Description

heikkitoivonen

opened on Oct 19, 2026

Issue body actions

Bug report

On CPython 3.15.0, formatting a ModuleNotFoundError raises a second exception when sys.path contains no existing directory.

import sys
import traceback
import importlib.resources.readers  # Load dependencies before clearing sys.path.

sys.path[:] = []

error = ModuleNotFoundError(
    "No module named 'example_missing'",
    name="example_missing",
)
print(traceback.format_exception_only(error))

Expected: Expected: A formatted ModuleNotFoundError. (["ModuleNotFoundError: No module named 'example_missing'\n"])

Actual: exception raised (FileNotFoundError: MultiplexedPath must contain at least one path)

The same condition formats successfully on CPython 3.14.8. A path containing only nonexistent directories also triggers the failure; adding an existing directory prevents it.

In _find_incompatible_extension_module() (https://github.com/python/cpython/blob/v3.15.0/Lib/traceback.py#L2057), filtering sys.path can leave no arguments for MultiplexedPath, whose exception escapes the formatter.

Found on Linux ARM64 using a uv-managed CPython 3.15.0 build, and reproduced on macOS

• Linux: Python 3.15.0 (main, Oct 14 2026, 01:07:17) [Clang 23.1.3 ]
• macOS: Python 3.15.0 (main, Oct 9 2026, 15:11:47) [Clang 22.1.3 ]

CPython versions tested on:

3.15

Operating systems tested on:

Linux

Linked PRs

• gh-159693: Format import errors with no usable search paths #159830

Activity

1. heikkitoivonen

   added

   type-bugAn unexpected behavior, bug, or errorAn unexpected behavior, bug, or error

   on Oct 19, 2026
2. picnixz

   added

   stdlibStandard Library Python modules in the Lib/ directoryStandard Library Python modules in the Lib/ directory

   on Oct 19, 2026
3. picnixz commented on Oct 19, 2026

   

   picnixz

   on Oct 19, 2026

   Member

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   I would personally think it's weird to have no sys.path at all but the fix should be fairly simple so it's fine to fix this as well. Did you encounter this in a legitimate session btw?
4. heikkitoivonen commented on Oct 19, 2026

   

   heikkitoivonen

   on Oct 19, 2026

   ContributorAuthor

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   I was adding support for 3.15.0 on my https://pythoncomplexity.com/ site, and Claude/Codex found this when working on unit tests.
5. picnixz commented on Oct 19, 2026

   

   picnixz

   on Oct 19, 2026

   Member

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   Yeah we don't wan't to do defensive programming actually. I honestly can't imagine a situation where you would empty your path for a long time. However, what I can expect is:

   • Empty sys.path for some reasons
   • Do something to fill it with some other stuff
   • Then hit an exception.
   • And the traceback formatting fails for some reason.

   I don't know if format_exception_only is called and if it raises, then is ignored in the interpreter somewhere and rewritten in C only just in case, but if we use format_exception_only implicitly when raising exceptions, then I would accept a fix. If we don't use traceback for formatting exceptions in the REPL or elsewhere, I would rather reject the fix.

   General rule: please don't report issues found by LLMs if they are not realistic in general (hence my question about a real situatio happening).

   Reacted by Stan Ulbrych
6. picnixz

   added

   pendingThe issue will be closed if no feedback is providedThe issue will be closed if no feedback is provided

   on Oct 19, 2026

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