httpx SyncHTTP11Connection ImportError on Python 3.9 when

The Fix

Upgrade to version 0.7.2 or later.

Based on closed encode/httpx issue #1393 · 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

@@ -98,8 +98,12 @@ def __init__(self) -> None:
         ), "The 'brotlipy' or 'brotli' library must be installed to use 'BrotliDecoder'"
         self.decompressor = brotli.Decompressor()
+        self.seen_data = False
 
     def decode(self, data: bytes) -> bytes:

repro.py

$ HTTPX_LOG_LEVEL=debug python debug/bug.py
getting result
DEBUG [2020-11-20 09:30:08] httpx._client - HTTP Request: POST https://httpbin.org/post "HTTP/1.1 200 OK"
DEBUG [2020-11-20 09:30:08] httpx._client - HTTP Request: POST https://httpbin.org/post "HTTP/1.1 200 OK"
DEBUG [2020-11-20 09:30:08] httpx._client - HTTP Request: POST https://httpbin.org/post "HTTP/1.1 200 OK"
DEBUG [2020-11-20 09:30:08] httpx._client - HTTP Request: POST https://httpbin.org/post "HTTP/1.1 200 OK"
DEBUG [2020-11-20 09:30:09] httpx._client - HTTP Request: POST https://httpbin.org/post "HTTP/1.1 200 OK"
DEBUG [2020-11-20 09:30:09] httpx._client - HTTP Request: POST https://httpbin.org/post "HTTP/1.1 200 OK"
DEBUG [2020-11-20 09:30:09] httpx._client - HTTP Request: POST https://httpbin.org/post "HTTP/1.1 200 OK"
DEBUG [2020-11-20 09:30:09] httpx._client - HTTP Request: POST https://httpbin.org/post "HTTP/1.1 200 OK"
DEBUG [2020-11-20 09:30:09] httpx._client - HTTP Request: POST https://httpbin.org/post "HTTP/1.1 200 OK"
Traceback (most recent call last):
  File "/Users/florimond/Developer/python-projects/httpx/debug/bug.py", line 30, in <module>
    main()
  File "/Users/florimond/Developer/python-projects/httpx/debug/bug.py", line 25, in main
    response = future.result()
  File "/Users/florimond/.pyenv/versions/3.9.0/lib/python3.9/concurrent/futures/_base.py", line 433, in result
    return self.__get_result()
  File "/Users/florimond/.pyenv/versions/3.9.0/lib/python3.9/concurrent/futures/_base.py", line 389, in __get_result
    raise self._exception
  File "/Users/florimond/.pyenv/versions/3.9.0/lib/python3.9/concurrent/futures/thread.py", line 52, in run
    result = self.fn(*self.args, **self.kwargs)
  File "/Users/florimond/Developer/python-projects/httpx/debug/bug.py", line 13, in do_post
    response = client.post("post")
  File "/Users/florimond/Developer/python-projects/httpx/httpx/_client.py", line 992, in post
    return self.request(
  File "/Users/florimond/Developer/python-projects/httpx/httpx/_client.py", line 733, in request
    return self.send(
  File "/Users/florimond/Developer/python-projects/httpx/httpx/_client.py", line 767, in send
    response = self._send_handling_auth(
  File "/Users/florimond/Developer/python-projects/httpx/httpx/_client.py", line 805, in _send_handling_auth
    response = self._send_handling_redirects(
  File "/Users/florimond/Developer/python-projects/httpx/httpx/_client.py", line 837, in _send_handling_redirects
    response = self._send_single_request(request, timeout)
  File "/Users/florimond/Developer/python-projects/httpx/httpx/_client.py", line 861, in _send_single_request
    (status_code, headers, stream, ext) = transport.request(
  File "/Users/florimond/Developer/python-projects/httpx/venv/lib/python3.9/site-packages/httpcore/_sync/connection_pool.py", line 200, in request
    response = connection.request(
  File "/Users/florimond/Developer/python-projects/httpx/venv/lib/python3.9/site-packages/httpcore/_sync/connection.py", line 88, in request
    self._create_connection(self.socket)
  File "/Users/florimond/Developer/python-projects/httpx/venv/lib/python3.9/site-packages/httpcore/_sync/connection.py", line 136, in _create_connection
    from .http11 import SyncHTTP11Connection
ImportError: cannot import name 'SyncHTTP11Connection' from partially initialized module 'httpcore._sync.http11' (most likely due to a circular import) (/Users/florimond/Developer/python-projects/httpx/venv/lib/python3.9/site-packages/httpcore/_sync/http11.py)

verify

Re-run: HTTPX_LOG_LEVEL=debug python debug/bug.py

fix.md

Option A — Upgrade to fixed release\nUpgrade to version 0.7.2 or later.\nWhen NOT to use: This fix is not applicable if using a different HTTP version or if the application relies on local imports for thread safety.\n\n

Why This Fix Works in Production

Trigger: $ HTTPX_LOG_LEVEL=debug python debug/bug.py
Mechanism: Fixed an ImportError related to SyncHTTP11Connection when using ThreadPoolExecutor on Python 3.9 by moving the local import to a global one.
Why the fix works: Fixed an ImportError related to SyncHTTP11Connection when using ThreadPoolExecutor on Python 3.9 by moving the local import to a global one. (first fixed release: 0.7.2).

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 3.9 in real deployments (not just unit tests).
Surfaces as: $ HTTPX_LOG_LEVEL=debug python debug/bug.py

Proof / Evidence

GitHub issue: #1393
Fix PR: https://github.com/encode/httpcore/pull/237
First fixed release: 0.7.2
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.85
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.29

Discussion

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

Jump to Sources Open on GitHub

“Woah, that's umm, weird”

@florimondmanca · 2020-11-20 · source

“Hi @florimondmanca! 1. Yes 2. Yes, with 4 workers; sometimes with 2-3 workers. 3. Not sure, sorry. I am not too familiar with it go…”

@senpos · 2020-11-20 · source

“Ah, also a funny bit of info: this only reproduces on http11, not http2. Eg Client(..., http2=True) makes the bug go away. 🤔”

@florimondmanca · 2020-11-20 · source

“Yup — moving the local import of SyncHTTP11Connection to a global one (which is fine — we don't need a local import there, since h11…”

@florimondmanca · 2020-11-20 · source

Failure Signature (Search String)

$ HTTPX_LOG_LEVEL=debug python debug/bug.py

Error Message

Stack trace

error.txt

Error Message
-------------
$ HTTPX_LOG_LEVEL=debug python debug/bug.py
getting result
DEBUG [2020-11-20 09:30:08] httpx._client - HTTP Request: POST https://httpbin.org/post "HTTP/1.1 200 OK"
DEBUG [2020-11-20 09:30:08] httpx._client - HTTP Request: POST https://httpbin.org/post "HTTP/1.1 200 OK"
DEBUG [2020-11-20 09:30:08] httpx._client - HTTP Request: POST https://httpbin.org/post "HTTP/1.1 200 OK"
DEBUG [2020-11-20 09:30:08] httpx._client - HTTP Request: POST https://httpbin.org/post "HTTP/1.1 200 OK"
DEBUG [2020-11-20 09:30:09] httpx._client - HTTP Request: POST https://httpbin.org/post "HTTP/1.1 200 OK"
DEBUG [2020-11-20 09:30:09] httpx._client - HTTP Request: POST https://httpbin.org/post "HTTP/1.1 200 OK"
DEBUG [2020-11-20 09:30:09] httpx._client - HTTP Request: POST https://httpbin.org/post "HTTP/1.1 200 OK"
DEBUG [2020-11-20 09:30:09] httpx._client - HTTP Request: POST https://httpbin.org/post "HTTP/1.1 200 OK"
DEBUG [2020-11-20 09:30:09] httpx._client - HTTP Request: POST https://httpbin.org/post "HTTP/1.1 200 OK"
Traceback (most recent call last):
  File "/Users/florimond/Developer/python-projects/httpx/debug/bug.py", line 30, in <module>
    main()
  File "/Users/florimond/Developer/python-projects/httpx/debug/bug.py", line 25, in main
    response = future.result()
  File "/Users/florimond/.pyenv/versions/3.9.0/lib/python3.9/concurrent/futures/_base.py", line 433, in result
    return self._
... (truncated) ...

Minimal Reproduction

repro.py

$ HTTPX_LOG_LEVEL=debug python debug/bug.py
getting result
DEBUG [2020-11-20 09:30:08] httpx._client - HTTP Request: POST https://httpbin.org/post "HTTP/1.1 200 OK"
DEBUG [2020-11-20 09:30:08] httpx._client - HTTP Request: POST https://httpbin.org/post "HTTP/1.1 200 OK"
DEBUG [2020-11-20 09:30:08] httpx._client - HTTP Request: POST https://httpbin.org/post "HTTP/1.1 200 OK"
DEBUG [2020-11-20 09:30:08] httpx._client - HTTP Request: POST https://httpbin.org/post "HTTP/1.1 200 OK"
DEBUG [2020-11-20 09:30:09] httpx._client - HTTP Request: POST https://httpbin.org/post "HTTP/1.1 200 OK"
DEBUG [2020-11-20 09:30:09] httpx._client - HTTP Request: POST https://httpbin.org/post "HTTP/1.1 200 OK"
DEBUG [2020-11-20 09:30:09] httpx._client - HTTP Request: POST https://httpbin.org/post "HTTP/1.1 200 OK"
DEBUG [2020-11-20 09:30:09] httpx._client - HTTP Request: POST https://httpbin.org/post "HTTP/1.1 200 OK"
DEBUG [2020-11-20 09:30:09] httpx._client - HTTP Request: POST https://httpbin.org/post "HTTP/1.1 200 OK"
Traceback (most recent call last):
  File "/Users/florimond/Developer/python-projects/httpx/debug/bug.py", line 30, in <module>
    main()
  File "/Users/florimond/Developer/python-projects/httpx/debug/bug.py", line 25, in main
    response = future.result()
  File "/Users/florimond/.pyenv/versions/3.9.0/lib/python3.9/concurrent/futures/_base.py", line 433, in result
    return self.__get_result()
  File "/Users/florimond/.pyenv/versions/3.9.0/lib/python3.9/concurrent/futures/_base.py", line 389, in __get_result
    raise self._exception
  File "/Users/florimond/.pyenv/versions/3.9.0/lib/python3.9/concurrent/futures/thread.py", line 52, in run
    result = self.fn(*self.args, **self.kwargs)
  File "/Users/florimond/Developer/python-projects/httpx/debug/bug.py", line 13, in do_post
    response = client.post("post")
  File "/Users/florimond/Developer/python-projects/httpx/httpx/_client.py", line 992, in post
    return self.request(
  File "/Users/florimond/Developer/python-projects/httpx/httpx/_client.py", line 733, in request
    return self.send(
  File "/Users/florimond/Developer/python-projects/httpx/httpx/_client.py", line 767, in send
    response = self._send_handling_auth(
  File "/Users/florimond/Developer/python-projects/httpx/httpx/_client.py", line 805, in _send_handling_auth
    response = self._send_handling_redirects(
  File "/Users/florimond/Developer/python-projects/httpx/httpx/_client.py", line 837, in _send_handling_redirects
    response = self._send_single_request(request, timeout)
  File "/Users/florimond/Developer/python-projects/httpx/httpx/_client.py", line 861, in _send_single_request
    (status_code, headers, stream, ext) = transport.request(
  File "/Users/florimond/Developer/python-projects/httpx/venv/lib/python3.9/site-packages/httpcore/_sync/connection_pool.py", line 200, in request
    response = connection.request(
  File "/Users/florimond/Developer/python-projects/httpx/venv/lib/python3.9/site-packages/httpcore/_sync/connection.py", line 88, in request
    self._create_connection(self.socket)
  File "/Users/florimond/Developer/python-projects/httpx/venv/lib/python3.9/site-packages/httpcore/_sync/connection.py", line 136, in _create_connection
    from .http11 import SyncHTTP11Connection
ImportError: cannot import name 'SyncHTTP11Connection' from partially initialized module 'httpcore._sync.http11' (most likely due to a circular import) (/Users/florimond/Developer/python-projects/httpx/venv/lib/python3.9/site-packages/httpcore/_sync/http11.py)

Environment

Python: 3.9

What Broke

Application crashes intermittently when using ThreadPoolExecutor with multiple workers, causing request failures.

Fix Options (Details)

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

Upgrade to version 0.7.2 or later.

When NOT to use: This fix is not applicable if using a different HTTP version or if the application relies on local imports for thread safety.

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

Fix reference: https://github.com/encode/httpcore/pull/237

First fixed release: 0.7.2

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

When NOT to Use This Fix

This fix is not applicable if using a different HTTP version or if the application relies on local imports for thread safety.

Verify Fix

verify

Re-run: HTTPX_LOG_LEVEL=debug python debug/bug.py

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.
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.

Version Compatibility Table

Version	Status
0.7.2	Fixed

Related Issues

No related fixes found.

Cluster: httpx:race-condition All hubs All clusters

Related clusters: Configuration error Data consistency Other

Sources

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

httpx SyncHTTP11Connection ImportError on Python 3.9 when using ThreadPoolExecutor (Fix)

The Fix

Why This Fix Works in Production

Why This Breaks in Prod

Proof / Evidence

Discussion

Failure Signature (Search String)

Error Message

Minimal Reproduction

Environment

What Broke

Fix Options (Details)

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

Get updates

When NOT to Use This Fix

Verify Fix

Did This Fix Work in Your Case?

Prevention

Version Compatibility Table

Related Issues

Sources