Issue #15767: Introduce ModuleNotFoundError, a subclass of

ImportError.

The exception is raised by import when a module could not be found.
Technically this is defined as no viable loader could be found for the
specified module. This includes ``from ... import`` statements so that
the module usage is consistent for all situations where import
couldn't find what was requested.

This should allow for the common idiom of::

  try:
    import something
  except ImportError:
    pass

to be updated to using ModuleNotFoundError and not accidentally mask
ImportError messages that should propagate (e.g. issues with a
loader).

This work was driven by the fact that the ``from ... import``
statement needed to be able to tell the difference between an
ImportError that simply couldn't find a module (and thus silence the
exception so that ceval can raise it) and an ImportError that
represented an actual problem.

2 files changed, 8 insertions, 4 deletions

diff --git a/Lib/test/test_importlib/import_/test_api.py b/Lib/test/test_importlib/import_/test_api.py
index 3d4cd94..6aca6dc 100644
--- a/Lib/test/test_importlib/import_/test_api.py
+++ b/Lib/test/test_importlib/import_/test_api.py
@@ -22,6 +22,10 @@ class APITest(unittest.TestCase):
     """Test API-specific details for __import__ (e.g. raising the right
     exception when passing in an int for the module name)."""
 
+    def test_raises_ModuleNotFoundError(self):
+        with self.assertRaises(ModuleNotFoundError):
+            util.import_('some module that does not exist')
+
     def test_name_requires_rparition(self):
         # Raise TypeError if a non-string is passed in for the module name.
         with self.assertRaises(TypeError):
diff --git a/Lib/test/test_importlib/import_/test_fromlist.py b/Lib/test/test_importlib/import_/test_fromlist.py
index c16c337..9aa27a0 100644
--- a/Lib/test/test_importlib/import_/test_fromlist.py
+++ b/Lib/test/test_importlib/import_/test_fromlist.py
@@ -69,16 +69,16 @@ class HandlingFromlist(unittest.TestCase):
                 self.assertTrue(hasattr(module, 'module'))
                 self.assertEqual(module.module.__name__, 'pkg.module')
 
-    def test_module_from_package_triggers_ImportError(self):
-        # If a submodule causes an ImportError because it tries to import
-        # a module which doesn't exist, that should let the ImportError
+    def test_module_from_package_triggers_ModuleNotFoundError(self):
+        # If a submodule causes an ModuleNotFoundError because it tries to import
+        # a module which doesn't exist, that should let the ModuleNotFoundError
         # propagate.
         def module_code():
             import i_do_not_exist
         with util.mock_modules('pkg.__init__', 'pkg.mod',
                                module_code={'pkg.mod': module_code}) as importer:
             with util.import_state(meta_path=[importer]):
-                with self.assertRaises(ImportError) as exc:
+                with self.assertRaises(ModuleNotFoundError) as exc:
                     import_util.import_('pkg', fromlist=['mod'])
                 self.assertEqual('i_do_not_exist', exc.exception.name)