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Excerpts from Design by contract - Wikipedia by Contributors to Wikimedia projects:
Design by contract (DbC), also known as contract programming, programming by contract and design-by-contract programming, is an approach for designing software.
A design by contract scheme
It prescribes that software designers should define formal, precise and verifiable interface specifications for software components, which extend the ordinary definition of abstract data types with preconditions, postconditions and invariants. These specifications are referred to as “contracts”, in accordance with a conceptual metaphor with the conditions and obligations of business contracts.
The DbC approach assumes all client components that invoke an operation on a server component will meet the preconditions specified as required for that operation.
Where this assumption is considered too risky (as in multi-channel or distributed computing), the inverse approach is taken, meaning that the server component tests that all relevant preconditions hold true (before, or while, processing the client component’ s request) and replies with a suitable error message if not.
Design by contract does not replace regular testing strategies, such as unit testing, integration testing and system testing. Rather, it complements external testing with internal self-tests that can be activated both for isolated tests and in production code during a test-phase.
The advantage of internal self-tests is that they can detect errors before they manifest themselves as invalid results observed by the client. This leads to earlier and more specific error detection.
The use of assertions can be considered to be a form of test oracle, a way of testing the design by contract implementation.