`ModuleType.__repr__` can fail on py312+

[AlexWaygood]

Bug report

Bug description:

Calling repr() on a ModuleType instance can fail with AttributeError on py312+, due to changes made in #98870. This is because the implementation of importlib._bootstrap._module_repr_from_spec assumes that all loaders will have a _path attribute. But if the module had a custom loader, that won't necessarily hold true: _path is an undocumented, internal, private attribute that CPython's import loaders have, but custom third-party loaders won't necessarily have.

An easy way to see this is the following (in a py312 venv with setuptools installed):

Python 3.12.0 (tags/v3.12.0:0fb18b0, Oct  2 2023, 13:03:39) [MSC v.1935 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> from setuptools._vendor import packaging
>>> repr(packaging)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<frozen importlib._bootstrap>", line 545, in _module_repr
  File "<frozen importlib._bootstrap>", line 830, in _module_repr_from_spec
AttributeError: 'VendorImporter' object has no attribute '_path'

setuptools implements the custom VendorImporter loader here: https://github.com/pypa/setuptools/blob/b1bf87be7869df40872e947e27296ef87e3125ae/setuptools/extern/__init__.py#L5-L70. Instances of VendorImporter do not have a _path attribute.

This kind of thing trips up external tools such as mypy's stubtest.py script, which calls repr() on arbitrary instances of types.ModuleType on certain code paths and (reasonably) expects that to always succeed: https://github.com/python/mypy/blob/1200d1d956e589a0a33c86ef8a7cb3f5a9b64f1f/mypy/stubtest.py#L224

Cc. @FFY00 and @jaraco, as participants in #98139 (the issue #98870 was linked to)

CPython versions tested on:

3.12

Operating systems tested on:

Windows

Linked PRs

• gh-112414: Fix AttributeError when calling repr() on a namespace package imported with a custom loader #112425
• gh-112414: Fix repr for custom loader #112436
• [3.12] gh-112414: Fix AttributeError when calling repr() on a namespace package imported with a custom loader (GH-112425) #112440
• gh-112414: Add additional unit tests for calling repr() on a namespace package #112475
• [3.12] gh-112414: Add additional unit tests for calling repr() on a namespace package (GH-112475) #112480

[AlexWaygood]

Maybe something like this would be a reasonable fix?

--- a/Lib/importlib/_bootstrap.py
+++ b/Lib/importlib/_bootstrap.py
@@ -826,8 +826,10 @@ def _module_repr_from_spec(spec):
     if spec.origin is None:
         if spec.loader is None:
             return f'<module {name!r}>'
-        else:
+        elif hasattr(spec.loader, "_path"):
             return f'<module {name!r} (namespace) from {list(spec.loader._path)}>'
+        else:
+            return f'<module {name!r} ({spec.loader!r})>'

[TeamSpen210]

Maybe it'd be better to do an isinstance() check. Then someone defining _path in their own loader wouldn't have the repr mysteriously change. Seems like an entirely reasonable attribute to have, and if it was a string you'd get rather silly results.

[AlexWaygood]

Maybe it'd be better to do an isinstance() check. Then someone defining _path in their own loader wouldn't have the repr mysteriously change. Seems like an entirely reasonable attribute to have, and if it was a string you'd get rather silly results.

Makes sense. That implies a diff like this:

--- a/Lib/importlib/_bootstrap.py
+++ b/Lib/importlib/_bootstrap.py
@@ -824,10 +824,16 @@ def _module_repr_from_spec(spec):
     """Return the repr to use for the module."""
     name = '?' if spec.name is None else spec.name
     if spec.origin is None:
-        if spec.loader is None:
+        loader = spec.loader
+        if loader is None:
             return f'<module {name!r}>'
+        elif (
+            _bootstrap_external is not None
+            and isinstance(loader, _bootstrap_external.NamespaceLoader)
+        ):
+            return f'<module {name!r} (namespace) from {list(loader._path)}>'
         else:
-            return f'<module {name!r} (namespace) from {list(spec.loader._path)}>'
+            # the loader could be a custom third-party object without a `_path` attribute
+            return f'<module {name!r} ({loader!r})>'

[AlexWaygood]

Another easy way to observe this bug: apply this diff to main:

diff --git a/Lib/test/test_importlib/import_/test___loader__.py b/Lib/test/test_importlib/import_/test___loader__.py
index 858b37effc..4ae60574ed 100644
--- a/Lib/test/test_importlib/import_/test___loader__.py
+++ b/Lib/test/test_importlib/import_/test___loader__.py
@@ -23,6 +23,7 @@ def test___loader__(self):
         with util.uncache('blah'), util.import_state(meta_path=[loader]):
             module = self.__import__('blah')
         self.assertEqual(loader, module.__loader__)
+        repr(module)

And then run python -m test test_importlib.import_.test___loader__ -v:

Test output

Using random seed: 2645428552
0:00:00 Run 1 test sequentially
0:00:00 [1/1] test_importlib.import_.test___loader__
test___loader__ (test.test_importlib.import_.test___loader__.Frozen_SpecLoaderAttributeTests.test___loader__) ... ERROR
test___loader__ (test.test_importlib.import_.test___loader__.Source_SpecLoaderAttributeTests.test___loader__) ... ERROR

======================================================================
ERROR: test___loader__ (test.test_importlib.import_.test___loader__.Frozen_SpecLoaderAttributeTests.test___loader__)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "C:\Users\alexw\coding\cpython\Lib\test\test_importlib\import_\test___loader__.py", line 26, in test___loader__
    repr(module)
  File "<frozen importlib._bootstrap>", line 545, in _module_repr
  File "<frozen importlib._bootstrap>", line 830, in _module_repr_from_spec
AttributeError: 'SpecLoaderMock' object has no attribute '_path'

======================================================================
ERROR: test___loader__ (test.test_importlib.import_.test___loader__.Source_SpecLoaderAttributeTests.test___loader__)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "C:\Users\alexw\coding\cpython\Lib\test\test_importlib\import_\test___loader__.py", line 26, in test___loader__
    repr(module)
  File "<frozen importlib._bootstrap>", line 545, in _module_repr
  File "<frozen importlib._bootstrap>", line 830, in _module_repr_from_spec
AttributeError: 'SpecLoaderMock' object has no attribute '_path'

----------------------------------------------------------------------
Ran 2 tests in 0.022s

FAILED (errors=2)
test test_importlib.import_.test___loader__ failed
test_importlib.import_.test___loader__ failed (2 errors)

== Tests result: FAILURE ==

1 test failed:
    test_importlib.import_.test___loader__

Total duration: 254 ms
Total tests: run=2
Total test files: run=1/1 failed=1
Result: FAILURE

I've filed #112425 to fix it.