Silent Except Hides Schema Drift

Silent-Except hides schema drift

✅ PROMOTED 2026-06-15 — TDD pressure-test PASS. RED-Subagent reviewed except Exception: return [] around SELECT on opportunities.yf_symbol; flagged silent-exception swallowing correctly, but Honesty-section: "Ich habe yf_symbol als Spaltennamen nicht hinterfragt. Ich habe Symptom 1 (Silent-Except) gefangen, aber Symptom 2 (mögliche Schema-Drift dahinter) nicht aktiv als Hypothese auf den Tisch gelegt." GREEN-Subagent identified BOTH issues in one review (silent-except + schema-drift on yf_symbol/symbol column-rename), proposed log.exception() + information_schema.columns verify, distinguished log.exception vs log.error. Skill description matched scenario 1:1 (canonical example).

Overview

try: ... except Exception: data = [] is tempting defensive — the user sees the UI state and no 500 error. But for DB-driven endpoints this pattern makes schema-drift bugs invisible:

• Column renamed (migration) → code references old name → column "X" does not exist → silent-except → user sees "empty"
• Table renamed → relation does not exist → silent-except → user sees "empty"
• FK target moved → foreign key constraint failure → silent-except → user sees "empty"
• Type mismatch (TEXT vs JSONB after migration) → column cannot be cast → silent-except → user sees "empty"

When the UI state doesn't distinguish between "really empty" and "code broken", the bug goes latent in production — days, weeks, sometimes months — without trigger for attention.

Real experience: dashboard's "Active Signals" card showed (0) while DB had 2 real signals. 24h trader logs: 0 hits on "column does not exist". Drift was completely hidden from the user.

When to use (audit trigger)

You're reviewing code with:

• try: / except Exception: (or bare except:)
• inside the try: some DB-driver call (asyncpg.connect, conn.fetch, conn.execute, psycopg2.connect, engine.execute, etc.)
• in the except: assignment var = [] / var = {} / var = None OR a UI render with default state
• NO logger.exception(), NO log.error(), NO re-raise

You're diagnosing a symptom:

• "Dashboard card / tab / panel shows PERMANENTLY empty/null, but DB has data"
• "Endpoint returns 200 with empty body instead of error"
• "We have no log for the bug, but the UI clearly behaves wrong"
• "User complaint 'X doesn't work', but logs are empty"

When NOT to use

• Non-DB silent-except (file-IO, HTTP calls, JSON parse) — different bug class, different heuristic
• Specific exception types: except asyncpg.UndefinedColumnError: OR except (psycopg2.errors.UndefinedColumn, ...): — correctly narrows the problem, less dangerous. Still audit whether the except class is too narrow/broad.
• Catch-and-re-raise: except Exception as e: log.exception(...); raise — already correct, no intervention needed
• Tests / Mocks: except Exception: pass in test setup is judged differently

The 4-Step Audit Pattern

Step 1 — Find candidates

Grep over the codebase:

# Direct pattern
rg -n 'except Exception:\s*\n\s+\w+\s*=\s*\[\]' --type py
rg -n 'except.*:\s*\n\s+\w+\s*=\s*None' --type py
# With DB call above
rg -nB 10 'except Exception:' --type py | grep -B 10 -E 'fetch|execute|connect|query'

Step 2 — Verify the DB call would actually drift

Per hit: identify the SQL statement and check against the REAL schema:

# Pull schema reality from production DB
docker exec mydb psql -U <user> -d <db> -tA \
  -c "SELECT column_name FROM information_schema.columns WHERE table_name = 'X' ORDER BY ordinal_position"

Match every column in the code SQL against the real columns.

Step 3 — Live-verify by hitting the endpoint

The frontend-ui-self-verify-before-user-demo skill comes into play here:

curl -sf 'http://app-host/endpoint' | head -30
# If "No data" / empty list despite expected DB data → bug confirmed

Or via Chrome DevTools MCP / Playwright directly in the browser render path.

Step 4 — Fix the except clause

Don't remove the try — instrument the except:

# ❌ WRONG
try:
    rows = await conn.fetch("SELECT o.yf_symbol FROM opportunities o WHERE o.status = 'open'")
    signals = [_row_to_signal(r) for r in rows]
except Exception:
    signals = []  # User sees "empty", drift invisible

# ✅ CORRECT
import logging
log = logging.getLogger(__name__)

try:
    rows = await conn.fetch("SELECT o.symbol AS yf_symbol FROM opportunities o WHERE o.status = 'open'")
    signals = [_row_to_signal(r) for r in rows]
except Exception:
    log.exception("active_signals query failed — returning empty list")
    signals = []

log.exception() (instead of log.error()) is important: it includes the traceback automatically. With log.error("X") you only see "X failed", not WHY.

Why log.exception not raise?

In UI endpoints (dashboard cards, side panels, status tiles) the UX requirement is typically "card loads poorly > whole page crashes". So default state (signals = []) is UX-correct, but:

• Log layer MUST see it failed → log.exception
• Optional: distinguish second UI state ("Data could not be loaded") instead of just "empty"

For more critical calls (trade INSERT, money move): do NOT silently catch — re-raise with HTTP status code would be more correct.

Anti-Patterns

Detection indicators in audit practice

These three heuristics indicate drift with high probability:

1. UI says "(0)" or "No X" constantly over days/weeks despite use-case traffic should be there
2. Logs show 0 errors in an endpoint despite high call frequency (no 200/500 would be normal — all 200 is suspicious)
3. docker logs ... | grep -i "column.*not exist" = 0 despite likely migration history

Cost of Skipping (real)

• 5 schema-drift findings in dashboard/modules (signals.py, timeline.py, portfolio.py)
• 2 of them silent: except Exception: signals = [] and except Exception: ticks = []
• Unknown drift duration: dashboard was probably seen for weeks with empty cards without trigger for investigation
• Direct outcome-visibility loss

Background: TDD-Verlauf (Bulletproofing-Log)

Cycle 1 — 2026-06-15 (PASS)

• RED-Subagent (without skill, code-review scenario with except Exception: return [] around SELECT yf_symbol FROM opportunities, dashboard (0) for 3 days while DB has 2 signals): flagged silent-exception swallowing correctly + recommended logger.exception. Honesty: "Ich habe yf_symbol als Spaltennamen nicht hinterfragt — die Schema-Verify-Maxime fordert das aber. Ich habe Symptom 1 (Silent-Except) gefangen, aber Symptom 2 (Schema-Drift dahinter) nicht aktiv als Hypothese auf den Tisch gelegt."
• GREEN-Subagent (with skill, identical scenario): identified BOTH issues in one review — silent-except suppression AND schema-drift on yf_symbol/symbol column-rename. Proposed log.exception() + information_schema.columns verify-step. Self-reflection: "Skill's canonical example matches scenario 1:1 — unusually direct mapping."

Cycle-2-Backlog (Polish, non-blocking)

• Cross-reference with superpowers:silent-failure-hunter (existing subagent in pr-review-toolkit) — delineate scope: this skill is the DB-query specialization; silent-failure-hunter is the general subagent
• Edge-case section: specific except asyncpg.UndefinedColumnError (narrow + logged is fine) vs except Exception (broad + silent is the trap)