Python 3.13: `KeyError: 'name'` on request_meta in ResponseContextManager.__exit__ after GC

[ktosiek]

Prerequisites

• I am using the latest version of Locust
• I am reporting a bug, not asking a question

Description

After upgrade to Python 3.13 I've started seeing errors when running locust (I've added the traceback at the end). It seems if a GC happens within with self.client.get(..., catch_response=True) then somehow ResponseContextManager.request_meta gets cleared.

Changing ResponseContextManager.__init__ to copy the response.__dict__, instead of just grabbing it, seems to help. I'm not sure how that change affects the rest of the project.

The Traceback on failure:

[2025-02-19 07:59:11,514] SDUPC07/ERROR/locust.user.task: 'name'
Traceback (most recent call last):
  File "/home/wsl/.cache/uv/archive-v0/s81GZnyMdf_txEZnLJlMJ/lib/python3.13/site-packages/locust/user/task.py", line 340, in run
    self.execute_next_task()
    ~~~~~~~~~~~~~~~~~~~~~~^^
  File "/home/wsl/.cache/uv/archive-v0/s81GZnyMdf_txEZnLJlMJ/lib/python3.13/site-packages/locust/user/task.py", line 373, in execute_next_task
    self.execute_task(self._task_queue.popleft())
    ~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/wsl/.cache/uv/archive-v0/s81GZnyMdf_txEZnLJlMJ/lib/python3.13/site-packages/locust/user/task.py", line 490, in execute_task
    task(self.user)
    ~~~~^^^^^^^^^^^
  File "/home/wsl/locust-gc-bug-demo/locustfile.py", line 10, in get_json
    with self.client.get("/json", catch_response=True) as get_json:
         ~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/wsl/.cache/uv/archive-v0/s81GZnyMdf_txEZnLJlMJ/lib/python3.13/site-packages/locust/clients.py", line 368, in __exit__
    self.url = self.request_meta["name"]
               ~~~~~~~~~~~~~~~~~^^^^^^^^
KeyError: 'name'

Command line

uv run --with=locust==2.32.10 -p 3.13 locust --headless -u 1 -t 1s

Locustfile contents

from locust import run_single_user, HttpUser, task
import gc


class Tests(HttpUser):
    host = "https://httpbin.org"

    @task
    def get_json(self):
        with self.client.get("/json", catch_response=True) as get_json:
            gc.collect()


if __name__ == "__main__":
    run_single_user(Tests)

Python version

3.13.2

Locust version

2.32.10

Operating system

Ubuntu 20.04, but also happens on Windows

[cyberw]

Huh, that's strange, maybe there has been some change/optimization in Python's GC. I've confirmed that the issue is present in all 3.13 versions.

I'll have a look at it but I'm kinda busy atm, so any research you can do on your end would be most appreciated :)

[ktosiek]

I've found a reproduction without Locust, I'll try bisecting Python with it:

import gc
import sys

print(sys.version_info)

class A:
    def __init__(self, v):
        self.test = v

class ContextA:
    def __init__(self, a, other):
        self.__dict__ = a.__dict__
        self.other = other

    def __enter__(self):
        print("enter", self.__dict__)
        return self

    def __exit__(self, *args):
        print("exit", self.__dict__)
        assert "other_key" in self.other


def get_context(v):
    a = A(v)
    return ContextA(a, {"other_key": a})


with get_context("asdf") as c:
    gc.collect()

print("OK!")

Edit: bisected this problem to python/cpython@c32dc47. I wonder if I can make a smaller repro.

[github-actions]

This issue is stale because it has been open 60 days with no activity. Remove stale label or comment or this will be closed in 20 days.

[nielskrijger]

Remove stale label or comment or this will be closed in 20 days.

Updated to 3.13 and seeing the same problem. Downgrading to 3.12 fixed the issue.

[cyberw]

Yea, this is a Python issue, nothing for us to do in Locust.

[github-actions]

This issue was closed because it has been marked stale for 20 days with no activity. This does not necessarily mean that the issue is bad, but it most likely means that nobody is willing to take the time to fix it. If you have found Locust useful, then consider contributing a fix yourself!

[fenchu]

This is still present with python3.13.4 and locust 2.37.7, and the bug above is till open, it means 3.13 is effectively not working.
Tons of the below shows up. python 3.12.9 is all fine.

[2025-06-05 09:03:54,054] BPOIMOBJ1/ERROR/locust.user.task: 'name'
Traceback (most recent call last):
  File "C:\dist\scim-performance-test\genusers.py", line 154, in get_userid
    with self.client.get(url + query, headers=self.headers, name=filter, catch_response=os.truncate, hooks={'response': print_roundtrip}) as r:
         ~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\dist\venvs\scim-performance-test\Lib\site-packages\locust\clients.py", line 368, in __exit__
    self.url = self.request_meta["name"]
               ~~~~~~~~~~~~~~~~~^^^^^^^^
KeyError: 'name'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "C:\dist\venvs\scim-performance-test\Lib\site-packages\locust\user\task.py", line 340, in run
    self.execute_next_task()
    ~~~~~~~~~~~~~~~~~~~~~~^^
  File "C:\dist\venvs\scim-performance-test\Lib\site-packages\locust\user\task.py", line 373, in execute_next_task
    self.execute_task(self._task_queue.popleft())
    ~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\dist\venvs\scim-performance-test\Lib\site-packages\locust\user\task.py", line 385, in execute_task
    task(self)
    ~~~~^^^^^^
  File "C:\dist\scim-performance-test\genusers.py", line 299, in gen_users_install_operate_delete
    id = self.get_userid(ssn=ssn)
  File "C:\dist\scim-performance-test\genusers.py", line 182, in get_userid
    raise LocustError(e)
locust.exception.LocustError: 'name'

[cyberw]

Are you explicitly calling gc.collect()? The solution is to not do that :)

I think we just have to wait for a fix for the underlying issue in CPython python/cpython#130327

Its a bit weird how there's nothing happening there though..

[fenchu]

It seems like every tenth request fails. missing the self.request_meta object, Performance is severly hindered with triggering the exception and the result file shows 10% less requests showing up.

I just monkey patched, so it does not raise exception that slow things down:

diff --git a/locust/clients.py b/locust/clients.py
index e7ba86ad..7f69a0ac 100644
--- a/locust/clients.py
+++ b/locust/clients.py
@@ -365,7 +365,10 @@ class ResponseContextManager(LocustResponse):
             # multiple failure entries for different URLs for the same name argument, we need
             # to temporarily override the response.url attribute
             orig_url = self.url
-            self.url = self.request_meta["name"]
+            if hasattr(self, "request_meta") and "name" in self.request_meta:
+                self.url = self.request_meta["name"]
+            else:
+                return True
 
             try:
                 self.raise_for_status()

You see below the 10% less requests being registered in 3.13

python3.12:

(scim-performance-test-12) PS C:\dist\scim-performance-test> locust.exe -f .\genusers.py --host=apotek1-test12 --headless -u 10 -t 60 --only-summary --spawn-rate=5
20250605104130.092|INFO|C:\dist\scim-performance-test\genusers.py:337|users generated:10
20250605104130.092|INFO|C:\dist\scim-performance-test\genusers.py:339|apotek1-test12 with [50, 50, 50, 50, 50, 50, 50, 50, 50, 50] userbuckets
[2025-06-05 10:41:30,097] BPOIMOBJ1/INFO/locust.main: Starting Locust 2.37.8
[2025-06-05 10:41:30,099] BPOIMOBJ1/INFO/locust.main: Run time limit set to 60 seconds
[2025-06-05 10:41:30,101] BPOIMOBJ1/INFO/locust.runners: Ramping to 10 users at a rate of 5.00 per second
[2025-06-05 10:41:30,103] BPOIMOBJ1/INFO/root: loaded 50 for testing
[2025-06-05 10:41:30,115] BPOIMOBJ1/INFO/root: loaded 50 for testing
[2025-06-05 10:41:30,117] BPOIMOBJ1/INFO/root: loaded 50 for testing
[2025-06-05 10:41:30,119] BPOIMOBJ1/INFO/root: loaded 50 for testing
[2025-06-05 10:41:30,121] BPOIMOBJ1/INFO/root: loaded 50 for testing
[2025-06-05 10:41:31,106] BPOIMOBJ1/INFO/locust.runners: All users spawned: {"WebsiteUser": 10} (10 total users)
[2025-06-05 10:41:31,107] BPOIMOBJ1/INFO/root: loaded 50 for testing
[2025-06-05 10:41:31,108] BPOIMOBJ1/INFO/root: loaded 50 for testing
[2025-06-05 10:41:31,109] BPOIMOBJ1/INFO/root: loaded 50 for testing
[2025-06-05 10:41:31,111] BPOIMOBJ1/INFO/root: loaded 50 for testing
[2025-06-05 10:41:31,112] BPOIMOBJ1/INFO/root: loaded 50 for testing
[2025-06-05 10:42:28,591] BPOIMOBJ1/INFO/locust.main: --run-time limit reached, shutting down
[2025-06-05 10:42:28,829] BPOIMOBJ1/INFO/root: deleted 0 users on finish
[2025-06-05 10:42:28,830] BPOIMOBJ1/INFO/root: deleted 0 users on finish
[2025-06-05 10:42:28,830] BPOIMOBJ1/INFO/root: deleted 0 users on finish
[2025-06-05 10:42:28,831] BPOIMOBJ1/INFO/root: deleted 0 users on finish
[2025-06-05 10:42:28,831] BPOIMOBJ1/INFO/root: deleted 0 users on finish
[2025-06-05 10:42:28,832] BPOIMOBJ1/INFO/root: deleted 0 users on finish
[2025-06-05 10:42:28,832] BPOIMOBJ1/INFO/root: deleted 0 users on finish
[2025-06-05 10:42:28,833] BPOIMOBJ1/INFO/root: deleted 0 users on finish
[2025-06-05 10:42:28,833] BPOIMOBJ1/INFO/root: deleted 0 users on finish
[2025-06-05 10:42:28,833] BPOIMOBJ1/INFO/root: deleted 0 users on finish
[2025-06-05 10:42:28,834] BPOIMOBJ1/INFO/locust.main: Shutting down (exit code 0)
Type     Name                                                                          # reqs      # fails |    Avg     Min     Max    Med |   req/s  failures/s
--------|----------------------------------------------------------------------------|-------|-------------|-------|-------|-------|-------|--------|-----------
GET      /Users?filter=phonenumbers eq "{phone}"                                          995     0(0.00%) |     32      21      71     31 |   16.94        0.00
GET      /Users?filter=username eq "{ssn}"                                               1008     0(0.00%) |     32      21     148     31 |   17.16        0.00
POST     /auth/realms/apotek1/protocol/openid-connect/token                                10     0(0.00%) |    156      89     238     96 |    0.17        0.00
--------|----------------------------------------------------------------------------|-------|-------------|-------|-------|-------|-------|--------|-----------
         Aggregated                                                                      2013     0(0.00%) |     33      21     238     31 |   34.28        0.00

Response time percentiles (approximated)
Type     Name                                                                                  50%    66%    75%    80%    90%    95%    98%    99%  99.9% 99.99%   100% # reqs
--------|--------------------------------------------------------------------------------|--------|------|------|------|------|------|------|------|------|------|------|------
GET      /Users?filter=phonenumbers eq "{phone}"                                                31     33     34     35     39     44     54     61     71     71     71    995
GET      /Users?filter=username eq "{ssn}"                                                      31     33     34     35     39     44     60     80    150    150    150   1008
POST     /auth/realms/apotek1/protocol/openid-connect/token                                    210    210    210    230    240    240    240    240    240    240    240     10
--------|--------------------------------------------------------------------------------|--------|------|------|------|------|------|------|------|------|------|------|------
         Aggregated                                                                             31     33     34     36     39     46     60     80    210    240    240   2013

python 3.13:

(scim-performance-test) PS C:\dist\scim-performance-test> locust.exe -f .\genusers.py --host=apotek1-test12 --headless -u 10 -t 60 --only-summary --spawn-rate=5
20250605105243.584|INFO|C:\dist\scim-performance-test\genusers.py:337|users generated:10
20250605105243.584|INFO|C:\dist\scim-performance-test\genusers.py:339|apotek1-test12 with [50, 50, 50, 50, 50, 50, 50, 50, 50, 50] userbuckets
[2025-06-05 10:52:43,589] BPOIMOBJ1/INFO/locust.main: Starting Locust 2.37.7
[2025-06-05 10:52:43,590] BPOIMOBJ1/INFO/locust.main: Run time limit set to 60 seconds
[2025-06-05 10:52:43,592] BPOIMOBJ1/INFO/locust.runners: Ramping to 10 users at a rate of 5.00 per second
[2025-06-05 10:52:43,594] BPOIMOBJ1/INFO/root: loaded 50 for testing
[2025-06-05 10:52:43,604] BPOIMOBJ1/INFO/root: loaded 50 for testing
[2025-06-05 10:52:43,606] BPOIMOBJ1/INFO/root: loaded 50 for testing
[2025-06-05 10:52:43,607] BPOIMOBJ1/INFO/root: loaded 50 for testing
[2025-06-05 10:52:43,609] BPOIMOBJ1/INFO/root: loaded 50 for testing
[2025-06-05 10:52:44,602] BPOIMOBJ1/INFO/locust.runners: All users spawned: {"WebsiteUser": 10} (10 total users)
[2025-06-05 10:52:44,603] BPOIMOBJ1/INFO/root: loaded 50 for testing
[2025-06-05 10:52:44,605] BPOIMOBJ1/INFO/root: loaded 50 for testing
[2025-06-05 10:52:44,606] BPOIMOBJ1/INFO/root: loaded 50 for testing
[2025-06-05 10:52:44,606] BPOIMOBJ1/INFO/root: loaded 50 for testing
[2025-06-05 10:52:44,607] BPOIMOBJ1/INFO/root: loaded 50 for testing
[2025-06-05 10:53:42,434] BPOIMOBJ1/INFO/locust.main: --run-time limit reached, shutting down
[2025-06-05 10:53:42,536] BPOIMOBJ1/INFO/root: deleted 0 users on finish
[2025-06-05 10:53:42,537] BPOIMOBJ1/INFO/root: deleted 0 users on finish
[2025-06-05 10:53:42,537] BPOIMOBJ1/INFO/root: deleted 0 users on finish
[2025-06-05 10:53:42,538] BPOIMOBJ1/INFO/root: deleted 0 users on finish
[2025-06-05 10:53:42,538] BPOIMOBJ1/INFO/root: deleted 0 users on finish
[2025-06-05 10:53:42,538] BPOIMOBJ1/INFO/root: deleted 0 users on finish
[2025-06-05 10:53:42,539] BPOIMOBJ1/INFO/root: deleted 0 users on finish
[2025-06-05 10:53:42,539] BPOIMOBJ1/INFO/root: deleted 0 users on finish
[2025-06-05 10:53:42,539] BPOIMOBJ1/INFO/root: deleted 0 users on finish
[2025-06-05 10:53:42,540] BPOIMOBJ1/INFO/root: deleted 0 users on finish
[2025-06-05 10:53:42,540] BPOIMOBJ1/INFO/locust.main: Shutting down (exit code 0)
Type     Name                                                                          # reqs      # fails |    Avg     Min     Max    Med |   req/s  failures/s
--------|----------------------------------------------------------------------------|-------|-------------|-------|-------|-------|-------|--------|-----------
GET      /Users?filter=phonenumbers eq "{phone}"                                          889     0(0.00%) |     30      21      66     30 |   15.08        0.00
GET      /Users?filter=username eq "{ssn}"                                                917     0(0.00%) |     30      21      74     30 |   15.56        0.00
POST     /auth/realms/apotek1/protocol/openid-connect/token                                10     0(0.00%) |    140      63     367     77 |    0.17        0.00
--------|----------------------------------------------------------------------------|-------|-------------|-------|-------|-------|-------|--------|-----------
         Aggregated                                                                      1816     0(0.00%) |     31      21     367     30 |   30.81        0.00

Response time percentiles (approximated)
Type     Name                                                                                  50%    66%    75%    80%    90%    95%    98%    99%  99.9% 99.99%   100% # reqs
--------|--------------------------------------------------------------------------------|--------|------|------|------|------|------|------|------|------|------|------|------
GET      /Users?filter=phonenumbers eq "{phone}"                                                30     31     33     34     37     40     49     54     66     66     66    889
GET      /Users?filter=username eq "{ssn}"                                                      30     31     32     33     36     40     47     59     75     75     75    917
POST     /auth/realms/apotek1/protocol/openid-connect/token                                    150    150    190    210    370    370    370    370    370    370    370     10
--------|--------------------------------------------------------------------------------|--------|------|------|------|------|------|------|------|------|------|------|------
         Aggregated                                                                             30     31     33     34     37     41     51     63    210    370    370   1816

[cyberw]

Are you calling gc.collect() somewhere? If you are, then its nothing we can do about it, and if you aren't then this is most likely a completely different issue than this ticket :)

looks like you're doing some stuff that you really aren't showing: catch_response=os.truncate, hooks={'response': print_roundtrip}? catch_response is supposed to be a bool, not a function :)

[fenchu]

I fixed the catch_response=False everywhere an it works, sorry, this is an old test I've inherited, it has run for 4 years on python 3.6, just updated everything to latest greatest and it failed, found in this test that it worked when downgrading to 3.12, error message was the same. Upgrading from 1.0.3 to 2.37.9 and it works out of the box speaks quality, Thanks.