Regression Testing After Bug Fixes

# Practical: Defect lifecycle, severity/priority, and density metrics in Python
from dataclasses import dataclass, field
from typing import List, Optional
from collections import Counter

@dataclass
class Defect:
    bug_id: str
    title: str
    severity: str        # Critical / High / Medium / Low
    priority: str        # High / Medium / Low
    status: str = "New"  # New→Open→Fixed→Retest→Closed / Deferred / Rejected
    module: str = "Unknown"
    defect_id: Optional[str] = None

    def transition(self, new_status: str, actor: str = ""):
        valid = {
            "New":     ["Open", "Rejected"],
            "Open":    ["Fixed", "Deferred"],
            "Fixed":   ["Retest"],
            "Retest":  ["Closed", "Open"],  # Open = Reopened
            "Closed":  [],
            "Deferred":["Open"],
            "Rejected":[],
        }
        if new_status not in valid.get(self.status, []):
            raise ValueError(f"Invalid transition: {self.status} → {new_status}")
        self.status = new_status
        print(f"  [{self.bug_id}] {self.title[:40]:<40} → {new_status}")

class DefectTracker:
    def __init__(self):
        self.defects: List[Defect] = []

    def add(self, defect: Defect):
        self.defects.append(defect)

    def open_by_severity(self) -> dict:
        open_defects = [d for d in self.defects if d.status not in ("Closed", "Rejected")]
        return dict(Counter(d.severity for d in open_defects))

    def defect_density(self, module: str, kloc: float) -> float:
        """Defects per 1000 lines of code in a module."""
        count = sum(1 for d in self.defects if d.module == module)
        return round(count / kloc, 2)

# ─── Usage ───────────────────────────────────────────────────────────────────
tracker = DefectTracker()
d1 = Defect("BUG-001", "Login fails with + sign in email", "Critical", "High", module="Auth")
d2 = Defect("BUG-002", "Broken logo on homepage", "Low", "High", module="UI")
d3 = Defect("BUG-003", "Checkout total off by $0.01 for discount codes", "Medium", "Medium", module="Checkout")

tracker.add(d1); tracker.add(d2); tracker.add(d3)

print("─── Defect Lifecycle Transitions ────────────────────────")
d1.transition("Open"); d1.transition("Fixed"); d1.transition("Retest"); d1.transition("Closed")
d2.transition("Open"); d2.transition("Fixed"); d2.transition("Retest"); d2.transition("Open")  # Reopened
d3.transition("Deferred")

print("\n─── Open Defects by Severity ─────────────────────────────")
print(tracker.open_by_severity())  # {'High': 1, 'Medium': 1}

print("\n─── Defect Density ───────────────────────────────────────")
print(f"Auth module (2 KLOC):     {tracker.defect_density('Auth', 2.0)} defects/KLOC")
print(f"Checkout module (3 KLOC): {tracker.defect_density('Checkout', 3.0)} defects/KLOC")

# ─── 5 Whys RCA Example ──────────────────────────────────────────────────────
print("\n─── 5 Whys RCA: BUG-001 ──────────────────────────────────")
whys = [
    "Login fails for emails containing '+'",
    "The + sign is not URL-encoded before being sent to the API",
    "Email validation uses a regex that strips special chars before encoding",
    "The regex was copied from an old internal tool that assumed ASCII-only input",
    "There is no input sanitization standard and no code review checklist for encoding",
]
for i, why in enumerate(whys, 1):
    print(f"  Why {i}: {why}")
print("  Root Cause → Add encoding standards checklist to code review process")