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I cannot reproduce the issue. IMO it has been fixed.
Moreover, you must join timers using timer.join(): a timer remains a thread.
Code:
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import os
import threading
os.system(f"grep ^VmRSS /proc/{os.getpid()}/status")
# warmup
for n in range(10):
 timer = threading.Timer(5, None)
 timer.start()
 timer.cancel()
 timer.join()
os.system(f"grep ^VmRSS /proc/{os.getpid()}/status")
for n in range(1000):
 timer = threading.Timer(5, None)
 timer.start()
 timer.cancel()
 timer.join()
os.system(f"grep ^VmRSS /proc/{os.getpid()}/status")
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Output on Linux with the main branch of Python (3.11):
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VmRSS:	 10924 kB
VmRSS:	 11104 kB
VmRSS:	 11104 kB
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