Design by Contract ni nini?

Question

Accepted Answer

**Design by Contract (DbC)** inakabidhi uhusiano kati ya njia na mwito wake kama makubaliano rasmi, yanayofafanuliwa na mambo matatu: **masharti ya awali** (kinachofaa kuhakikisha mwito), **masharti ya baada** (kinachohakikisha njia kwa upande wake) na **invariants** (kinachobaki kweli kwa kitu kila wakati).

## Jukumu tatu

```text
PRECONDITION  → caller's duty: inputs/state the method requires to run correctly
POSTCONDITION → method's duty: what it promises on return (if precondition held)
INVARIANT     → always-true property of the object, before and after every method
```

## Katika kod

```python
class Account:
    def __init__(self, balance):
        self._balance = balance
        self._check_invariant()

def withdraw(self, amount):
        assert amount > 0, "PRE: amount must be positive"        # precondition
        assert amount <= self._balance, "PRE: sufficient funds"  # precondition
        old = self._balance
        self._balance -= amount
        assert self._balance == old - amount, "POST: balance reduced"  # postcond
        self._check_invariant()

def _check_invariant(self):
        assert self._balance >= 0, "INVARIANT: balance never negative"
```

Kama *sharti la awali* lifeli, **mwito** ana hitilafu. Kama *sharti la baada* au *invariant* ilifeli, **njia** ina hitilafu — makubaliano yanabainisha kuokolea kwa usahihi.

## Uhusiano na LSP

DbC inafanya Liskov kuwa sahihi: aina ndogo inaweza **kuzumia masharti ya awali** na **kuimarisha masharti ya baada**, lakini sio kinyume chake. Hii ni kanuni ya kubadilika kwa dhahiri, ikionyeshwa kama makubaliano.

## Mabadiliko

```text
✓ Bugs surface at the boundary, with clear blame   ✓ Contracts double as documentation
⚠️ Runtime checks cost performance (often disabled in prod via assertions)
⚠️ Not all languages enforce it natively (Eiffel does; most use asserts/validation)
```

## Kwa nini ni muhimu

Makubaliano hubadilisha mawazo yasiyo sahihi ("kiasi kinapaswa kuwa chanya, nadhani") kuwa kanuni zenye wazi, zinazoweza kuangaliwa — na zinabainisha kuokolea kwa usahihi wakati kitu kikubwa kilivyobadilika.

Zinasokeza Liskov kubadilika na kubuni ya API wenye nguvu: makubaliano ya njia ni interface yake ya kweli, inaaminifu kuliko sahihi ya aina tu.