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By the way, the filename of the warning is not really useful. Example script test.py:
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f = open(__file__)
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At exit, Python displays:
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sys:1: ResourceWarning: unclosed file <_io.TextIOWrapper name='test.py' mode='r' encoding='UTF-8'>
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"sys" is not a real file, _Py_DisplaySourceLine() would not be able to display the Python line (the correct filename is test.py).
Python traceback when the warning is emitted, according to gdb:
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(gdb) py-bt
Traceback (most recent call first):
 <built-in method _dealloc_warn of _io.FileIO object at remote 0x7ffff1a04368>
 <built-in method _dealloc_warn of _io.BufferedReader object at remote 0x7ffff1ae4838>
 <built-in method close of _io.TextIOWrapper object at remote 0x7ffff1b97dc0>
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We are in PyImport_Cleanup(), called by Py_Finalize().