Merge for issue #26367

Lib/importlib/_bootstrap.py

@@ -922,9 +922,12 @@ def _sanity_check(name, package, level):
         raise TypeError('module name must be str, not {}'.format(type(name)))
     if level < 0:
         raise ValueError('level must be >= 0')
-    if package:
+    if level > 0:
         if not isinstance(package, str):
             raise TypeError('__package__ not set to a string')
+        elif not package:
+            raise ImportError('attempted relative import with no known parent '
+                              'package')
         elif package not in sys.modules:
             msg = ('Parent module {!r} not loaded, cannot perform relative '
                    'import')

Lib/test/test_importlib/import_/test_relative_imports.py

@@ -218,6 +218,11 @@ class RelativeImports:
             self.__import__('a', {'__package__': '', '__spec__': None},
                             level=1)
 
+    def test_relative_import_no_package_exists_absolute(self):
+        with self.assertRaises(ImportError):
+            self.__import__('sys', {'__package__': '', '__spec__': None},
+                            level=1)
+
 
 (Frozen_RelativeImports,
  Source_RelativeImports

Misc/NEWS

@@ -185,6 +185,9 @@ Core and Builtins
 
 Library
 -------
+- Issue #26367: importlib.__init__() raises ImportError like
+  builtins.__import__() when ``level`` is specified but without an accompanying
+  package specified.
 
 - Issue #26309: In the "socketserver" module, shut down the request (closing
   the connected socket) when verify_request() returns false.  Patch by Aviv