A precondition is a Boolean condition that must be true before a use case, method, or function begins. In a use case, it records what must already hold or have happened; in programming by contract, it defines what the caller must establish before calling. section 3.6.2 section 6.3.2
Use a precondition to make responsibilities and expected behavior explicit:
- Have the caller check the condition before making the call. section 6.3.2
- Document the condition clearly, and provide a public way to check it when it belongs to a class method. section 6.3.2
- Treat a satisfied precondition as a promise that the function will behave correctly; an unsatisfied one releases that guarantee. section 6.3.2 section 6.4
Circular buffer
For a CircularBuffer, add() requires self._count < self._capacity, so the caller must avoid adding when the buffer is full. remove() requires self._count > 0, so the caller must avoid removing from an empty buffer. Once the relevant condition holds, add() must insert at the tail and advance it, while remove() must remove from the head, advance it, and return the value. section 6.3.2
Preconditions form one part of a programming-by-contract design alongside postconditions and class invariants. Python has no native contract support, but these ideas can still be implemented with existing language features, especially for critical classes. section 6.3
