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str.maketrans on a mutating dict is not thread-safe #145142
Description
Crash report
What happened?
A reproducer (in the format of Lib/tests/test_free_threading/xxx.py):
import unittest
from test.support import threading_helper
threading_helper.requires_working_threading(module=True)
class ItertoolsThreading(unittest.TestCase):
@threading_helper.reap_threads
def test_maketrans(self):
number_of_iterations = 10
number_of_attempts=100
for _ in range(number_of_iterations):
print(f'iteration {_}')
d = {2000: 'a'}
def work_iterator(dct):
for idx in range(number_of_attempts):
str.maketrans(dct)
idx = 2000 + idx # outside range of small ints
dct[idx] = chr(idx%16)
dct.pop(idx, None)
threading_helper.run_concurrently(work_iterator, nthreads=10, args=[d])
if __name__ == "__main__":
unittest.main()
print('done')At first glance it seems that the iteration using PyDict_Next is not safe (but maybe there are more unsafe parts):
cpython/Objects/unicodeobject.c
Line 13158 in 24cc998
(found when reviewing #145129)
CPython versions tested on:
CPython main branch
Operating systems tested on:
Windows
Output from running 'python -VV' on the command line:
No response
Activity
colesbury commentedon Feb 23, 2026
Yes,
PyDict_Nextisn't thread-safe if there are concurrent modifications of the dict. We should wrap the loop with a critical section onxhere.Do you want to make the change?
eendebakpt commentedon Feb 23, 2026
I am adding the easy label for newcomers. If not picked up, I will make the change.
VanshAgarwal24036 commentedon Feb 23, 2026
I am working on this issue.
pythongh-145142: Make str.maketrans safe under free-threading
Merge branch 'main' into pythongh-145142-dict-next-critical-section
Merge remote-tracking branch 'upstream/main' into pythongh-145142-dic…
Merge branch 'main' into pythongh-145142-dict-next-critical-section
pythongh-145142: Make str.maketrans safe under free-threading
Merge branch 'main' into pythongh-145142-dict-next-critical-section
gh-145142: Make str.maketrans safe under free-threading (gh-145157)
pythongh-145142: Make str.maketrans safe under free-threading (python…
[3.14] gh-145142: Make str.maketrans safe under free-threading (gh-14…
pythongh-145142: Make str.maketrans safe under free-threading (python…
pythongh-145142: Make str.maketrans safe under free-threading (python…