Revert #1575 and catch further KeyErrors (#1576)

Provide `ModuleSpec.submodule_search_locations` to the `path` argument of `PathFinder.find_spec()`

astroid/interpreter/_import/util.py

@@ -2,8 +2,12 @@
 # For details: https://github.com/PyCQA/astroid/blob/main/LICENSE
 # Copyright (c) https://github.com/PyCQA/astroid/blob/main/CONTRIBUTORS.txt
 
+from __future__ import annotations
+
+import pathlib
 import sys
 from functools import lru_cache
+from importlib._bootstrap_external import _NamespacePath
 from importlib.util import _find_spec_from_path
 
 
@@ -18,18 +22,46 @@ def is_namespace(modname: str) -> bool:
     # That's unacceptable here, so we fallback to _find_spec_from_path(), which does
     # not, but requires instead that each single parent ('astroid', 'nodes', etc.)
     # be specced from left to right.
-    components = modname.split(".")
-    for i in range(1, len(components) + 1):
-        working_modname = ".".join(components[:i])
+    processed_components = []
+    last_submodule_search_locations: _NamespacePath | None = None
+    for component in modname.split("."):
+        processed_components.append(component)
+        working_modname = ".".join(processed_components)
         try:
-            # Search under the highest package name
-            # Only relevant if package not already on sys.path
-            # See https://github.com/python/cpython/issues/89754 for reasoning
-            # Otherwise can raise bare KeyError: https://github.com/python/cpython/issues/93334
-            found_spec = _find_spec_from_path(working_modname, components[0])
+            # Both the modname and the path are built iteratively, with the
+            # path (e.g. ['a', 'a/b', 'a/b/c']) lagging the modname by one
+            found_spec = _find_spec_from_path(
+                working_modname, path=last_submodule_search_locations
+            )
         except ValueError:
             # executed .pth files may not have __spec__
             return True
+        except KeyError:
+            # Intermediate steps might raise KeyErrors
+            # https://github.com/python/cpython/issues/93334
+            # TODO: update if fixed in importlib
+            # For tree a > b > c.py
+            # >>> from importlib.machinery import PathFinder
+            # >>> PathFinder.find_spec('a.b', ['a'])
+            # KeyError: 'a'
+
+            # Repair last_submodule_search_locations
+            if last_submodule_search_locations:
+                # TODO: py38: remove except
+                try:
+                    # pylint: disable=unsubscriptable-object
+                    last_item = last_submodule_search_locations[-1]
+                except TypeError:
+                    last_item = last_submodule_search_locations._recalculate()[-1]
+                # e.g. for failure example above, add 'a/b' and keep going
+                # so that find_spec('a.b.c', path=['a', 'a/b']) succeeds
+                assumed_location = pathlib.Path(last_item) / component
+                last_submodule_search_locations.append(str(assumed_location))
+            continue
+
+        # Update last_submodule_search_locations
+        if found_spec and found_spec.submodule_search_locations:
+            last_submodule_search_locations = found_spec.submodule_search_locations
 
     if found_spec is None:
         return False

tests/unittest_manager.py

@@ -125,6 +125,9 @@ def test_submodule_homonym_with_non_module(self) -> None:
             util.is_namespace("tests.testdata.python3.data.parent_of_homonym.doc")
         )
 
+    def test_module_is_not_namespace(self) -> None:
+        self.assertFalse(util.is_namespace("tests.testdata.python3.data.all"))
+
     def test_implicit_namespace_package(self) -> None:
         data_dir = os.path.dirname(resources.find("data/namespace_pep_420"))
         contribute = os.path.join(data_dir, "contribute_to_namespace")