
)However if you’ve never done it will be hard to read. It’s easy to read to someone who has the background, and it is easier to refactor and less error prone because the context of a nullable value (the if statement checks) is factored out from the core logic. So if the core logic needs to be changed it can be done so without touching the if statement checking for None.
if (balance := user.get_balance()) and (credit := balance.credit_amount()):
discount_program = choose_discount(credit)