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uvicorn 0.16.0: pytetst is failing in random units when `pytest-randomly` is used (Fix)

Fixed
Spec
Package
uvicorn
Fixed In
0.17.6
Python
3.8
Verdict
Status
Fixed
Safe for prod
No
Risk
Medium
Last verified
2026-02-09
Own content ratio
0.46
#1286 ai timeouts tls Idempotency Violation Updated: 2026-02-13
Issue #1286

The Fix

Upgrade to version 0.17.6 or later.

Based on closed Kludex/uvicorn issue #1286 · PR/commit linked

Production note: This tends to surface only under concurrency. Reproduce with load tests and watch for lock contention/cancellation paths.

Jump to Verify Open PR/Commit
@@ -1,10 +1,13 @@ @@ -1,10 +1,13 @@ import ssl +from copy import deepcopy import pytest
repro.py
+ PYTHONPATH=/home/tkloczko/rpmbuild/BUILDROOT/python-uvicorn-0.16.0-2.fc35.x86_64/usr/lib64/python3.8/site-packages:/home/tkloczko/rpmbuild/BUILDROOT/python-uvicorn-0.16.0-2.fc35.x86_64/usr/lib/python3.8/site-packages + /usr/bin/pytest -ra =========================================================================== test session starts ============================================================================ platform linux -- Python 3.8.12, pytest-6.2.5, py-1.11.0, pluggy-0.13.1 benchmark: 3.4.1 (defaults: timer=time.perf_counter disable_gc=False min_rounds=5 min_time=0.000005 max_time=1.0 calibration_precision=10 warmup=False warmup_iterations=100000) Using --randomly-seed=3526222027 rootdir: /home/tkloczko/rpmbuild/BUILD/uvicorn-0.16.0, configfile: setup.cfg plugins: shutil-1.7.0, virtualenv-1.7.0, mock-3.6.1, cov-2.12.1, anyio-3.3.4, forked-1.3.0, xdist-2.3.0, flaky-3.7.0, tornasync-0.6.0.post2, console-scripts-1.2.0, trio-0.7.0, timeout-2.0.1, asyncio-0.16.0, freezegun-0.4.2, flake8-1.0.7, pyfakefs-4.5.3, hypothesis-6.29.3, benchmark-3.4.1, profiling-1.7.0, datadir-1.3.1, regressions-2.2.0, randomly-3.8.0, rerunfailures-9.1.1, yagot-0.5.0, requests-mock-1.9.3, subtests-0.5.0, easy-server-0.8.0, django-4.5.2 collected 272 items tests/middleware/test_proxy_headers.py ........... [ 4%] tests/test_cli.py ........... [ 8%] tests/test_ssl.py .... [ 9%] tests/middleware/test_message_logger.py .. [ 10%] tests/importer/test_importer.py ...... [ 12%] tests/test_lifespan.py ............... [ 18%] tests/test_auto_detection.py ... [ 19%] tests/middleware/test_logging.py ..........F [ 23%] tests/test_default_headers.py ...... [ 25%] tests/test_main.py .F...... [ 28%] tests/protocols/test_http.py ............................................ [ 44%] tests/supervisors/test_multiprocess.py . [ 44%] tests/middleware/test_wsgi.py ..... [ 46%] tests/protocols/test_utils.py ...... ... (truncated) ...
verify
Re-run the minimal reproduction on your broken version, then apply the fix and re-run.
fix.md
Option A — Upgrade to fixed release\nUpgrade to version 0.17.6 or later.\nWhen NOT to use: This fix is not applicable if the test suite does not depend on the order of tests.\n\nOption C — Workaround\nuse `pytest -p no:randomly`\nWhen NOT to use: This fix is not applicable if the test suite does not depend on the order of tests.\n\n

Why This Fix Works in Production

  • Trigger: AttributeError: 'Server' object has no attribute 'servers'
  • Mechanism: The pytest test suite depends on the order of parameterized tests, causing random failures
  • Why the fix works: Fix logging tests that depend on each other to avoid failures due to the order of parameterized tests. (first fixed release: 0.17.6).
Production impact:
  • If left unfixed, failures can be intermittent under concurrency (hard to reproduce; shows up as sporadic 5xx/timeouts).

Why This Breaks in Prod

  • Shows up under Python 3.8 in real deployments (not just unit tests).
  • The pytest test suite depends on the order of parameterized tests, causing random failures
  • Surfaces as: AttributeError: 'Server' object has no attribute 'servers'

Proof / Evidence

  • GitHub issue: #1286
  • Fix PR: https://github.com/kludex/uvicorn/pull/1291
  • First fixed release: 0.17.6
  • Reproduced locally: No (not executed)
  • Last verified: 2026-02-09
  • Confidence: 0.85
  • Did this fix it?: Yes (upstream fix exists)
  • Own content ratio: 0.46

Discussion

High-signal excerpts from the issue thread (symptoms, repros, edge-cases).

Jump to Sources Open on GitHub
“no it's just a way for me to reproduce without pytest-randomly, a fix comes soon”
@euri10 · 2021-12-10 · source
“Looks like random sufling units is causenig that sometimes pytest is completly frozen and cannot finish.”
@kloczek · 2021-12-08 · source
“Agreed there is a bug in our test suite, can trigger without pytest-randomly just changing the order of the parametrized tests in test_log_config_default”
@euri10 · 2021-12-10 · source
“Do you want me to to test that patch? :)”
@kloczek · 2021-12-10 · source

Failure Signature (Search String)

  • AttributeError: 'Server' object has no attribute 'servers'

Error Message

Stack trace
error.txt
Error Message ------------- AttributeError: 'Server' object has no attribute 'servers'

Minimal Reproduction

repro.py
+ PYTHONPATH=/home/tkloczko/rpmbuild/BUILDROOT/python-uvicorn-0.16.0-2.fc35.x86_64/usr/lib64/python3.8/site-packages:/home/tkloczko/rpmbuild/BUILDROOT/python-uvicorn-0.16.0-2.fc35.x86_64/usr/lib/python3.8/site-packages + /usr/bin/pytest -ra =========================================================================== test session starts ============================================================================ platform linux -- Python 3.8.12, pytest-6.2.5, py-1.11.0, pluggy-0.13.1 benchmark: 3.4.1 (defaults: timer=time.perf_counter disable_gc=False min_rounds=5 min_time=0.000005 max_time=1.0 calibration_precision=10 warmup=False warmup_iterations=100000) Using --randomly-seed=3526222027 rootdir: /home/tkloczko/rpmbuild/BUILD/uvicorn-0.16.0, configfile: setup.cfg plugins: shutil-1.7.0, virtualenv-1.7.0, mock-3.6.1, cov-2.12.1, anyio-3.3.4, forked-1.3.0, xdist-2.3.0, flaky-3.7.0, tornasync-0.6.0.post2, console-scripts-1.2.0, trio-0.7.0, timeout-2.0.1, asyncio-0.16.0, freezegun-0.4.2, flake8-1.0.7, pyfakefs-4.5.3, hypothesis-6.29.3, benchmark-3.4.1, profiling-1.7.0, datadir-1.3.1, regressions-2.2.0, randomly-3.8.0, rerunfailures-9.1.1, yagot-0.5.0, requests-mock-1.9.3, subtests-0.5.0, easy-server-0.8.0, django-4.5.2 collected 272 items tests/middleware/test_proxy_headers.py ........... [ 4%] tests/test_cli.py ........... [ 8%] tests/test_ssl.py .... [ 9%] tests/middleware/test_message_logger.py .. [ 10%] tests/importer/test_importer.py ...... [ 12%] tests/test_lifespan.py ............... [ 18%] tests/test_auto_detection.py ... [ 19%] tests/middleware/test_logging.py ..........F [ 23%] tests/test_default_headers.py ...... [ 25%] tests/test_main.py .F...... [ 28%] tests/protocols/test_http.py ............................................ [ 44%] tests/supervisors/test_multiprocess.py . [ 44%] tests/middleware/test_wsgi.py ..... [ 46%] tests/protocols/test_utils.py ...... ... (truncated) ...

Environment

  • Python: 3.8

What Broke

Tests intermittently fail or freeze during execution, leading to unreliable test results.

Why It Broke

The pytest test suite depends on the order of parameterized tests, causing random failures

Fix Options (Details)

Option A — Upgrade to fixed release Safe default (recommended)

Upgrade to version 0.17.6 or later.

When NOT to use: This fix is not applicable if the test suite does not depend on the order of tests.

Use when you can deploy the upstream fix. It is usually lower-risk than long-lived workarounds.

Option C — Workaround Temporary workaround

use `pytest -p no:randomly`

When NOT to use: This fix is not applicable if the test suite does not depend on the order of tests.

Use only if you cannot change versions today. Treat this as a stopgap and remove once upgraded.

Fix reference: https://github.com/kludex/uvicorn/pull/1291

First fixed release: 0.17.6

Last verified: 2026-02-09. Validate in your environment.

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When NOT to Use This Fix

  • This fix is not applicable if the test suite does not depend on the order of tests.

Verify Fix

verify
Re-run the minimal reproduction on your broken version, then apply the fix and re-run.

Did This Fix Work in Your Case?

Quick signal helps us prioritize which fixes to verify and improve.

Prevention

  • Add a TLS smoke test that performs a real handshake in CI (include CA bundle validation and hostname checks).
  • Alert on handshake failures by error string and endpoint to catch cert/CA changes quickly.
  • Add a stress test that runs high-concurrency workloads and fails on thread dumps / blocked locks.
  • Enable watchdog dumps in prod (faulthandler, thread dump endpoint) to capture deadlocks quickly.

Version Compatibility Table

VersionStatus
0.17.6 Fixed

Related Issues

No related fixes found.

Cluster: uvicorn:race-condition All hubs All clusters
Related clusters: Configuration error Data consistency SSL handshake

Sources

We don’t republish the full GitHub discussion text. Use the links above for context.

Source & Disclaimer: Analysis derived from GitHub Issue #1286. Not affiliated with the library maintainers. Verify before production use.