Requirement

The Requirement class is the atomic unit of a V&V specification. Each requirement captures a single, verifiable statement about expected system behavior, with metadata for traceability and quality checking.

Purpose

Requirements are the foundation of any V&V specification. vnvspec requirements follow INCOSE "Guide to Writing Requirements" (GtWR) conventions: each statement should use shall-language, be unambiguous, testable, and traceable to standards clauses.

Key Fields

Field	Type	Description
id	str	Unique identifier, e.g. "REQ-001"
statement	str	The requirement in shall-language
rationale	str	Why this requirement exists
priority	Priority	One of "blocking", "high", "medium", "low"
verification_method	VerificationMethod	One of "test", "analysis", "inspection", "demonstration", "simulation"
standards	dict[str, list[str]]	Mapping of standard name to clause IDs
acceptance_criteria	list[str]	Concrete pass/fail criteria

GtWR Quality Checking

The check_quality() method runs INCOSE GtWR rule checks against the requirement and returns a list of RuleViolation objects. An empty list means the requirement passes all checks.

from vnvspec import Requirement

req = Requirement(
    id="REQ-001",
    statement="The system should work.",
)
violations = req.check_quality()
for v in violations:
    print(f"{v.rule}: {v.message}")

Common violations include missing shall-language, vague terms ("should", "may"), missing rationale, and missing acceptance criteria.

Standards Traceability

The standards field lets you link a requirement to specific clauses in safety standards:

req = Requirement(
    id="REQ-002",
    statement="The model shall be robust to domain shift.",
    verification_method="test",
    standards={"iso_pas_8800": ["6.2.1"], "iso_21448": ["5.3"]},
)

API reference: vnvspec.core.requirement.Requirement