A better error message from importlib.resources.files() when module spec is None

importlib_resources/_adapters.py

@@ -160,9 +160,9 @@ def files(self):
         return CompatibilityFiles.SpecPath(self.spec, self._reader)
 
 
-def wrap_spec(package):
+def wrap_spec(spec):
     """
     Construct a package spec with traversable compatibility
     on the spec/loader/reader.
     """
-    return SpecLoaderAdapter(package.__spec__, TraversableResourcesLoader)
+    return SpecLoaderAdapter(spec, TraversableResourcesLoader)

importlib_resources/_common.py

@@ -113,7 +113,13 @@ def from_package(package: types.ModuleType):
     # deferred for performance (python/cpython#109829)
     from .future.adapters import wrap_spec
 
-    spec = wrap_spec(package)
+    if package.__spec__ is None:
+        raise TypeError(
+            f"Cannot access resources for '{package.__name__ or package!r}' "
+            "as it does not appear to correspond to an importable module (its __spec__ is None)."
+        )
+
+    spec = wrap_spec(package.__spec__)
     reader = spec.loader.get_resource_reader(spec.name)
     return reader.files()
 

importlib_resources/tests/test_resource.py

@@ -1,3 +1,4 @@
+import types
 import unittest
 from importlib import import_module
 
@@ -234,5 +235,17 @@ class ResourceFromNamespaceZipTests(
     MODULE = 'namespacedata01'
 
 
+class ResourceFromMainModuleWithNoneSpecTests(unittest.TestCase):
+    # `__main__.__spec__` can be `None` depending on how it is populated.
+    # https://docs.python.org/3/reference/import.html#main-spec
+    def test_main_module_with_none_spec(self):
+        mainmodule = types.ModuleType("__main__")
+
+        self.assertIsNone(mainmodule.__spec__)
+
+        with self.assertRaises(TypeError, msg="Cannot access resources for '__main__' as it does not appear to correspond to an importable module (its __spec__ is None)."):
+            resources.files(mainmodule)
+
+
 if __name__ == '__main__':
     unittest.main()