icontract_deal_class_with_precondition_postcondition_validation.py

1import icontract
2
3class Deal:
4    def __init__(self, amount: int, balance: int) -> None:
5        self.amount = amount
6        self.balance = balance
7
8    def __repr__(self) -> str:
9        return f"Deal(amount={self.amount}, balance={self.balance})"
10
11@icontract.require(lambda amount, balance: 0 < amount <= balance)
12@icontract.ensure(lambda result, amount, balance: result.balance == balance - amount)
13def make_deal(amount: int, balance: int) -> Deal:
14    return Deal(amount=amount, balance=balance - amount)
15
16# This works
17deal = make_deal(amount=10, balance=100)
18print(deal)
19
20# This raises a ViolationError because amount > balance
21try:
22    make_deal(amount=110, balance=100)
23except icontract.ViolationError as err:
24    print(err)