STINNER Victor added the comment:
The problem is tricky.
The modules are deleted in a random order at exit. The problem is that showing
the warning requires to import the linecache module. But during Python
finalization, we may or may not be able to import modules.
Py_Finalize() calls PyImport_Cleanup() which starts by setting sys.path and
sys.meta_path to None. The _find_spec() function of importlib._bootstrap fails
because sys.meta_path is None. In fact, find_spec() starts by emiting a warning
which calls formatwarning() which tries to import linecache again... and then
importlib emits again a warning... "import linecache" is tried twice and then
everything fails.
A workaround would be to unload the __main__ module first, before clearing sys
attributes.
Another workaround would be to not try to import modules in
warnings.formatwarning() if Python is exiting.
The tricky exit code was already discussed in the issue #19421 ("FileIO
destructor imports indirectly the io module at exit"). See also my more general
issue #21788 ("Rework Python finalization").
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nosy: +haypo
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<http://bugs.python.org/issue21925>
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