PreConditions

Preconditions are conditions that must be true before a function or operation is executed. In software engineering, they describe the valid state of inputs and the environment, forming part of the contract between a caller and the callee. Common preconditions include non-null arguments, value ranges, or order relationships between inputs.

Preconditions are often contrasted with postconditions, which describe what must hold after execution, and with invariants, which hold throughout a component’s lifetime. In design by contract, preconditions are the caller’s responsibility: the callee may assume that preconditions are satisfied and need not check them. Some languages and tools also support runtime checks or static verification to enforce preconditions, in which case violations typically raise exceptions or errors.

There are several ways preconditions are expressed. They may be documented as part of a function’s specification, enforced via assertions or contract features, or implemented as explicit input validation. Documentation-only preconditions rely on callers to adhere to the contract, while contract-enforced preconditions actively check inputs and report failures when violated.

Examples of common preconditions include a function dividing two numbers with denominator not equal to zero, a method that accesses an array at a given index within bounds, or a string operation requiring non-null, properly formatted input. Best practices favor simple, verifiable preconditions and minimizing strong assumptions that degrade modularity. However, preconditions do not guarantee correctness after a call; they define what must be true beforehand to ensure safe and predictable execution.