Python ModuleNotFoundError safe restart decision response

When this happens

Use this when a restart might fix the symptom but could also interrupt users or jobs. Do not stop at the screen message; validate whether reload, restart, queue drain, or rollback is safer first.

Symptom checklist

• ModuleNotFoundError appears repeatedly in the Python UI or logs.
• venv, pip freeze, stack trace, encoding, path differs between successful and failed requests.
• The issue appears only after separating code error from environment/package conflict.
• It often follows deploys, permission changes, configuration edits, or data refreshes.

Likely causes

• ModuleNotFoundError specifically changes the investigation surface for Python: verify the exact failing object, route, user, and timestamp before applying the broader pattern.
• The deployed runtime does not contain the package, class, image, or module that local development used.
• Lockfiles or dependency caches select a different version than expected.
• ESM/CJS, classpath, Composer autoload, or Python virtualenv boundaries are mixed.
• A registry or image pull permission fails before application code starts.
• Build and runtime environments use different package managers or versions.
• For the safe restart decision case, the first useful clue is whether reload, restart, queue drain, or rollback is safer.

First 1-minute checks

1. Write down the first failure time, latest change, affected user, path, and object ID.
2. Compare venv, pip freeze, stack trace, encoding, path for success and failure in the same window.
3. Test the hypothesis: dependency or import failure caused by missing package, incompatible module system, stale lockfile, classpath, image registry, or isolated runtime.
4. Classify this as safe restart decision: whether reload, restart, queue drain, or rollback is safer.
5. Capture current values before changing configuration.

First evidence

Treat ModuleNotFoundError as a safe restart decision case. First collect evidence for whether reload, restart, queue drain, or rollback is safer.

Output examples

Normal output

Config test passes and active requests or jobs can be drained safely.

Failing output

The service is crash-looping, has invalid config, or still has in-flight critical jobs.

Output-to-action branches

• A restart might fix the symptom but could also interrupt users or jobs.
  Run config tests, inspect active jobs, choose reload when possible, and prepare rollback.
• The working and failing outputs differ.
  Act on the differing layer first: For ModuleNotFoundError, apply the fix only after reproducing the same condition and saving the before/after evidence for this exact code.
• Command output is normal but users still fail.
  Separate browser cache, cookies, permissions, and network location before declaring it fixed.

Do not do this

• Do not restart blindly before saving logs and validating configuration.
• Do not change multiple layers before identifying the failing layer.
• Do not delete production data, grant broad permissions, or disable security controls as a first response.

Evidence quality

Auto-generated operator draft: includes issue-specific causes, commands, output branches, and unsafe-action warnings. Official-source links and real incident validation are queued for enrichment.

Commands to run first

grep -R "ModuleNotFoundError" ./logs

python -m pip freeze

python -X dev script.py

python -m traceback

grep -R "ModuleNotFoundError" .

npm ls || true

python -m pip freeze || true

composer diagnose || true

mvn -q dependency:tree || true

systemctl status SERVICE --no-pager && journalctl -u SERVICE -n 100 --no-pager

Fix order

1. Record the full ModuleNotFoundError message, failing URL, user, object ID, and latest change.
2. Collect issue-specific evidence for dependency or import failure caused by missing package, incompatible module system, stale lockfile, classpath, image registry, or isolated runtime.
3. Compare the failing case with a successful case before editing settings.
4. If this is the safe restart decision branch, Run config tests, inspect active jobs, choose reload when possible, and prepare rollback.
5. Re-check with the same command and URL, then record the normal output.

Actions by cause

• For ModuleNotFoundError, apply the fix only after reproducing the same condition and saving the before/after evidence for this exact code.
• Reproduce from a clean install rather than a warmed local cache.
• Pin dependency versions and regenerate the lockfile deliberately.
• Align module system, classpath, autoload, or virtualenv configuration.
• Authenticate registry/image pulls before restarting workloads.
• Add a build-time check for the missing module or class.
• For the safe restart decision branch, Run config tests, inspect active jobs, choose reload when possible, and prepare rollback.

Evidence links

• Python exceptions official
• Python asyncio development mode official

Verification metadata

• operator-draft
• official-reference-linked
• 2026-07-23

Update queue

• Review cadence
  weekly-source-review
• Next enrichment
  Add one official-source check and one real output example for Python ModuleNotFoundError.

Environment-specific checks

• Shared hosting, proxies, VPNs, or CDN layers can change code error from environment/package conflict results.
• Do not trust only the Python UI; compare command output.
• Japanese hosting panels may show completion before DNS or SSL fully propagates.
• Test from both office and external networks.

Prevent it next time

• Store normal examples for venv, pip freeze, stack trace, encoding, path.
• Add venv reproduction, dependency pinning, explicit encoding, type checks to the release checklist.
• Keep recurring errors in the same note format.
• Split alerts by error rate, latency, certificates, disk, and permission changes.

Related notes

• Python ModuleNotFoundError first triage response
• Python ImportError first triage response
• Python KeyError first triage response
• Python TypeError first triage response
• Python UnicodeDecodeError first triage response
• Python pip conflict first triage response
• Python virtualenv first triage response
• Python asyncio first triage response