The Fix
pip install urllib3==1.25
Based on closed urllib3/urllib3 issue #1032 · PR/commit linked
Production note: Most teams hit this during upgrades or environment changes. Roll out with a canary and smoke critical endpoints (health, OpenAPI/docs) before 100%.
@@ -275,6 +275,11 @@ def test_contextmanager(self):
self.assertRaises(Empty, old_pool_queue.get, block=False)
+ def test_mixed_case_url(self):
+ pool = HTTPConnectionPool('Example.com')
+ response = pool.request('GET', "http://Example.com")
repro.py
Traceback (most recent call last):
File "minimum.py", line 5, in <module>
response = pool.request('GET', "http://Example.com")
File "/usr/local/lib/python3.5/dist-packages/urllib3/request.py", line 66, in request
**urlopen_kw)
File "/usr/local/lib/python3.5/dist-packages/urllib3/request.py", line 87, in request_encode_url
return self.urlopen(method, url, **extra_kw)
File "/usr/local/lib/python3.5/dist-packages/urllib3/connectionpool.py", line 549, in urlopen
raise HostChangedError(self, url, retries)
urllib3.exceptions.HostChangedError: HTTPConnectionPool(host='Example.com', port=None): Tried to open a foreign host with url: http://Example.com
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\npip install urllib3==1.25\nWhen NOT to use: This fix should not be used if the application relies on case-sensitive hostnames.\n\n
Why This Fix Works in Production
- Trigger: response = pool.request('GET', "http://Example.com")
- Mechanism: HostChangedError occurs due to the hostname not being downcased in the ConnectionPool
- Why the fix works: Fixes the HostChangedError by applying lowercasing to all hosts given to the ConnectionPool. (first fixed release: 1.25).
Production impact:
- If left unfixed, the same config can fail only in production (env differences), causing startup failures or partial feature outages.
Why This Breaks in Prod
- Shows up under Python 3.5 in real deployments (not just unit tests).
- HostChangedError occurs due to the hostname not being downcased in the ConnectionPool
- Surfaces as: Traceback (most recent call last):
Proof / Evidence
- GitHub issue: #1032
- Fix PR: https://github.com/urllib3/urllib3/pull/1033
- First fixed release: 1.25
- Reproduced locally: No (not executed)
- Last verified: 2026-02-09
- Confidence: 0.95
- Did this fix it?: Yes (upstream fix exists)
- Own content ratio: 0.43
Discussion
High-signal excerpts from the issue thread (symptoms, repros, edge-cases).
“Hmmm”
“I'm not sure why I was using ConnectionPool, I will use PoolManager instead”
“So the biggest issue here is probably that we should forcibly lowercase the hostname in the base ConnectionPool class. I'd accept a patch that does…”
“@Lukasa Is a blind lower() call appropriate? I thought that function was too smart for it's own good with some non-ASCII characters.”
Failure Signature (Search String)
- response = pool.request('GET', "http://Example.com")
Error Message
Stack trace
error.txt
Error Message
-------------
Traceback (most recent call last):
File "minimum.py", line 5, in <module>
response = pool.request('GET', "http://Example.com")
File "/usr/local/lib/python3.5/dist-packages/urllib3/request.py", line 66, in request
**urlopen_kw)
File "/usr/local/lib/python3.5/dist-packages/urllib3/request.py", line 87, in request_encode_url
return self.urlopen(method, url, **extra_kw)
File "/usr/local/lib/python3.5/dist-packages/urllib3/connectionpool.py", line 549, in urlopen
raise HostChangedError(self, url, retries)
urllib3.exceptions.HostChangedError: HTTPConnectionPool(host='Example.com', port=None): Tried to open a foreign host with url: http://Example.com
Minimal Reproduction
repro.py
Traceback (most recent call last):
File "minimum.py", line 5, in <module>
response = pool.request('GET', "http://Example.com")
File "/usr/local/lib/python3.5/dist-packages/urllib3/request.py", line 66, in request
**urlopen_kw)
File "/usr/local/lib/python3.5/dist-packages/urllib3/request.py", line 87, in request_encode_url
return self.urlopen(method, url, **extra_kw)
File "/usr/local/lib/python3.5/dist-packages/urllib3/connectionpool.py", line 549, in urlopen
raise HostChangedError(self, url, retries)
urllib3.exceptions.HostChangedError: HTTPConnectionPool(host='Example.com', port=None): Tried to open a foreign host with url: http://Example.com
Environment
- Python: 3.5
What Broke
Requests to a pool with uppercase hostnames result in HostChangedError exceptions.
Why It Broke
HostChangedError occurs due to the hostname not being downcased in the ConnectionPool
Fix Options (Details)
Option A — Upgrade to fixed release Safe default (recommended)
pip install urllib3==1.25
When NOT to use: This fix should not be used if the application relies on case-sensitive hostnames.
Use when you can deploy the upstream fix. It is usually lower-risk than long-lived workarounds.
Fix reference: https://github.com/urllib3/urllib3/pull/1033
First fixed release: 1.25
Last verified: 2026-02-09. Validate in your environment.
When NOT to Use This Fix
- This fix should not be used if the application relies on case-sensitive hostnames.
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 CI check that diffs key outputs after upgrades (OpenAPI schema snapshots, JSON payload shapes, CLI output).
- Upgrade behind a canary and run integration tests against the canary before 100% rollout.
- Track RSS + object counts after deployments; alert on monotonic growth and GC pressure.
- Add a long-running test that repeats the failing call path and asserts stable memory.
Version Compatibility Table
| Version | Status |
|---|---|
| 1.25 | Fixed |
Related Issues
No related fixes found.
Sources
We don’t republish the full GitHub discussion text. Use the links above for context.